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A Dictionary of Science, Sixth Edition (Oxford Paperback Reference)
A Dictionary of Science SIXTH EDITION 1 1 Great Clarendon Street, Oxford OX2 6DP Oxford University Press is a depa...
Author: John Daintith and Elizabeth Martin (Editors)
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A Dictionary of
Science SIXTH EDITION
Great Clarendon Street, Oxford OX2 6DP Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offices in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries Published in the United States by Oxford University Press Inc., New York © Market House Books Ltd, 1984, 1991, 1996, 1999, 2005, 2010 The moral rights of the author have been asserted Database right Oxford University Press (maker) First published 1984 as Concise Science Dictionary Second edition 1991 Third edition 1996 Fourth edition 1999 Fifth edition 2005 Sixth edition 2010 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose this same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Data available Typeset by Market House Books Ltd. Printed in Great Britain by Clays Ltd, St Ives plc ISBN 978–0–19–956146–9 10 9 8 7 6 5 4 3 2 1
A Dictionary of Science
Atomic Theory Chronology The Big-Bang Theory (Feature) Biochemistry Chronology Cell Biology Chronology Cosmology Chronology Crystal Defects (Feature) Electronics Chronology El Niño (Feature) Explosives Chronology Genetically Modified Organisms (Feature) Learning in Animals (Feature) Microscopy Chronology Moon Exploration Chronology Magnetic Resonance Imaging (Feature) Plastics Chronology Polymers (Feature) Refrigeration (Feature) Solar System (Feature) Optical Astronomical Telescopes (Feature) Vitamins Chronology
63 86 90 144 200 210 275 284 308 352 470 528 541 569 638 648 702 766 809 863
Appendices SI units Fundamental constants The solar system Geological time scale Simplified classification of land plants Simplified classification of the animal kingdom The periodic table Useful websites
Preface This sixth edition of A Dictionary of Science, like its predecessors, aims to provide school and first-year university students with accurate explanations of any unfamiliar words they might come across in the course of their studies, in their own or adjacent disciplines. For example, students of the physical sciences will find all they are likely to need to know about the life sciences, and vice versa. The dictionary is also designed to provide non-scientists with a useful reference source to explain the scientific terms that they may encounter in their work or in their general reading. At this level the dictionary provides full coverage of terms, concepts, and laws relating to physics, chemistry, biology, biochemistry, palaeontology, and the earth sciences. There is also coverage of key terms in astronomy, cosmology, mathematics, biotechnology, and computer technology. In addition, the dictionary includes: •
over 160 short biographical entries on the most important scientists in the history of the subject
ten features (each of one or two pages) on concepts of special significance in modern science
ten chronologies showing the development of selected concepts, fields of study, and industries
eight Appendices, including the periodic table, tables of SI units and conversion tables to and from other systems of units, summary classifications of the plant and animal kingdoms, and useful websites.
For this sixth edition nearly 700 new entries have been added to the text, incorporating recent advances in all the major fields and increased coverage of astronomy, forensic chemistry, and computing. In compiling the dictionary, the contributors and editors have made every effort to make the entries as concise and comprehensible as possible, always bearing in mind the needs of the readers. Particular features of the book are its lack of unnecessary scientific jargon and its extensive network of cross-references. An asterisk placed before a word used in an entry indicates that this word can be looked up in the dictionary and will provide further explanation or clarification. However, not every word that is defined in the dictionary has an asterisk placed before it when it is used in an entry. Some entries simply refer the reader to another entry, indicating either that they are synonyms or abbreviations or that they are most conveniently explained in one of the dictionary's longer articles. Synonyms and abbreviations are usually placed within brackets immediately after the headword. Terms that are explained within an entry are highlighted by being printed in boldface type. Where appropriate, the entries have been supplemented by fully labelled line-drawings or tables in situ. JD EM 2009
Credits Editors John Daintith BSc, PhD Elizabeth Martin MA
Advisers B. S. Beckett BSc, BPhil, MA(Ed) R. A. Hands BSc Michael Lewis MA
Contributors Richard Batley PhD, MInstP Tim Beardsley BA Lionel Bender BSc John Clark BSc H. M. Clarke MA, MSc W. M. Clarke BSc Derek Cooper PhD, FRIC John Cullerne DPhil R. Cutler BSc E. K. Daintith BSc D. E. Edwards BSc, MSc A. J. H. Goddard PhD, MIMechE, FInstNucE, FInstP, MSRP William Gould BA
Malcolm Hart BSc, MIBiol Robert S. Hine BSc, MSc Elaine Holmes BSc, PhD Valerie Illingworth BSc, MPhil Anne Lockwood BSc J. Valerie Neal BSc, PhD R. A. Prince MA Richard Rennie BSc, MSc, PhD Michael Ruse BSc, PhD Jackie Smith BA Brian Stratton BSc, MSc Elizabeth Tootill BSc, MSc David Eric Ward BSc, MSc, PhD Edmund Wright DPhil
A aa See . AAR See   . AAS See   . ab- A preÜx attached to the name of a practical electrical unit to provide a name for a unit in the electromagnetic system of units (see  ), e.g. abampere, abcoulomb, abvolt. The preÜx is an abbreviation of the word 'absolute' as this system is also known as the absolute system. Compare -. In modern practice both absolute and electrostatic units have been replaced by *SI units. abdomen The posterior region of the body trunk of animals. In vertebrates it contains the stomach and intestines and the organs of excretion and reproduction. It is particularly well deÜned in mammals, being separated from the *thorax by the *diaphragm. In many arthropods, such as insects and spiders, it may be segmented. Abelian group See . aberration 1. (in optics) A defect in the image formed by a lens or curved mirror. In chromatic aberration the image formed by a lens (but not a mirror) has coloured fringes as a result of the different extent to which light of different colours is refracted by glass. It is corrected by using an *achromatic lens. In spherical aberration, the rays from the object come to a focus in slightly different positions as a result of the curvature of the lens or mirror. For a mirror receiving light strictly parallel with its axis, this can be corrected by using a parabolic surface rather than a spherical surface. Spherical aberration in lenses is minimized by making both surfaces contribute equally to the ray deviations, and can be lessened (though with reduced image brightness) by the use of diaphragms to let light pass only through the centre part of the lens. See also ; . 2. (in astronomy) The apparent displacement in the position of a star as a result of the earth's motion round the sun. Light appears to come from a point that is slightly displaced in the direction of the earth's mo-
tion. The angular displacement α = v/c, where v is the earth's orbital velocity and c is the speed of light.
abiogenesis The origin of living from nonliving matter, as by *biopoiesis. See also  . abiotic factor Any of the nonliving factors that make up the abiotic environment in which living organisms occur. They include all the aspects of climate, geology, and atmosphere that may affect the biotic environment. Compare  . abomasum The fourth and Ünal chamber of the stomach of ruminants. It leads from the *omasum and empties into the small intestine. The abomasum is referred to as the 'true stomach' as it is in this chamber that protein digestion occurs, in acidic conditions. See R. ABO system One of the most important human *blood group systems. The system is based on the presence or absence of *antigens A and B on the surface of red blood cells and of *antibodies against these in blood serum. A person whose blood contains either or both these antibodies cannot receive a transfusion of blood containing the corresponding antigens as this would cause the red cells to clump (see ). The table overleaf illustrates the basis of the system: people of blood group O are described as 'universal donors' as they can give blood to those of any of the other groups. See also  . abscisic acid A naturally occurring plant hormone that appears to be involved primarily in seed maturation, stress responses (e.g. to heat and waterlogging), and in regulating closure of leaf pores (stomata). In seeds, it promotes the synthesis of storage protein and prevents premature germination. In leaves, abscisic acid is produced in large amounts when the plant lacks sufÜcient water, promoting closure of stomata and hence reducing further water losses. It was formerly believed to play a central role in *abscission, hence the name.
abscissa
2 Antigens on red cell surface
Antibodies in serum
Blood group of people donor can receive blood from
Blood group of people donor can give blood to
anti-B
A, O
A, AB
anti-A
B, O
B, AB
A, B, AB, O
anti-A and anti-B
ABO system. abscissa See C . abscission The separation of a leaf, fruit, or other part from the body of a plant. It involves the formation of an abscission zone, at the base of the part, within which a layer of cells (abscission layer) breaks down. This process is suppressed so long as sufÜcient amounts of *auxin, a plant hormone, Ûow from the part through the abscission zone. However, if the auxin Ûow declines, for example due to injury or ageing, abscission is activated. Ethylene (*ethene) acts as the primary trigger for abscission, inducing cells in the abscission zone to produce cellulase enzymes that degrade cell walls. absolute 1. Not dependent on or relative to anything else, e.g. *absolute zero. 2. Denoting a temperature measured on an absolute scale, a scale of temperature based on absolute zero. The usual absolute scale now is that of thermodynamic *temperature; its unit, the kelvin, was formerly called the degree absolute (°A) and is the same size as the degree Celsius. In British engineering practice an absolute scale with Fahrenheit-size degrees has been used: this is the Rankine scale. absolute alcohol See . absolute conÜguration A way of denoting the absolute structure of an optical isomer (see  ). Two conventions are in use: The D–L convention relates the structure of the molecule to some reference molecule. In the case of sugars and similar compounds, the dextrorotatory form of glyceraldehyde (HOCH2CH(OH)CHO), 2,3dihydroxypropanal) was used. The rule is as follows. Write the structure of this molecule down with the asymmetric carbon in the centre, the –CHO group at the top, the –OH on the right, the –CH2OH at the bottom, and the –H on the left. Now imagine that the cen-
tral carbon atom is at the centre of a tetrahedron with the four groups at the corners and that the –H and –OH come out of the paper and the –CHO and –CH2OH groups go into the paper. The resulting three-dimensional structure was taken to be that of d-glyceraldehyde and called D-glyceraldehyde. Any compound that contains an asymmetric carbon atom having this conÜguration belongs to the D-series. One having the opposite conÜguration belongs to the L-series. It is important to note that the preÜxes D- and Ldo not stand for dextrorotatory and laevorotatory (they are not the same as d- and l-). In fact the arbitrary conÜguration assigned to D-glyceraldehyde is now known to be the correct one for the dextrorotatory form, although this was not known at the time. However, all D-compounds are not dextrorotatory. For instance, the acid obtained by oxidizing the –CHO group of glyceraldehyde is glyceric acid (1,2-dihydroxypropanoic acid). By convention, this belongs to the D-series, but it is in fact laevorotatory; i.e. its name can be written as D-glyceric acid or l-glyceric acid. To avoid confusion it is better to use + (for dextrorotatory) and – (for laevorotatory), as in D-(+)-glyceraldehyde and D-(–)-glyceric acid. The D–L convention can also be used with alpha amino acids (compounds with the –NH2 group on the same carbon as the –COOH group). In this case the molecule is imagined as being viewed along the H–C bond between the hydrogen and the asymmetric carbon atom. If the clockwise order of the other three groups is –COOH, –R, –NH2, the amino acid belongs to the D-series; otherwise it belongs to the L-series. This is known as the CORN rule. The R–S convention is a convention based on priority of groups attached to the chiral carbon atom. The order of priority is I, Br, Cl, SO3H, OCOCH3, OCH3, OH, NO2, NH2,
absolute value
3 CHO H
HCOH
structure in 3 dimensions
planar formula
Fischer projection
D-(+)-glyceraldehyde (2,3-dihydroxypropanal) COOH
H C CH3
H COOH NH2
D-alanine (R is CH2 in the CORN rule). The molecule is viewed with H on top
R–configuration
S–configuration
R–S system. The lowest priority group is behind the chiral carbon atom
Absolute configuration. COOCH3, CONH2, COCH3, CHO, CH2OH, C6H5, C2H5, CH3, H, with hydrogen lowest. The molecule is viewed with the group of lowest priority behind the chiral atom. If the clockwise arrangement of the other three groups is in descending priority, the compound belongs to the R-series; if the descending order is anticlockwise it is in the S-series. D-(+)-glyceraldehyde is R-(+)-glyceraldehyde. See illustration.
without reference to a tuned musical instrument.
absolute space Space that exists as a background to events and processes and is not affected by objects or other entities in the universe. The idea underpins Newtonian physics, although many physicists have always regarded absolute space as an undesirable concept and suggested, as in *Mach's principle, that fundamental physics should be described by *relational theories.
• Information about IUPAC nomenclature for the R–S system
absolute temperature See ; .
absolute expansivity See . absolute permittivity See .
absolute time Time that exists independently of any events or processes in the universe. Like *absolute space, absolute time is a basic concept in Newtonian physics.
absolute pitch (perfect pitch) The ability of a person to identify and reproduce a note
absolute value (modulus) The square root of the sum of the squares of the real
absolute humidity See .
numbers in a *complex number, i.e. the absolute value of the complex number z = x + iy is |z| = √(x2 + y2).
absolute zero Zero of thermodynamic *temperature (0 kelvin) and the lowest temperature theoretically attainable. It is the temperature at which the kinetic energy of atoms and molecules is minimal. It is equivalent to –273.15°C or –459.67°F. See also - ; . absorbed dose See . absorptance Symbol α. The ratio of the radiant or luminous Ûux absorbed by a body to the Ûux falling on it. Formerly called absorptivity, the absorptance of a *black body is by deÜnition 1. absorption 1. (in chemistry) The take up of a gas by a solid or liquid, or the take up of a liquid by a solid. Absorption differs from *adsorption in that the absorbed substance permeates the bulk of the absorbing substance. 2. (in physics) The conversion of the energy of electromagnetic radiation, sound, streams of particles, etc., into other forms of energy on passing through a medium. A beam of light, for instance, passing through a medium, may lose intensity because of two effects: scattering of light out of the beam, and absorption of photons by atoms or molecules in the medium. When a photon is absorbed, there is a transition to an excited state. 3. (in biology) The movement of Ûuid or a dissolved substance across a plasma membrane. In many animals, for example, soluble food material is absorbed into cells lining the alimentary canal and thence into the blood. In plants, water and mineral salts are absorbed from the soil by the *roots. See ;  . absorption coefÜcient 1. (in physics) See L' . 2. (in chemistry) The volume of a given gas, measured at standard temperature and pressure, that will dissolve in unit volume of a given liquid. absorption indicator See  . absorption spectrum See . absorptivity See . ABS plastic Any of a class of plastics based on acrylonitrile–butadiene–styrene copolymers. abundance 1. The ratio of the total mass of a speciÜed element in the earth's crust to
the total mass of the earth's crust, often expressed as a percentage. For example, the abundance of aluminium in the earth's crust is about 8%. 2. The ratio of the number of atoms of a particular isotope of an element to the total number of atoms of all the isotopes present, often expressed as a percentage. For example, the abundance of uranium–235 in natural uranium is 0.71%. This is the natural abundance, i.e. the abundance as found in nature before any enrichment has taken place.
abyssal zone The lower depths of the ocean (below approximately 2000 metres), where there is effectively no light penetration. Abyssal organisms are adapted for living under high pressures in cold dark conditions. See also  . a.c. See  . acac The symbol for the *acetylacetonato ligand, used in formulae. accelerant A Ûammable material used to start and spread a Üre in cases of arson. Petrol and parafÜn are the substances commonly used. Traces of accelerant are detectable by gas chromatography in forensic work. acceleration Symbol a. The rate of increase of speed or velocity. It is measured in m s–2. For a body moving linearly with constant acceleration a from a speed u to a speed v, a = (v – u)/t = (v2 – u2)/2s where t is the time taken and s the distance covered. If the acceleration is not constant it is given by dv/dt = d2s/dt2. If the motion is not linear the vector character of displacement, velocity, and acceleration must be considered. See also  . acceleration of free fall Symbol g. The acceleration experienced by any massive object falling freely in the earth's gravitational Üeld. Experimentally this is almost constant for all positions near the earth's surface, independent of the nature of the falling body (provided air resistance is eliminated). This is taken to indicate the strict proportionality of *weight (the force causing the acceleration) and inertial *mass, on the basis of Newton's second law of motion (see N'   ). There is some variation of g with latitude, because of the earth's rotation and because the earth is
not completely spherical. The standard value is taken as 9.806 65 m s–2. The acceleration of free fall is also called the acceleration due to gravity.
accelerator 1. (in physics) An apparatus for increasing the kinetic energies of charged particles, used for research in nuclear and particle physics. See ;  ; ; . 2. (in chemistry) A substance that increases the rate of a chemical reaction, i.e. a catalyst.
• A list of world particle accelerators and accelerator laboratories, with links, from the ELSA website at the University of Bonn
acceptor 1. (in chemistry and biochemistry) A compound, molecule, ion, etc., to which electrons are donated in the formation of a coordinate bond. 2. (in biochemistry) A *receptor that binds a hormone without any apparent biological response. 3. (in physics) A substance that is added as an impurity to a *semiconductor because of its ability to accept electrons from the valence bands, causing p-type conduction by the mobile positive holes left. Compare . acceptor levels Energy levels of an acceptor atom in a *semiconductor, such as aluminium, in silicon. These energy levels are very near the top of the valence band, and therefore cause p-type conduction. See also  . access point (wireless access point) A device that acts as the core of a wireless network. It communicates with wireless nodes within its range and provides the necessary facilities for them to network successfully. Access points commonly also manage a link to a wired network, allowing their nodes to link with corporate networks, the Internet, etc. acclimation The physiological changes occurring in an organism in response to a change in a particular environmental factor (e.g. temperature), especially under laboratory conditions. Thermal acclimation studies reveal how such properties as metabolic rate, muscle contractility, nerve conduction, and heart rate differ between cold- and warm-acclimated members of the same species. These changes occur naturally during *acclimatization and equip the organism for living in, say, cold or warm conditions.
acentric acclimatization 1. The progressive adaptation of an organism to any change in its natural environment that subjects it to physiological stress. 2. The overall sum of processes by which an organism attempts to compensate for conditions that would substantially reduce the amount of oxygen delivered to its cells. Compare . accommodation 1. (in animal physiology) Focusing: the process by which the focal length of the *lens of the eye is changed so that clear images of objects at a range of distances are displayed on the retina. In humans and some other mammals accommodation is achieved by reÛex adjustments in the shape of the lens brought about by relaxation and contraction of muscles within the *ciliary body. 2. (in animal behaviour) Adjustments made by an animal's nervous or sensory systems in response to continuously changing environmental conditions. accretion The way in which collisions with relatively slow-moving smaller objects add to the mass of a larger celestial object. The process accelerates as the increased mass strengthens the gravitational Üeld of the larger object. For example, the planets are thought to have formed by the accretion of dust particles onto *planetesimals. Other accreting objects probably include black holes and protostars. accretion disc A disc-shaped rotating mass formed by gravitational attraction. See  ;  ;  . accumulator (secondary cell; storage battery) A type of *voltaic cell or battery that can be recharged by passing a current through it from an external d.c. supply. The charging current, which is passed in the opposite direction to that in which the cell supplies current, reverses the chemical reactions in the cell. The common types are the *lead–acid accumulator and the *nickel–iron and nickel–cadmium accumulators. See also – . acellular Describing tissues or organisms that are not made up of separate cells but often have more than one nucleus (see ). Examples of acellular structures are muscle Übres. Compare . acentric Describing an aberrant chromosome fragment that lacks a centromere. Such fragments are normally lost because they are unable to orientate properly during cell division.
acetaldehyde See . acetaldol See  . acetals Organic compounds formed by addition of alcohol molecules to aldehyde molecules. If one molecule of aldehyde (RCHO) reacts with one molecule of alcohol (R′OH) a hemiacetal is formed (RCH(OH)OR′). The rings of aldose sugars are hemiacetals. Further reaction with a second alcohol molecule produces a full acetal (RCH(OR′)2). It is common to refer to both types of compounds simply as 'acetals'. The formation of acetals is reversible; acetals can be hydrolysed back to aldehydes in acidic solutions. In synthetic organic chemistry aldehyde groups are often converted into acetal groups to protect them before performing other reactions on different groups in the molecule. See also .
• Information about IUPAC nomenclature
R + R1OH
hemiacetal
H alcohol
acetylacetonato The ion (CH3COCHCOCH3)–, functioning as a bidentate ligand coordinating through the two oxygen atoms. In formulae, the symbol acac is used. acetylation See . acetyl chloride See  . acetylcholine (ACh) One of the main *neurotransmitters of the vertebrate nervous system. It is released at some (cholinergic) nerve endings and may be excitatory or inhibitory; it initiates muscular contraction at *neuromuscular junctions. Once acetylcholine has been released it has only a transitory effect because it is rapidly broken down by the enzyme *cholinesterase. acetylcholinesterase See .
test) A *presumptive test for blood in which a small amount of acetone (propenal) is added to the bloodstain, followed by a drop of hydrochloric acid. Haemoglobin produces derivatives such as haematin and haemin, forming small characteristic crystals that can be identiÜed under a microscope.
Formation of acetals. acetamide See . acetanilide A white crystalline primary amide of ethanoic acid, CH3CONHC6H5; r.d. 1.2; m.p. 114.3°C; b.p. 304°C. It is made by reacting phenylamine (aniline) with excess ethanoic acid or ethanoic anhydride and is used in the manufacture of dyestuffs and rubber. The full systematic name is Nphenylethanamide. acetate See . acetate process See . acetic acid See  . acetoacetic acid See - . acetoacetic ester See  -. acetone See ;  . acetone–chlor–haemin test (Wagenaar
acetyl coenzyme A (acetyl CoA) A compound formed in the mitochondria when an acetyl group (CH3CO–), derived from the breakdown of fats, proteins, or carbohydrates (via *glycolysis), combines with the thiol group (–SH) of *coenzyme A. Acetyl CoA feeds into the energy generating *Krebs cycle and also plays a role in the synthesis and oxidation of fatty acids. acetylene See . acetylenes See . acetyl group See  . acetylide See . achene A dry indehiscent fruit formed from a single carpel and containing a single seed. An example is the feathery achene of clematis. Variants of the achene include the *caryopsis, *cypsela, *nut, and *samara. See also . Acheson process An industrial process for the manufacture of graphite by heating coke mixed with clay. The reaction involves the production of silicon carbide, which loses silicon at 4150°C to leave graphite. The process was patented in 1896 by the US inventor Edward Goodrich Acheson (1856–1931).
achiral Describing a molecule that does not contain a *chirality element. achondrite A stony meteorite that has no spherical silicate particles (chondrules) found in the meteorites called chondrites. Achondrites do not contain iron or nickel and have a coarser crystal structure than chondrites. achromatic lens A lens that corrects for chromatic *aberration by using a combination of two lenses, made of different kinds of glass, such that their *dispersions neutralize each other although their *refractions do not. The aberration can be reduced further by using an apochromatic lens, which consists of three or more different kinds of glass. aciclovir (acyclovir; acycloguanosine) A drug used to treat cold sores, shingles, genital blisters, or other lesions caused by herpesvirus infection. It is an analogue of the base guanine and acts by interfering with DNA replication of the virus. acid 1. A type of compound that contains hydrogen and dissociates in water to produce positive hydrogen ions. The reaction, for an acid HX, is commonly written: HX ˆ H+ + X– In fact, the hydrogen ion (the proton) is solvated, and the complete reaction is: HX + H2O ˆ H3O+ + X– The ion H3O+ is the oxonium ion (or hydroxonium ion or hydronium ion). This deÜnition of acids comes from the Arrhenius theory. Such acids tend to be corrosive substances with a sharp taste, which turn litmus red and give colour changes with other *indicators. They are referred to as protonic acids and are classiÜed into strong acids, which are almost completely dissociated in water (e.g. sulphuric acid and hydrochloric acid), and weak acids, which are only partially dissociated (e.g. ethanoic acid and hydrogen sulphide). The strength of an acid depends on the extent to which it dissociates, and is measured by its *dissociation constant. See also . 2. In the Lowry–Brønsted theory of acids and bases (1923), the deÜnition was extended to one in which an acid is a proton donor, and a base is a proton acceptor. For example, in HCN + H2O ˆ H3O+ + CN– the HCN is an acid, in that it donates a proton to H2O. The H2O is acting as a base in ac-
acid anhydrides cepting a proton. Similarly, in the reverse reaction H3O+ is an acid and CN– a base. In such reactions, two species related by loss or gain of a proton are said to be conjugate. Thus, in the reaction above HCN is the conjugate acid of the base CN–, and CN– is the conjugate base of the acid HCN. Similarly, H3O+ is the conjugate acid of the base H2O. An equilibrium, such as that above, is a competition for protons between an acid and its conjugate base. A strong acid has a weak conjugate base, and vice versa. Under this deÜnition water can act as both acid and base. Thus in NH3 + H2O ˆ NH4+ + OH– the H2O is the conjugate acid of OH–. The deÜnition also extends the idea of acid–base reaction to solvents other than water. For instance, liquid ammonia, like water, has a high dielectric constant and is a good ionizing solvent. Equilibria of the type NH3 + Na+Cl– ˆ Na+NH2– + HCl can be studied, in which NH3 and HCl are acids and NH2– and Cl– are their conjugate bases. 3. A further extension of the idea of acids and bases was made in the Lewis theory (G. N. Lewis, 1923). In this, a Lewis acid is a compound or atom that can accept a pair of electrons and a Lewis base is one that can donate an electron pair. This deÜnition encompasses 'traditional' acid–base reactions. In HCl + NaOH → NaCl + H2O the reaction is essentially H+ + :OH– → H:OH i.e. donation of an electron pair by OH–. But it also includes reactions that do not involve ions, e.g. H3N: + BCl3 → H3NBCl3 in which NH3 is the base (donor) and BCl3 the acid (acceptor). The Lewis theory establishes a relationship between acid–base reactions and *oxidation–reduction reactions. See HSAB . See also  ; ; .
acid anhydrides (acyl anhydrides) Compounds that react with water to form an acid. For example, carbon dioxide reacts with water to give carbonic acid: CO2(g) + H2O(aq) ˆ H2CO3(aq) A particular group of acid anhydrides are anhydrides of carboxylic acids. They have a general formula of the type R.CO.O.CO.R′,
acid–base balance
_ _ _C O _ _ C O _ R' R
HO HO
_ _ _C O O_ _ _C O R' R
– H2O
acid anhydride
Acid anhydride. Formation of a carboxylic acid anhydride.
where R and R′ are alkyl or aryl groups. For example, the compound ethanoic anhydride (CH3.CO.O.CO.CH3) is the acid anhydride of ethanoic (acetic) acid. Organic acid anhydrides can be produced by dehydrating acids (or mixtures of acids). They are usually made by reacting an acyl halide with the sodium salt of the acid. They react readily with water, alcohols, phenols, and amines and are used in *acylation reactions.
acid–base balance The regulation of the concentrations of acids and bases in blood and other body Ûuids so that the pH remains within a physiologically acceptable range. This is achieved by the presence of natural *buffer systems, such as the haemoglobin, hydrogencarbonate ions, and carbonic acid in mammalian blood. By acting in conjunction, these effectively mop up excess acids and bases and therefore prevent any large shifts in blood pH. The acid–base balance is also inÛuenced by the selective removal of certain ions by the kidneys and the rate of removal of carbon dioxide from the lungs. acid–base indicator See . acid dissociation constant See . acid dye See . acid halides See  . acidic 1. Describing a compound that is an acid. 2. Describing a solution that has an excess of hydrogen ions. 3. Describing a compound that forms an acid when dissolved in water. Carbon dioxide, for example, is an acidic oxide. acidic hydrogen (acid hydrogen) A hydrogen atom in an *acid that forms a positive ion when the acid dissociates. For instance, in methanoic acid HCOOH ˆ H+ + HCOO– the hydrogen atom on the carboxylate group is the acidic hydrogen (the one bound di-
rectly to the carbon atom does not dissociate).
acidic stains See . acidimetry Volumetric analysis using standard solutions of acids to determine the amount of base present. acidity constant See . acid rain Precipitation having a pH value of less than about 5.0, which has adverse effects on the fauna and Ûora on which it falls. Rainwater typically has a pH value of 5.6, due to the presence of dissolved carbon dioxide (forming carbonic acid). Acid rain results from the emission into the atmosphere of various pollutant gases, in particular sulphur dioxide and various oxides of nitrogen, which originate from the burning of fossil fuels and from car exhaust fumes, respectively. These gases dissolve in atmospheric water to form sulphuric and nitric acids in rain, snow, or hail (wet deposition). Alternatively, the pollutants are deposited as gases or minute particles (dry deposition). Both types of acid deposition affect plant growth – by damaging the leaves and impairing photosynthesis and by increasing the acidity of the soil, which results in the leaching of essential nutrients. This acid pollution of the soil also leads to acidiÜcation of water draining from the soil into lakes and rivers, which become unable to support Üsh life. Lichens are particularly sensitive to changes in pH and can be used as indicators of acid pollution (see  ). acid rock A low-density igneous rock containing a preponderance (more than 65%) of light-coloured *silicate minerals. Examples include granite and rhyolite. acid salt A salt of a polybasic acid (i.e. an acid having two or more acidic hydrogens) in which not all the hydrogen atoms have been replaced by positive ions. For example, the dibasic acid carbonic acid (H2CO3) forms acid salts (hydrogencarbonates) containing the ion HCO3–. Some salts of monobasic acids are also known as acid salts. For instance, the compound potassium hydrogendiÛuoride, KHF2, contains the ion [F...H–F]–, in which there is hydrogen bonding between the Ûuoride ion F– and a hydrogen Ûuoride molecule. acid value A measure of the amount of free acid present in a fat, equal to the number of milligrams of potassium hydroxide
actinic radiation
needed to neutralize this acid. Fresh fats contain glycerides of fatty acids and very little free acid, but the glycerides decompose slowly with time and the acid value increases.
acinus The smallest unit of a multilobular gland, such as the pancreas. Each acinus in the pancreas is made up of a hollow cluster of acinar cells, which produce the digestive enzymes secreted in pancreatic juice. Minute ducts from the pancreatic acini eventually drain into the pancreatic duct. acoustics 1. The study of sound and sound waves. 2. The characteristics of a building, especially an auditorium, with regard to its ability to enable speech and music to be heard clearly within it. For this purpose there should be no obtrusive echoes or resonances and the reverberation time should be near the optimum for the hall. Echoes are reduced by avoiding sweeping curved surfaces that could focus the sound and by breaking up large plane surfaces or covering them with sound-absorbing materials. Resonance is avoided by avoiding simple ratios for the main dimensions of the room, so that no one wavelength of sound is a factor of more than one of them. If the reverberation time is too long, speech will sound indistinct and music will be badly articulated, with one note persisting during the next. However, if it is too short, music sounds dead. It is long in a bare room with hard walls, and can be deliberately reduced by carpets, soft furnishings and sound-absorbent ('acoustic') felt. Reverberation times tend to be reduced by the presence of an audience and this must be taken into account in the design of the building.
• The website of the Acoustical Society of America • The website of the UK Institute of Acoustics
acoustoelectronic devices (electroacoustic devices) Devices in which electronic signals are converted into acoustic waves. Acoustoelectronic devices are used in constructing *delay lines and also in converting digital data from computers for transmission by telephone lines. acquired characteristics Features that are developed during the lifetime of an individual, e.g. the enlarged arm muscles of a tennis player. Such characteristics are not genetically controlled and cannot be passed
on to the next generation. See also L; -L.
acquired immune deÜciency syndrome See AIDS. Acrilan A trade name for a synthetic Übre. See  . acrolein See . acromegaly A chronic condition developing in adulthood due to overproduction of (or oversensitivity to) *growth hormone, usually caused by a tumour in the pituitary gland. This leads to a gradual enlargement of the bones, causing characteristic coarsening of the facial features and large hands and feet. acrosome See . acrylamide An inert gel (polyacrylamide) employed as a medium in *electrophoresis. It is used particularly in the separation of macromolecules, such as nucleic acids and proteins. acrylate See . acrylic acid See  . acrylic resins Synthetic resins made by polymerizing esters or other derivatives of acrylic acid (propenoic acid). Examples are poly(propenonitrile) (e.g. Acrilan), and poly(methyl 2-methylpropenoate) (polymethyl methacrylate, e.g. Perspex). acrylonitrile See . ACTH (adrenocorticotrophic hormone; corticotrophin) A hormone, produced by the anterior *pituitary gland, that controls secretion of certain hormones (the *corticosteroids) by the adrenal glands. Its secretion, which is controlled by corticotrophinreleasing hormone and occurs in short bursts every few hours, is increased by stress. actin A contractile protein found in muscle tissue, in which it occurs in the form of Ülaments (called thin Ülaments). Each thin Ülament consists of two chains of globular actin molecules, around which is twisted a strand of *tropomyosin and interspersed *troponin. Units of muscle Übre (see ) consist of actin and *myosin Ülaments, which interact to bring about muscle contraction. Actin is also found in the microÜlaments that form part of the *cytoskeleton of all cells. actinic radiation Electromagnetic radiation that is capable of initiating a chemical
reaction. The term is used especially of ultraviolet radiation and also to denote radiation that will affect a photographic emulsion.
actinides See . actinium Symbol Ac. A radioactive metallic element belonging to group 3 of the periodic table; a.n. 89; mass number of most stable isotope 227 (half-life 21.7 years); m.p. 1050 ± 50°C; b.p. 3200°C (estimated). Actinium–227 occurs in natural uranium to an extent of about 0.715%. Actinium–228 (halflife 6.13 hours) also occurs in nature. There are 22 other artiÜcial isotopes, all radioactive and all with very short half-lives. Its main use is as a source of alpha particles. The element was discovered by A. Debierne in 1899.
• Information from the WebElements site
actinium series See  . Actinobacteria (Actinomycetes; Actinomycota) A phylum of Gram-positive mostly anaerobic nonmotile bacteria. Many species are fungus-like, with Ülamentous cells producing reproductive spores on aerial branches similar to the spores of certain moulds. The phylum includes bacteria of the genera Actinomyces, some species of which cause disease in animals (including humans); and Streptomyces, which are a source of many important antibiotics (including streptomycin). actinoid contraction A smooth decrease in atomic or ionic radius with increasing proton number found in the *actinoids. actinoids (actinides) A series of elements in the *periodic table, generally considered to range in atomic number from thorium (90) to lawrencium (103) inclusive. The actinoids all have two outer s-electrons (a 7s2 conÜguration), follow actinium, and are classiÜed together by the fact that increasing proton number corresponds to Ülling of the 5f level. In fact, because the 5f and 6d levels are close in energy the Ülling of the 5f orbitals is not smooth. The outer electron conÜgurations are as follows: 89 actinium (Ac) 6d 17s2 90 thorium (Th) 6d 27s2 91 protactinium (Pa) 5f 26d17s2 92 uranium (Ur) 5f 36d7s2 93 neptunium (Np) 5f 57s2 (or 5f 46d17s2) 94 plutonium (Pu) 5f 67s2 95 americium (Am) 5f 77s2
96 curium (Cm) 5f 76d1s2 97 berkelium (Bk) 5f 86d7s2 (or 5f 97s2) 98 californium (Cf) 5f 107s2 99 einsteinium (Es) 5f 117s2 100 fermium (Fm) 5f 127s2 101 mendelevium (Md) 5f 137s2 102 nobelium (Nb) 5f 147s2 103 lawrencium (Lw) 5f 146d1s2 The Ürst four members (Ac to Ur) occur naturally. All are radioactive and this makes investigation difÜcult because of self-heating, short lifetimes, safety precautions, etc. Like the *lanthanoids, the actinoids show a smooth decrease in atomic and ionic radius with increasing proton number. The lighter members of the series (up to americium) have f-electrons that can participate in bonding, unlike the lanthanoids. Consequently, these elements resemble the transition metals in forming coordination complexes and displaying variable valency. As a result of increased nuclear charge, the heavier members (curium to lawrencium) tend not to use their inner f-electrons in forming bonds and resemble the lanthanoids in forming compounds containing the M3+ ion. The reason for this is pulling of these inner electrons towards the centre of the atom by the increased nuclear charge. Note that actinium itself does not have a 5f electron, but it is usually classiÜed with the actinoids because of its chemical similarities. See also  .
actinometer Any of various instruments for measuring the intensity of electromagnetic radiation. Recent actinometers use the *photoelectric effect but earlier instruments depended either on the Ûuorescence produced by the radiation on a screen or on the amount of chemical change induced in some suitable substance. actinomorphy See  . Actinomycetes See A. action at a distance The direct interaction between bodies that are not in physical contact with each other. The concept involves the assumption that the interactions are instantaneous. This assumption is not consistent with the special theory of *relativity, which states that nothing (including interactions) can travel through space faster than the *speed of light in a vacuum. For this reason it is more logical to describe interactions between bodies by *quantum Üeld theories or by the exchange of virtual parti-
active galaxy
action potential The change in electrical potential that occurs across a plasma membrane during the passage of a nerve *impulse. As an impulse travels in a wavelike manner along the *axon of a nerve, it causes a localized and transient switch in electric potential across the membrane from –60 mV (millivolts; the *resting potential) to +45 mV. The change in electric potential is caused by an inÛux of sodium ions. Nervous stimulation of a muscle Übre has a similar effect. action spectrum A graphical plot of the efÜciency of electromagnetic radiation in producing a photochemical reaction against the wavelength of the radiation used. For example, the action spectrum for photosynthesis using light shows a peak in the region 670–700 nm. This corresponds to a maximum absorption in the absorption *spectrum of chlorophylls in this region. activated adsorption Adsorption that involves an activation energy. This occurs in certain cases of chemisorption. activated alumina See  . activated charcoal See . activated complex The association of atoms of highest energy formed in the *transition state of a chemical reaction. activation analysis An analytical technique that can be used to detect most elements when present in a sample in milligram quantities (or less). In neutron activation analysis the sample is exposed to a Ûux of thermal neutrons in a nuclear reactor. Some of these neutrons are captured by nuclides in the sample to form nuclides of the same atomic number but a higher mass number. These newly formed nuclides emit gamma radiation, which can be used to identify the element present by means of a gamma-ray spectrometer. Activation analysis has also been employed using highenergy charged particles, such as protons or alpha particles.
• An overview of neutron activation analysis from the Archaeometry Laboratory, Missouri Research Reactor • Information about neutron activation analysis from the Worcester Polytechnic Institute
activation energy Symbol Ea. The minimum energy required for a chemical reaction to take place. In a reaction, the reactant molecules come together and chemical bonds are stretched, broken, and formed in producing the products. During this process the energy of the system increases to a maximum, then decreases to the energy of the products (see illustration). The activation energy is the difference between the maximum energy and the energy of the reactants; i.e. it is the energy barrier that has to be overcome for the reaction to proceed. The activation energy determines the way in which the rate of the reaction varies with temperature (see A ). It is usual to express activation energies in joules per mole of reactants. An activation energy greater than 200 kJ mol–1 suggests that a bond has been completely broken in forming the transition state (as in the SN1 reaction). A lower Ügure suggests incomplete breakage (as in the SN2 reaction).
cles (see  ) rather than theories based on action at a distance.
products ∆H
reactants
Activation energy. Reaction profile (for an endothermic reaction).
activator 1. A type of *transcription factor involved in assembling proteins to form an initiation complex at the *promoter of a gene in readiness for transcription. Compare . 2. A substance that – by binding to an allosteric site on an enzyme (see ) – enables the active site of the enzyme to bind to the substrate. 3. Any compound that potentiates the activity of a drug or other foreign substance in the body. active device 1. An electronic component, such as a transistor, that is capable of ampliÜcation. 2. An artiÜcial *satellite that receives information and retransmits it after ampliÜcation. 3. A radar device that emits microwave radiation and provides information about a distant body by receiving a reÛection of this radiation. Compare  . active galactic nucleus See  . active galaxy A galaxy that contains an
active immunity
active galactic nucleus (AGN), i.e. a central region that gives off a great deal of electromagnetic radiation. This emission is thought to be due to the accretion of matter into a supermassive *black hole, at the centre of the galaxy. See also  .
active immunity *Immunity acquired due to the body's response to a foreign antigen. active mass See  . active site (active centre) 1. A site on the surface of a catalyst at which activity occurs. 2. The site on the surface of an *enzyme molecule that binds and acts on the substrate molecule. The properties of an active site are determined by the threedimensional arrangement of the polypeptide chains of the enzyme and their constituent amino acids. These govern the nature of the interaction that takes place and hence the degree of substrate speciÜcity and susceptibility to *inhibition.
Kp = γBpBγCpC/γApA The equilibrium constant of a reaction in solution is Kc = γBXBγCXC/γAXA The activity coefÜcients thus act as correction factors for the pressures or concentrations. The activity is given by an equation µ = µŠ + R T ln a where µ is chemical potential. See also . 2. Symbol A. The number of atoms of a radioactive substance that disintegrate per unit time. The specific activity (a) is the activity per unit mass of a pure radioisotope. See  .
activity series See  . acyclic Describing a compound that does not have a ring in its molecules. acyclovir (acycloguanosine) See . acyl anhydrides See  .
active transport The movement of substances through membranes in living cells, often against a *concentration gradient: a process requiring metabolic energy. Organic molecules and inorganic ions are transported into and out of both cells and their organelles. The substance binds to a *transport protein embedded in the membrane, which carries it through the membrane and releases it on the opposite side. Active transport serves chieÛy to maintain the normal balance of ions in cells, especially the concentration gradients of sodium and potassium ions crucial to the activity of nerve and muscle cells. Compare  .
acylation The process of introducing an acyl group (RCO–) into a compound. The usual method is to react an alcohol with an acyl halide or a carboxylic acid anhydride; e.g. RCOCl + R′OH → RCOOR′ + HCl The introduction of an acetyl group (CH3CO–) is acetylation, a process used for protecting –OH groups in organic synthesis.
activity 1. Symbol a. A thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For example, in a reaction AˆB+C the true equilibrium constant is given by K = aBaC/aA where aA, aB, and aC are the activities of the components, which function as concentrations (or pressures) corrected for nonideal behaviour. Activity coefficients (symbol γ) are deÜned for gases by γ = a/p (where p is pressure) and for solutions by γ = aX (where X is the mole fraction). Thus, the equilibrium constant of a gas reaction has the form
acyl group A group of the type RCO–, where R is an organic group. An example is the acetyl group CH3CO–.
acyl Üssion The breaking of the carbon– oxygen bond in an acyl group. It occurs in the hydrolysis of an *ester to produce an alcohol and a carboxylic acid. acylglycerol See .
acyl halides (acid halides) Organic compounds containing the group –CO.X, where X is a halogen atom (see formula). Acyl chlorides, for instance, have the general formula RCOCl. The group RCO– is the acyl group. In systematic chemical nomenclature acylhalide names end in the sufÜx -oyl; for example, ethanoyl chloride, CH3COCl. Acyl O R
Acyl halide. X is a halogen atom.
halides react readily with water, alcohols, phenols, and amines and are used in *acylation reactions. They are made by replacing the –OH group in a carboxylic acid by a halogen using a halogenating agent such as PCl5.
Ada A high-level computer programming language developed in the late 1970s for the US military. It was originally employed in missile control systems and is now used in various other real-time applications. Ada was named after Augusta Ada Lovelace (1815–52), the mathematician daughter of Lord Byron, who worked with Charles *Babbage on his mechanical computer, the 'analytical engine'.
• The Ada 2005 Language Reference Manual
adamantane A colourless crystalline hydrocarbon, C10H16; m.p. 269°C. It is found in certain petroleum fractions. The structure contains three symmetrically fused cyclohexane rings. Adams, John Couch (1819–92) British astronomer who became professor of astronomy and geometry at Cambridge University in 1858. He is best known for his prediction (1845) of the existence and position of the planet *Neptune, worked out independently the following year by Urbain Leverrier (1811–77). The planet was discovered in 1846 by Johann Galle (1812–1910), using Leverrier's Ügures. Adams's priority was not acknowledged. Adams catalyst A dark brown powder, a hydrated form of platinum(IV) oxide (PtO2), produced by heating chloroplatinic acid (H2PtCl6) with sodium nitrate (NaNO3). Platinum nitrate is produced, and this decomposes to platinum(IV) oxide with evolution of NO2 and oxygen. It is used in hydrogenations of alkenes to alkanes, nitro compounds to aminos, and ketones to alcohols. The actual catalyst is not the oxide but Ünely divided *platinum black, which forms during the hydrogenation reaction. adaptation 1. (in evolution) Any change in the structure or functioning of successive generations of a population that makes it better suited to its environment. *Natural selection of heritable adaptations ultimately leads to the development of new species. Increasing adaptation of a species to a particu-
addition reaction lar environment tends to diminish its ability to adapt to any sudden change in that environment. 2. (in physiology) The alteration in the degree of sensitivity (either an increase or a decrease) of a sense organ to suit conditions more extreme than normally encountered. An example is the adjustment of the eye to vision in very bright or very dim light.
adaptive optics (AO) Computer-aided techniques for making virtually instantaneous corrective adjustments to the shape of a deformable mirror in a ground-based optical or near-infrared telescope in order to reduce or eliminate distortions of an image caused by dynamic turbulence in the earth's atmosphere. Adaptive optics also has applications outside astronomy. For example, in ophthalmology it can be used to measure aberration within the human eye and help to provide sharp images of the retina at the cellular level. adaptive radiation (divergent evolution) The evolution from one species of animals or plants of a number of different forms. As the original population increases in size it spreads out from its centre of origin to exploit new habitats and food sources. In time this results in a number of populations each adapted to its particular habitat: eventually these populations will differ from each other sufÜciently to become new species. A good example of this process is the evolution of the Australian marsupials into species adapted as carnivores, herbivores, burrowers, Ûiers, etc. On a smaller scale, the adaptive radiation of the Galapagos Ünches provided Darwin with crucial evidence for his theory of evolution (see D' ). addition polymerization See . addition reaction A chemical reaction in which one molecule adds to another. Addition reactions occur with unsaturated compounds containing double or triple bonds, and may be *electrophilic or *nucleophilic. An example of electrophilic addition is the reaction of hydrogen chloride with an alkene, e.g. HCl + CH2:CH2 → CH3CH2Cl An example of nucleophilic addition is the addition of hydrogen cyanide across the carbonyl bond in aldehydes to form *cyanohydrins. Addition–elimination reactions are ones in which the addition is followed by
elimination of another molecule (see  ).
additive A substance added to another substance or material to improve its properties in some way. Additives are often present in small amounts and are used for a variety of purposes, as in preventing corrosion, stabilizing polymers, etc. Food additives are used to enhance the taste and colour of foods and improve their texture and keeping qualities. See  . additive process See . adduct A compound formed by an addition reaction. The term is used particularly for compounds formed by coordination between a Lewis acid (acceptor) and a Lewis base (donor). See . adenine A *purine derivative. It is one of the major component bases of *nucleotides and the nucleic acids *DNA and *RNA. adenosine A nucleoside comprising one adenine molecule linked to a D-ribose sugar molecule. The phosphate-ester derivatives of adenosine, AMP, ADP, and *ATP, are of fundamental biological importance as carriers of chemical energy. adenosine diphosphate (ADP) See ATP. adenosine monophosphate (AMP) See ATP;  AMP. adenosine triphosphate See ATP;  AMP. adenovirus One of a group of DNAcontaining viruses found in rodents, fowl, cattle, monkeys, and humans. In humans they produce acute respiratory-tract infections with symptoms resembling those of the common cold. They are also implicated in the formation of tumours (see ). adenylate cyclase The enzyme that catalyses the formation of *cyclic AMP. It is bound to the inner surface of the plasma membrane. Many hormones and other chemical messengers exert their physiological effects by increased synthesis of cyclic AMP through the activation of adenylate cyclase. The hormone binds to a receptor on the outer surface of the plasma membrane, which then activates adenylate cyclase on the inner surface via a *G protein. ADH See  . adhesive A substance used for joining surfaces together. Adhesives are generally col-
loidal solutions, which set to gels. There are many types including animal glues (based on collagen), vegetable mucilages, and synthetic resins (e.g. *epoxy resins).
adiabatic approximation An approximation used in *quantum mechanics when the time dependence of parameters such as the inter-nuclear distance between atoms in a molecule is slowly varying. This approximation means that the solution of the *Schrödinger equation at one time goes continuously over to the solution at a later time. This approximation was formulated by Max Born and the Soviet physicist Vladimir Alexandrovich Fock (1898–1974) in 1928. The *Born–Oppenheimer approximation is an example of the adiabatic approximation. adiabatic demagnetization A technique for cooling a paramagnetic salt, such as potassium chrome alum, to a temperature near *absolute zero. The salt is placed between the poles of an electromagnet and the heat produced during magnetization is removed by liquid helium. The salt is then isolated thermally from the surroundings and the Üeld is switched off; the salt is demagnetized adiabatically and its temperature falls. This is because the demagnetized state, being less ordered, involves more energy than the magnetized state. The extra energy can come only from the internal, or thermal, energy of the substance. It is possible to obtain temperatures as low as 0.005 K in this way. adiabatic process Any process that occurs without heat entering or leaving a system. In general, an adiabatic change involves a fall or rise in temperature of the system. For example, if a gas expands under adiabatic conditions, its temperature falls (work is done against the retreating walls of the container). The adiabatic equation describes the relationship between the pressure (p) of an ideal gas and its volume (V), i.e. pVγ = K, where γ is the ratio of the principal speciÜc *heat capacities of the gas and K is a constant. adipic acid See  . adipose tissue A body tissue comprising cells containing *fat and oil. It is found chieÛy below the skin (see  ) and around major organs (such as the kidneys and heart), acting as an energy reserve, providing insulation and protection, and generating heat. Secretion of the hor-
mone *leptin by adipose tissue regulates the amount of adipose tissue and adjusts the body's energy balance. See  ; .
admittance Symbol Y. The reciprocal of *impedance. It is measured in siemens. adolescence The period in human development that occurs during the teenage years, between the end of childhood and the start of adulthood, and is characterized by various physical and emotional changes associated with development of the reproductive system. It starts at puberty, when the reproductive organs begin to function, and is marked by the start of menstruation (see  ) in females and the appearance of the *secondary sexual characteristics in both sexes. In males the secondary sexual characteristics are controlled by the hormone testosterone and include deepening of the voice due to larynx enlargement, the appearance of facial and pubic hair, rapid growth of the skeleton and muscle, and an increase in *sebaceous gland secretions. In females the secondary sexual characteristics are controlled by oestrogens and include growth of the breasts, broadening of the pelvis, redistribution of fat in the body, and appearance of pubic hair. ADP See ATP. adrenal cortex The outer layer of the *adrenal gland, in which several steroid hormones, the *corticosteroids, are produced. adrenal glands A pair of endocrine glands situated immediately above the kidneys (hence they are also known as the suprarenal glands). The inner portion of the adrenals, the medulla, secretes the hormones *adrenaline and *noradrenaline; the outer cortex secretes small amounts of sex hormones (*androgens and *oestrogens) and various *corticosteroids, which have a wide range of effects on the body. See also ACTH. adrenaline (epinephrine) A hormone, produced by the medulla of the *adrenal glands, that increases heart activity, improves the power and prolongs the action of muscles, and increases the rate and depth of breathing to prepare the body for 'fright, Ûight, or Üght'. At the same time it inhibits digestion and excretion and stimulates mobilization of body fat (*lipolysis) and energy metabolism (*glycolysis). Similar effects are produced by stimulation of the *sympathetic nervous system. Adrenaline causes these
ADSL effects by binding to *adrenoceptors on target cells. It can be administered by injection to relieve bronchial asthma and reduce blood loss during surgery by constricting blood vessels.
adrenal medulla The inner part of the *adrenal gland, in which *adrenaline is produced. adrenergic 1. Describing a cell (especially a neuron) or a cell receptor that is stimulated by *adrenaline, *noradrenaline, or related substances. See . 2. Describing a nerve Übre or neuron that releases adrenaline or noradrenaline when stimulated. Compare . adrenoceptor (adrenoreceptor; adrenergic receptor) Any cell receptor that binds and is activated by the catecholamines adrenaline or noradrenaline. Adrenoceptors are therefore crucial in mediating the effects of catecholamines as neurotransmitters or hormones. There are two principal types of adrenoceptor, alpha (α) and beta (β). The alpha adrenoceptors fall into two main subtypes: α1-adrenoceptors, which mediate the contraction of smooth muscle and hence cause constriction of blood vessels; and α2adrenoceptors, which occur, for example, in presynaptic neurons at certain nerve synapses, where they inhibit release of noradrenaline from the neuron. The beta adrenoceptors also have two main subtypes: β1-adrenoceptors, which stimulate cardiac muscle causing a faster and stronger heartbeat; and β2-adrenoceptors, which mediate relaxation of smooth muscle in blood vessels, bronchi, the uterus, bladder, and other organs. Activation of β2-adrenoceptors thus causes widening of the airways (bronchodilation) and blood vessels (vasodilation). See also  . adrenocorticotrophic hormone See ACTH. Adrian, Edgar Douglas, Baron (1889–1977) British neurophysiologist, who became a professor at Cambridge in 1937, where he remained until his retirement. He is best known for his work on nerve impulses, establishing that messages are conveyed by changes in the frequency of the impulses. He shared the 1932 Nobel Prize for physiology or medicine with Sir Charles *Sherrington for this work. ADSL (asymmetric digital subscriber line) A mechanism by which *broadband commu-
adsorbate
nication via the Internet can be made available via pre-existing telephone lines, while allowing simultaneous use of the line for normal telephone calls. Data communication via ADSL is asymmetric in that upstream (transmitting) communication is slower than downstream (receiving) communication, typically half as fast. Commonly available downstream data rates in the UK are 512 Kbps, 1 Mbps, and 2 Mbps. Faster rates are available in other countries. ADSL coexists with standard telephone operation on the same line by the use of band separation Ülters at each telephone socket.
adsorbate A substance that is adsorbed on a surface. adsorbent A substance on the surface of which a substance is adsorbed. adsorption The formation of a layer of gas, liquid, or solid on the surface of a solid or, less frequently, of a liquid. There are two types depending on the nature of the forces involved. In chemisorption a single layer of molecules, atoms, or ions is attached to the adsorbent surface by chemical bonds. In physisorption adsorbed molecules are held by the weaker *van der Waals' forces. Adsorption is an important feature of surface reactions, such as corrosion, and heterogeneous catalysis. The property is also utilized in adsorption *chromatography. adsorption indicator (absorption indicator) A type of indicator used in reactions that involve precipitation. The yellow dye Ûuorescein is a common example, used for the reaction NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq) As silver nitrate solution is added to the sodium chloride, silver chloride precipitates. As long as Cl– ions are in excess, they adsorb on the precipitate particles. At the end point, no Cl– ions are left in solution and negative Ûuorescein ions are then adsorbed, giving a pink colour to the precipitate. The technique is sometimes known as Fajans' method. adsorption isotherm An equation that describes how the amount of a substance adsorbed onto a surface depends on its pressure (if a gas) or its concentration (if in a solution), at a constant temperature. Several adsorption isotherms are used in surface chemistry including the *BET isotherm and the *Langmuir adsorption isotherm. The different isotherms correspond to different as-
sumptions about the surface and the adsorbed molecules.
adulterant See  . advanced gas-cooled reactor (AGR) See  . adventitious Describing organs or other structures that arise in unusual positions. For example, ivy has adventitious roots growing from its stems. aerial (antenna) The part of a radio or television system from which radio waves are transmitted into the atmosphere or space (transmitting aerial) or by which they are received (receiving aerial). A directional or directive aerial is one in which energy is transmitted or received more effectively from some directions than others, whereas an omnidirectional aerial transmits and receives equally well in all directions. aerobe See  . aerobic respiration A type of *respiration in which foodstuffs (usually carbohydrates) are completely oxidized to carbon dioxide and water, with the release of chemical energy, in a process requiring atmospheric oxygen. The reaction can be summarized by the equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy The chemical energy released is stored mainly in the form of *ATP. The Ürst stage of aerobic respiration is *glycolysis, which takes place in the cytosol of cells and also occurs in fermentations and other forms of *anaerobic respiration. Further oxidation in the presence of oxygen is via the *Krebs cycle and *electron transport chain, enzymes for which are located in the *mitochondria of eukaryote cells. Most organisms have aerobic respiration (i.e. they are aerobes); exceptions include certain bacteria and yeasts. aerodynamics The study of the motion of gases (particularly air) and the motion of solid bodies in air. Aerodynamics is particularly concerned with the motion and stability of aircraft. Another application of aerodynamics is to the *Ûight of birds and insects. The branch of aerodynamics concerned with the Ûow of gases through compressors, ducts, fans, oriÜces, etc., is called internal aerodynamics. Aerodynamic drag is the force that opposes the motion of a body moving relative to a gas and is a function of the density of the gas, the square of the relative velocity, the
Compare . 2. (in botany) The arrangement of the parts of a Ûower bud, especially of the sepals and petals.
lift rarefaction zone wing
compression zone
aetiology The study of causes, especially the causes of medical conditions. afferent Carrying (nerve impulses, blood, etc.) from the outer regions of a body or organ towards its centre. The term is usually applied to types of nerve Übres or blood vessels. Compare .
section through an aircraft wing lift drag
weight forces on an aircraft
Aerodynamics. surface area of the body, and a quantity called the drag coefficient, which is a function of the *Reynolds number. Aerodynamic lift is an upward force experienced by a body moving through a gas and is a function of the same variables as aerodynamic drag.
aerogel A low-density porous transparent material that consists of more than 90% air. Usually based on metal oxides or silica, aerogels are used as drying agents and insulators. aerogenerator See  . aerosol A colloidal dispersion of a solid or liquid in a gas. The commonly used aerosol sprays contain an inert propellant liqueÜed under pressure. Halogenated alkanes containing chlorine and Ûuorine (*chloroÛuorocarbons, or CFCs) were formerly used in aerosol cans. They have now largely been replaced by volatile hydrocarbons because of their effect on the *ozone layer. aerospace The earth's atmosphere and the space beyond it. aerotaxis See . aestivation 1. (in zoology) A state of inactivity occurring in some animals, notably lungÜsh, during prolonged periods of drought or heat. Feeding, respiration, movement, and other bodily activities are considerably slowed down. See also .
aÛatoxin Any of four related toxic compounds produced by the mould Aspergillus Ûavus. AÛatoxins bind to DNA and prevent replication and transcription. They can cause acute liver damage and cancers: humans may be poisoned by eating stored peanuts and cereals contaminated with the mould. AFM See   . afterbirth The *placenta, *umbilical cord, and *extraembryonic membranes, which are expelled from the womb after a mammalian fetus is born. In most nonhuman mammals the afterbirth, which contains nutrients and might otherwise attract predators, is eaten by the female. after-heat Heat produced by a nuclear reactor after it has been shut down. The afterheat is generated by radioactive substances formed in the fuel elements. agamospermy See . agar An extract of certain species of red seaweeds that is used as a gelling agent in microbiological *culture media, foodstuffs, medicines, and cosmetic creams and jellies. Nutrient agar consists of a broth made from beef extract or blood that is gelled with agar and used for the cultivation of bacteria, fungi, and some algae. agate A variety of *chalcedony that forms in rock cavities and has a pattern of concentrically arranged bands or layers that lie parallel to the cavity walls. These layers are frequently alternating tones of brownishred. Moss agate does not show the same banding and is a milky chalcedony containing mosslike or dendritic patterns formed by inclusions of manganese and iron oxides. Agates are used in jewellery and for ornamental purposes. ageing See . age of the earth The time since the earth
age of the universe
emerged as a planet of the sun, estimated by *dating techniques to be about 4.6 × 109 years. The oldest known rocks on earth are estimated by their *radioactive age to be about 3.5 × 109 years old. The earth is older than this because of the long time it took to cool. An estimate for the cooling time is included in the estimate for the age of the earth.
age of the universe A time determined by the reciprocal of the value of the *Hubble constant to be about 13.7 billion years. The calculation of the Hubble constant, and hence the age of the universe, depends on which theory of *cosmology is used. Usually, the age of the universe is calculated by assuming that the *expansion of the universe can be described by the *big-bang theory. agglutination The clumping together by antibodies of microscopic foreign particles, such as red blood cells or bacteria, so that they form a visible pellet-like precipitate. Agglutination is a speciÜc reaction, i.e. a particular antigen will only clump in the presence of its speciÜc antibody; it therefore provides a means of identifying unknown bacteria and determining *blood group. When blood of incompatible blood groups (e.g. group A and group B – see ABO ) is mixed together agglutination of the red cells occurs (haemagglutination). This is due to the reaction between antibodies in the plasma (agglutinins) and *agglutinogens (antigens) on the surface of the red cells. agglutinogen Any of the antigens that are present on the outer surface of red blood cells (erythrocytes). There are more than 100 different agglutinogens and they form the basis for identifying the different *blood groups. Antibodies in the plasma, known as agglutinins, react with the agglutinogens in blood of an incompatible blood group (see ). aggression Behaviour aimed at intimidating or injuring another animal of the same or a competing species. Aggression between individuals of the same species often starts with a series of ritualized displays or contests that can end at any stage if one of the combatants withdraws, leaving the victor with access to a disputed resource (e.g. food, a mate, or *territory) or with increased social dominance (see ). It is also often seen in *courtship. Aggression or threat displays usually appear to exaggerate the performer's size or strength; for example, many
Üsh erect their Üns and mammals and birds may erect hairs or feathers. Special markings may be prominently exhibited, and intention movements may be made: dogs bare their teeth, for example. Some animals have evolved special structures for use in aggressive interactions (e.g. antlers in deer) but these are seldom used to cause actual injury; the opponent usually Ûees Ürst or adopts *appeasement postures. Fights 'to the death' are comparatively rare. See  ;  ; .
AGN Active galactic nucleus. See  . agnathan Any jawless craniate animal. Agnathans were formerly classiÜed in the subphylum (or superclass) Agnatha, with the living representatives – lampreys and hagÜshes – constituting the class Cyclostomata. However, it is now accepted that the lampreys are more closely related to the jawed vertebrates than to the hagÜshes, and the agnathans are now placed in two distinct clades of the subphylum *Craniata, the hagÜshes forming the class Myxini (Hyperotreti) and the lampreys the Hyperoartia. The closest living relatives of the latter are the jawed vertebrates (Gnathostomata), which with the lampreys constitute the superclass Vertebrata. Fossil agnathans, covered in an armour of bony plates, are the oldest known fossil vertebrates. They have been dated from the Silurian and Devonian periods, 440–345 million years ago. agonist A drug, hormone, neurotransmitter, or other signal molecule that forms a complex with a *receptor site, thereby triggering an active response from a cell. Compare . agonistic behaviour Any form of behaviour associated with *aggression, including threat, attack, *appeasement, or Ûight. It is often associated with defence of a territory; for example, a threat display by the defending individual is often met with an appeasement display from the intruder, thus avoiding harmful conÛict. AGR Advanced gas-cooled reactor. See  . agranulocyte Any white blood cell (see ) with a nongranular cytoplasm and a large spherical nucleus; *lymphocytes and *monocytes are examples. Agranulo-
cytes are produced either in the lymphatic system or in the bone marrow and account for 30% of all leucocytes. Compare .
agriculture The study and practice of cultivating land for the growing of crops and the rearing of livestock. The increasing demands for food production since the mid-20th century have seen many developments in agricultural technology and practices that have greatly increased crop and livestock production. However, these advances in modern intensive farming techniques have had their impact on the environment, particularly with increased use of *fertilizers and *pesticides. The now widespread practice of crop monoculture (in which one crop is grown densely over an extensive area) has required an increase in the use of *pesticides, as monoculture provides an ideal opportunity for crop pests. Monoculture also requires vast areas of land, which has meant that natural habitats have been destroyed. *Deforestation has resulted from the clearing of forests for crop production and cattle rearing. Advances in technology have included ploughing machines with hydraulic devices that can control the depth to which the soil is ploughed, and seed drills that automatically implant seeds in the soil so that ploughing is not necessary. Food supply in many less-developed countries relies on subsistence farming, in which the crops and livestock produced are used solely to feed the farmer and his family. In such countries a system known as slash and burn is common, in which the vegetation in an area is cut down and then burnt, thus returning the minerals to the soil. The area can then be used for crop cultivation until the soil fertility drops, at which point it is then abandoned for a number of years and another site is cultivated. The selective *breeding of crop plants and farm animals has had an enormous impact on productivity in agriculture. Modern varieties of crop plants have increased nutritional value and greater resistance to disease, while animals have been selectively bred to enhance their yields of milk, meat, and other products. Developments in genetic engineering have enabled the introduction to commercial cultivation of genetically modiÜed crop plants, such as tomatoes and soya, which contain foreign genes to enhance crop growth, nutritional properties, or storage characteristics. Ge-
netic modiÜcation can also confer resistance to herbicides, thereby allowing more effective weed control, as well as improved resistance to insects and other pests and to diseases. The application of similar technology to animal production is being researched. See also    (Feature).
Agrobacterium tumefaciens A Gramnegative soil bacterium that infects a wide range of plants and causes *galls, especially at the root/stem junction (crown gall). It is of interest because the bacterial cells contain a *plasmid, the Ti plasmid (tumour-inducing plasmid), a segment of which is transferred to cells of the plant host. This T-DNA (transfer DNA) segment, which comprises the genes responsible for the gall, becomes integrated into the genome of infected plant cells. Possession of the Ti plasmid has made A. tumefaciens an important tool in genetic engineering for the introduction of foreign genes into plant tissue. The tumourinducing genes are usually replaced with the gene of interest, and a marker gene (e.g. the antibiotic resistance gene) is added to enable selection of transformed cells. See    (Feature). AI 1. See  . 2. See  . AIDS (acquired immune deÜciency syndrome) A disease of humans characterized by defective cell-mediated *immunity and increased susceptibility to infections. It is caused by the retrovirus *HIV (human immunodeÜciency virus). This infects and destroys helper *T cells, which are essential for combating infections. HIV is transmitted in blood, semen, and vaginal Ûuid; the major routes of infection are unprotected vaginal and anal intercourse, intravenous drug abuse, and the administration of contaminated blood and blood products. A person infected with HIV is described as HIVpositive; after the initial infection the virus can remain dormant for up to ten years before AIDS develops. *Antiviral drugs can delay the development of full-blown AIDS, in some cases for many years. air See ' . air bladder See  . air mass (in meteorology) An area of the atmosphere that in the horizontal Üeld possesses more or less uniform properties, especially temperature and humidity, and
extends for hundreds of kilometres. The transition zone at which one air mass meets another is known as a *front. Air masses develop over extensive areas of the earth's surface, known as source regions, where conditions are sufÜciently uniform to impart similar characteristics to the overlying air. These areas are chieÛy areas of high pressure. As an air mass moves away from its source region it undergoes modiÜcation.
air pollution (atmospheric pollution) The release into the atmosphere of substances that cause a variety of harmful effects to the natural environment. Most air pollutants are gases that are released into the troposphere, which extends about 8 km above the surface of the earth. The burning of *fossil fuels, for example in power stations, is a major source of air pollution as this process produces such gases as sulphur dioxide and carbon dioxide. Released into the atmosphere, carbon dioxide is the major contributor to the *greenhouse effect; methane, derived from livestock and rice cultivation, is another signiÜcant greenhouse gas. Sulphur dioxide and nitrogen oxides, released in car exhaust fumes, are air pollutants that are responsible for the formation of *acid rain; nitrogen oxides also contribute to the formation of *photochemical smog. See also  ; . air sac 1. Any one of a series of thin-walled sacs in birds that are connected to the lungs and increase the efÜciency of ventilation. Some of the air sacs penetrate the internal cavities of bones. 2. A structural extension to the *trachea in insects, which increases the surface area available for the exchange of oxygen and carbon dioxide in respiration. alabaster See . alanine See  . albedo 1. The ratio of the radiant Ûux reÛected by a surface to that falling on it. 2. The probability that a neutron entering a body of material will be reÛected back through the same surface as it entered. albinism Hereditary lack of pigmentation (see ) in an organism. Albino animals and human beings have no colour in their skin, hair, or eyes (the irises appear pink from underlying blood vessels). The *allele responsible is *recessive to the allele for normal pigmentation.
albumen See . albumin One of a group of globular proteins that are soluble in water but form insoluble coagulates when heated. Albumins occur in egg white (the protein component of which is known as albumen), blood, milk, and plants. Serum albumins, which constitute about 55% of blood plasma protein, help regulate the osmotic pressure and hence plasma volume. They also bind and transport fatty acids. α-lactalbumin is a protein in milk. albuminous cell See  . alburnum See . alcoholic fermentation See . alcohols Organic compounds that contain the –OH group. In systematic chemical nomenclature alcohol names end in the sufÜx -ol. Examples are methanol, CH3OH, and ethanol, C2H5OH. Primary alcohols have two hydrogen atoms on the carbon joined to the –OH group (i.e. they contain the group –CH2–OH); secondary alcohols have one hydrogen on this carbon (the other two bonds being to carbon atoms, as in (CH3)2CHOH); tertiary alcohols have no hydrogen on this carbon (as in (CH3)3COH): see formulae. The different types of alcohols may differ in the way they react chemically. For example, with potassium dichromate(VI) in sulphuric acid the following reactions occur primary alcohol → aldehyde → carboxylic acid secondary alcohol → ketone tertiary alcohol – no reaction Other characteristics of alcohols are reaction with acids to give *esters and dehydration to give *alkenes or *ethers. Alcohols that have two –OH groups in their molecules are
OH H C H H primary alcohol (methanol) H
_ _ OH _ C _ CH
secondary alcohol (propan-2-ol)
_ _ OH C_ _ CH CH
tertiary alcohol (2-methylpropan-2-ol)
Examples of alcohols.
diols (or dihydric alcohols), those with three are triols (or trihydric alcohols), etc.
aldehydes Organic compounds that contain the group –CHO (the aldehyde group; i.e. a carbonyl group (C=O) with a hydrogen atom bound to the carbon atom). In systematic chemical nomenclature, aldehyde names end with the sufÜx -al. Examples of aldehydes are methanal (formaldehyde), HCOH, and ethanal (acetaldehyde), CH3CHO. Aldehydes are formed by oxidation of primary *alcohols; further oxidation yields carboxylic acids. They are reducing agents and tests for aldehydes include *Fehling's test and *Tollens reagent. Aldehydes have certain characteristic addition and condensation reactions. With sodium hydrogensulphate(IV) they form addition compounds of the type [RCOH(SO3)H]– Na+. Formerly these were known as bisulphite addition compounds. They also form addition compounds with hydrogen cyanide to give *cyanohydrins and with alcohols to give *acetals and undergo condensation reactions to yield *oximes, *hydrazones, and *semicarbazones. Aldehydes readily polymerize. See also .
• Information about IUPAC nomenclature O R
aldehyde group
C H
Aldehyde structure. aldohexose See . aldol See  . aldol reaction A reaction of aldehydes of the type 2RCH2CHO ˆ RCH2CH(OH)CHRCHO where R is a hydrocarbon group. The resulting compound is a hydroxy-aldehyde, i.e. an aldehyde–alcohol or aldol, containing alcohol (–OH) and aldehyde (–CHO) groups on adjacent carbon atoms. The reaction is basecatalysed, the Ürst step being the formation of a carbanion of the type RHC–CHO, which adds to the carbonyl group of the other aldehyde molecule. For the carbanion to form, the aldehyde must have a hydrogen atom on the carbon next to the carbonyl group.
Aldols can be further converted to other products; in particular, they are a source of unsaturated aldehydes. For example, the reaction of ethanal gives 3-hydroxybutenal (acetaldol): 2CH3CHO ˆ CH3CH(OH)CH2CHO This can be further dehydrated to 2-butenal (crotonaldehyde): CH3CH(OH)CH2CHO → H2O + CH3CH:CHCHO
aldose See . aldosterone A hormone produced by the adrenal glands (see ) that controls excretion of sodium by the kidneys and thereby maintains the balance of salt and water in the body Ûuids. See also . algae Any of various unrelated simple organisms that contain chlorophyll (and can therefore carry out photosynthesis) and live in aquatic habitats and in moist situations on land. The algal body may be unicellular or multicellular (Ülamentous, ribbon-like, or platelike). Molecular studies have conÜrmed that red and green algae are related to modern plants, whereas other algal groups, such as brown algae, are not. Algae are assigned to separate phyla based primarily on the composition of the cell wall, the nature of the stored food reserves, and the other photosynthetic pigments present. See B; C; P; R. The organisms formerly known as bluegreen algae are now classiÜed as bacteria (see C). algal bloom (bloom) The rapid increase in populations of algae and other phytoplankton, in particular the *Cyanobacteria, that occurs in inland water systems, such as lakes. The density of the organisms may be such that it may prevent light from passing to lower depths in the water system. Blooms are caused by an increase in levels of nitrate, a mineral ion essential for algal and bacterial growth. The source of increased nitrate may be from agricultural *fertilizers, which are leached into water systems from the land, or *sewage efÛuent. Blooms contribute to the eutrophication of water systems. See also . algebra The branch of mathematics in which variable quantities and numbers are represented by symbols. Statements are usu-
algebraic sum
ally made in the form of equations, which are manipulated into convenient forms and solved according to a set of strictly logical rules.
algebraic sum The total of a set of quantities paying due regard to sign, e.g. the algebraic sum of 3 and –4 is –1. Algol An early high-level block-structured computer programming language. Algol 60 dates from about 1960. Algol 68, of 1968, is a more powerful abstract language. The Ünal version, Algol W, was the precursor of Pascal. The name is short for algorithmic language (see ). algorithm A method of solving a problem, involving a Ünite series of steps. In computing practice the algorithm denotes the expression on paper of the proposed computing process (often by means of a Ûowchart) prior to the preparation of the program. If no algorithm is possible a *heuristic solution has to be sought. alicyclic compound A compound that contains a ring of atoms and is aliphatic. Cyclohexane, C6H12, is an example. alignment (in bioinformatics) The process of matching up base sequences (e.g. of genes) or amino acid sequences (of proteins) to reveal similarities and differences between them. It enables researchers to compare, for example, a newly sequenced gene or protein fragment with well-characterized sequences and is a key step in identifying the nature, possible function, and evolutionary relationships of novel genes and proteins. Alignment is performed by any of various computer programs and makes use of the vast amount of sequence data stored on public databases, which can be accessed via the Internet. In a pairwise alignment, just two sequences are compared, whereas multiple sequence alignment compares three or more. The program compares the sequences and computes the best alignment(s), allowing for gaps and mismatches. alimentary canal (digestive tract; gut) A tubular organ in animals that is divided into a series of zones specialized for the ingestion, *digestion, and *absorption of food and for the elimination of indigestible material (see illustration overleaf). In most animals the canal has two openings, the mouth (for the intake of food) and the *anus (for the elimination of waste). Simple animals, such as cnidarians (e.g. Hydra and jellyÜsh) and
Ûatworms, have only one opening to their alimentary canal, which must serve both functions.
aliphatic compounds Organic compounds that are *alkanes, *alkenes, or *alkynes or their derivatives. The term is used to denote compounds that do not have the special stability of *aromatic compounds. All noncyclic organic compounds are aliphatic. Cyclic aliphatic compounds are said to be alicyclic. alizarin An orange-red dye, C14H8O4, which forms *lakes when heavy metal salts are added to its alkaline solutions. It occurs naturally in madder, but is generally synthesized from *anthraquinone. alkali A *base that dissolves in water to give hydroxide ions. alkali metals (group 1 elements) The elements of group 1 (formerly IA) of the *periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). All have a characteristic electron conÜguration that is a noble gas structure with one outer s-electron. They are typical metals (in the chemical sense) and readily lose their outer electron to form stable M+ ions with noble-gas conÜgurations. All are highly reactive, with the reactivity (i.e. metallic character) increasing down the group. There is a decrease in ionization energy from lithium (520 kJ mol–1) to caesium (380 kJ mol–1). The second ionization energies are much higher and divalent ions are not formed. Other properties also change down the group. Thus, there is an increase in atomic and ionic radius, an increase in density, and a decrease in melting and boiling point. The standard electrode potentials are low and negative, although they do not show a regular trend because they depend both on ionization energy (which decreases down the group) and the hydration energy of the ions (which increases). All the elements react with water (lithium slowly; the others violently) and tarnish rapidly in air. They can all be made to react with chlorine, bromine, sulphur, and hydrogen. The hydroxides of the alkali metals are strongly alkaline (hence the name) and do not decompose on heating. The salts are generally soluble. The carbonates do not decompose on heating, except at very high temperatures. The nitrates (except for lithium) decompose to give the nitrite and oxygen:
alkaline-earth metals
2MNO3(s) → 2MNO2(s) + O2(g) Lithium nitrate decomposes to the oxide. In fact lithium shows a number of dissimilarities to the other members of group 1 and in many ways resembles magnesium (see  ). In general, the stability of salts of oxo acids increases down the group (i.e. with increasing size of the M+ ion). This trend occurs because the smaller cations (at the top of the group) tend to polarize the oxo anion more effectively than the larger cations at the bottom of the group.
alkalimetry Volumetric analysis using standard solutions of alkali to determine the amount of acid present.
alkaline 1. Describing an alkali. 2. Describing a solution that has an excess of hydroxide ions (i.e. a pH greater than 7). alkaline-earth metals (group 2 elements) The elements of group 2 (formerly IIA) of the *periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The elements are sometimes referred to as the 'alkaline earths', although strictly the 'earths' are the oxides of the elements. All have a characteristic electron conÜguration that is a noblegas structure with two outer s-electrons. They are typical metals (in the chemical sense) and readily lose both outer electrons
salivary glands mouth
oesophagus (gullet)
stomach liver gall bladder
pyloric sphincter
pancreatic duct duodenum small intestine ileum
colon caecum appendix
rectum anus
The human alimentary canal.
to form stable M2+ ions; i.e. they are strong reducing agents. All are reactive, with the reactivity increasing down the group. There is a decrease in both Ürst and second ionization energies down the group. Although there is a signiÜcant difference between the Ürst and second ionization energies of each element, compounds containing univalent ions are not known. This is because the divalent ions have a smaller size and larger charge, leading to higher hydration energies (in solution) or lattice energies (in solids). Consequently, the overall energy change favours the formation of divalent compounds. The third ionization energies are much higher than the second ionization energies, and trivalent compounds (containing M3+) are unknown. Beryllium, the Ürst member of the group, has anomalous properties because of the small size of the ion; its atomic radius (0.112 nm) is much less than that of magnesium (0.16 nm). From magnesium to radium there is a fairly regular increase in atomic and ionic radius. Other regular changes take place in moving down the group from magnesium. Thus, the density and melting and boiling points all increase. Beryllium, on the other hand, has higher boiling and melting points than calcium and its density lies between those of calcium and strontium. The standard electrode potentials are negative and show a regular small decrease from magnesium to barium. In some ways beryllium resembles aluminium (see  ). All the metals are rather less reactive than the alkali metals. They react with water and oxygen (beryllium and magnesium form a protective surface Ülm) and can be made to react with chlorine, bromine, sulphur, and hydrogen. The oxides and hydroxides of the metals show the increasing ionic character in moving down the group: beryllium hydroxide is amphoteric, magnesium hydroxide is only very slightly soluble in water and is weakly basic, calcium hydroxide is sparingly soluble and distinctly basic, strontium and barium hydroxides are quite soluble and basic. The hydroxides decompose on heating to give the oxide and water: M(OH)2(s) → MO(s) + H2O(g) The carbonates also decompose on heating to the oxide and carbon dioxide: MCO3(s) → MO(s) + CO2(g) The nitrates decompose to give the oxide:
2M(NO3)2(s) → 2MO(s) + 4NO2(g) + O2(g) As with the *alkali metals, the stability of salts of oxo acids increases down the group. In general, salts of the alkaline-earth elements are soluble if the anion has a single charge (e.g. nitrates, chlorides). Most salts with a doubly charged anion (e.g. carbonates, sulphates) are insoluble. The solubilities of salts of a particular acid tend to decrease down the group. (Solubilities of hydroxides increase for larger cations.)
alkaloid One of a group of nitrogenous organic compounds, mostly derived from plants, and having diverse pharmacological properties. They are biosynthesized from amino acids and classiÜed according to some structural feature. A simple classiÜcation is into: the pyridine group (e.g. coniine, nicotine) the tropine group (e.g. atropine, cocaine) the quinoline group (e.g. quinine, strychnine, brucine) the isoquinoline group (e.g. morphine, codeine) the phenylethylamine group (e.g. methamphetamine, mescaline, ephedrine) the indole group (e.g. tryptamine, lysergic acid) the purine group (e.g. caffeine, theobromine, theophylline) alkanal An aliphatic aldehyde. alkanes (parafÜns) Saturated hydrocarbons with the general formula CnH2n+2. In systematic chemical nomenclature alkane names end in the sufÜx -ane. They form a *homologous series (the alkane series) methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12), etc. The lower members of the series are gases; the high-molecular weight alkanes are waxy solids. Alkanes are present in natural gas and petroleum. They can be made by heating the sodium salt of a carboxylic acid with soda lime: RCOO–Na+ + Na+OH– → Na2CO3 + RH Other methods include the *Wurtz reaction and *Kolbe's method. Generally the alkanes are fairly unreactive. They form haloalkanes with halogens when irradiated with ultraviolet radiation.
• Information about IUPAC nomenclature • Further details about nomenclature
alkanol An aliphatic alcohol.
alkenes (oleÜnes; oleÜns) Unsaturated hydrocarbons that contain one or more double carbon–carbon bonds in their molecules. In systematic chemical nomenclature alkene names end in the sufÜx -ene. Alkenes that have only one double bond form a homologous series (the alkene series) starting ethene (ethylene), CH2:CH2, propene, CH3CH:CH2, etc. The general formula is CnH2n. Higher members of the series show isomerism depending on position of the double bond; for example, butene (C4H8) has two isomers, which are (1) but-1-ene (C2H5CH:CH2) and (2) but-2-ene (CH3CH:CHCH3): see formulae. Alkenes can be made by dehydration of alcohols (passing the vapour over hot pumice): RCH2CH2OH – H2O → RCH:CH2 An alternative method is the removal of a hydrogen atom and halogen atom from a haloalkane by potassium hydroxide in hot alcoholic solution: RCH2CH2Cl + KOH → KCl + H2O + RCH:CH2 Alkenes typically undergo *addition reactions to the double bond. They can be tested for by the *Baeyer test. See also ;  ; ; Z .
H C
CH3 C H2
H C H3C
but-1-ene
Alkenes. Butene isomers. alkoxides Compounds formed by reaction of alcohols with sodium or potassium metal. Alkoxides are saltlike compounds containing the ion R–O–. alkyd resin A type of *polyester resin used in paints and other surface coating. The original alkyd resins were made by copolymerizing phthalic anhydride with glycerol, to give a brittle cross-linked polymer. The properties of such resins can be modiÜed by adding
monobasic acids or alcohols during the polymerization.
alkylation A chemical reaction that introduces an *alkyl group into an organic molecule. The *Friedel–Crafts reaction results in alkylation of aromatic compounds. alkylbenzenes Organic compounds that have an alkyl group bound to a benzene ring. The simplest example is methylbenzene (toluene), CH3C6H5. Alkyl benzenes can be made by the *Friedel–Crafts reaction. alkyl group A group obtained by removing a hydrogen atom from an alkane, e.g. methyl group, CH3–, derived from methane. alkyl halides See . alkynes (acetylenes) Unsaturated hydrocarbons that contain one or more triple carbon–carbon bonds in their molecules. In systematic chemical nomenclature alkyne names end in the sufÜx -yne. Alkynes that have only one triple bond form a *homologous series: ethyne (acetylene), CH≡CH, propyne, CH3CH≡CH, etc. They can be made by the action of potassium hydroxide in alcohol solution on haloalkanes containing halogen atoms on adjacent carbon atoms; for example: RCHClCH2Cl + 2KOH → 2KCl + 2H2O + RCH≡CH Like *alkenes, alkynes undergo addition reactions.
allantois One of the membranes that develops in embryonic reptiles, birds, and mammals as a growth from the hindgut. It acts as a urinary bladder for the storage of waste excretory products in the egg (in reptiles and birds) and as a means of providing the embryo with oxygen (in reptiles, birds, and mammals) and food (in mammals; see ). See also  . allele (allelomorph) One of the alternative forms of a gene. In a diploid cell there are usually two alleles of any one gene (one from each parent), which occupy the same relative position (*locus) on *homologous chromosomes. These alleles may be the same, or one allele may be *dominant to the other (known as the *recessive), i.e. it determines which aspects of a particular characteristic the organism will display. Within a popula-
allelomorph
tion there may be many alleles of a gene; each has a unique nucleotide sequence.
allelomorph See . allelopathy The secretion by plants of chemicals, such as phenolic and terpenoid compounds, that inhibit the growth or germination of other plants, with which they are competing. For example, the aromatic oils released by certain shrubs of the Californian chaparral pass into the soil and inhibit the growth of herbaceous species nearby. Some plants produce chemicals that are toxic to grazing herbivorous animals. allenes Compounds that contain the group >C=C=C800°C) to produce oxygen 2BaO2 → 2BaO + O2
bromide See .
Britannia metal A silvery alloy consisting of 80–90% tin, 5–15% antimony, and sometimes small percentages of copper, lead, and zinc. It is used in bearings and some domestic articles. British thermal unit (Btu) The Imperial unit of heat, being originally the heat required to raise the temperature of 1lb of water by 1°F. 1 Btu is now deÜned as 1055.06 joules. broadband Communication by a system that supports a wide range of frequencies, so that identical messages can be carried simultaneously. See also ADSL. broken symmetry A situation in which the lowest-energy state of a many-body system or *vacuum state of a relativistic *quantum Üeld theory has a lower symmetry than the equations deÜning the system. Examples in solid-state physics include ferromagnetism, antiferromagnetism, and superconductivity. In particle physics, the Weinberg– Salam model (see  ) is an important example of a relativistic quantum Üeld theory with broken symmetry. A result associated with broken symmetry is Goldstone's theorem. This states that a relativistic quantum Üeld theory having continuous symmetry that is broken must include the existence of massless particles called Goldstone bosons. In many-body theory Goldstone bosons are *collective excitations. An exception to Goldstone's theorem is provided in the case of broken *gauge theories, such as the Weinberg–Salam model, in which the Goldstone bosons become massive bosons known as *Higgs bosons. In many-body theory, long-range forces provide the analogous exception to Goldstone's theorem, with the Higgs bosons being excitations with a nonzero gap. Such Higgs bosons are found in superconductors. bromate A salt or ester of a bromic acid.
bromic(V) acid A colourless liquid, HBrO3, made by adding sulphuric acid to barium bromate. It is a strong acid. bromination A chemical reaction in which a bromine atom is introduced into a molecule. See also . bromine Symbol Br. A *halogen element; a.n. 35; r.a.m. 79.909; r.d. 3.13; m.p. –7.2°C; b.p. 58.78°C. It is a red volatile liquid at room temperature, having a red-brown vapour. Bromine is obtained from brines in the USA (displacement with chlorine); a small amount is obtained from sea water in Anglesey. Large quantities are used to make 1,2dibromoethane as a petrol additive. It is also used in the manufacture of many other compounds. Chemically, it is intermediate in reactivity between chlorine and iodine. It forms compounds in which it has oxidation states of 1, 3, 5, or 7. The liquid is harmful to human tissue and the vapour irritates the eyes and throat. The element was discovered in 1826 by Antoine Balard.
bromoethane (ethyl bromide) A colourless Ûammable liquid, C2H5Br; r.d. 1.46; m.p. –119°C; b.p. 38.4°C. It is a typical *haloalkane, which can be prepared from ethene and hydrogen bromide. Bromoethane is used as a refrigerant. bromoform See ; . bromomethane (methyl bromide) A colourless volatile nonÛammable liquid, CH3Br; r.d. 1.68; m.p. –93°C; b.p. 3.56°C. It is a typical *haloalkane. N-bromosuccinimide (NBS) A crystalline solid, C4O2NBr, used extensively as a reagent for electrophilic addition of bromine. It acts by producing a small constant supply of bromine in solution C4O2NBr + H+ + Br– → C4O2NH + Br2. bromothymol blue An acid–base *indicator that is yellow in acid solutions and blue in alkaline solutions. It changes colour over the pH range 6–8. bronchiole A Üne respiratory tube in the
lungs of reptiles, birds, and mammals. It is formed by the subdivision of a *bronchus and in reptiles and mammals it terminates in a number of *alveoli.
bronchus (bronchial tube) One of the major air tubes in the *lung. The *trachea divides into two main bronchi, one for each lung, which split into smaller bronchi and then into *bronchioles. The walls of the bronchi are stiffened by rings of cartilage. bronze Any of a group of alloys of copper and tin, sometimes with lead and zinc present. The amount of tin varies from 1% to 30%. The alloy is hard and easily cast and extensively used in bearings, valves, and other machine parts. Various improved bronzes are produced by adding other elements; for instance, phosphor bronzes contain up to 1% phosphorus. In addition certain alloys of copper and metals other than tin are called bronzes – aluminium bronze is a mixture of copper and aluminium. Other special bronzes include *bell metal, *gun metal, and *beryllium bronze. Brown, Robert (1773–1858) British botanist, born in Scotland. After serving as an army medical ofÜcer he met botanist Joseph Banks (1743–1820) in 1798. Three years later Banks recommended him as naturalist on a survey of the Australian coast, during which he collected 4000 plant specimens; it took him Üve years to classify them. During this work he was the Ürst to distinguish between gymnosperms and angiosperms. Then in 1827, while observing pollen grains in water, he discovered *Brownian movement. brown algae See P. brown dwarf An astronomical object with a mass intermediate between the mass of a giant planet and that of a small star. The mass of a brown dwarf is large enough to generate energy by gravitational pressure, but not large enough to sustain nuclear fusion. The energy is radiated as electromagnetic radiation. Brown dwarfs are faint objects, which are expected to shine for about 100 million years before cooling. Their masses lie between a few times the mass of Jupiter and 80 times the mass of Jupiter. It has been suggested that brown dwarfs may contribute to the *missing mass of the universe. brown earth (brown forest soil) A type of soil that is characteristic of those mid-
deciduous woodland vegetation litter layer
horizon A
grey-brown porous crumb structure
transition to parent material
Brown earth profile. latitude parts of the world that were originally covered with deciduous woodland. It is rich in organic matter derived from the annual leaf fall of deciduous trees and from associated shrubs, herbs, and grasses. Brown earths occur in the NE USA, N China, central Japan, and NW and central Europe. They are important agriculturally, possessing a good crumb structure, mild acidity, and free drainage, and consequently most of the original forest has long since been cleared for agricultural use.
brown fat A darker coloured region of *adipose tissue found in newborn and hibernating animals (in which it may also be called the hibernating gland). Compared to normal white *fat, deposits of brown fat are more richly supplied with blood vessels and have numerous mitochondria (hence the brown colour, due to the high concentrations of cytochrome oxidase). They can also be more rapidly converted to heat energy – a process that takes place in the fat cells themselves – especially during arousal from hibernation and during cold stress in young animals. Since the deposits are strategically placed near major blood vessels, the heat they generate warms the blood returning to the heart. Some types of obesity in humans may be linked to a lack of brown fat in affected individuals. See .
Brownian movement
Brownian movement The continuous random movement of microscopic solid particles (of about 1 micrometre in diameter) when suspended in a Ûuid medium. First observed by Robert Brown in 1827 when studying pollen grains in water, it was originally thought to be the manifestation of some vital force. It was later recognized to be a consequence of bombardment of the pollen by the continually moving molecules of the liquid. The smaller the particles the more extensive is the motion. The effect is also visible in particles of smoke suspended in a still gas and in the material of dead cells.
• A transcript of part of Robert Brown's original 1829 paper • Jean Perrin's paper
brown-ring test A test for ionic nitrates. The sample is dissolved and iron(II) sulphate solution added in a test tube. Concentrated sulphuric acid is then added slowly so that it forms a separate layer. A brown ring (of Fe(NO)SO4) at the junction of the liquids indicates a positive result. brucite A mineral form of *magnesium hydroxide, Mg(OH)2. brush An electrical contact to a moving commutator on a motor or generator. It is made of a specially prepared form of carbon and is kept in contact with the moving part by means of a spring. brush border A region of surface epithelium that possesses densely packed microvilli (see ), rather like the bristles of a brush. This greatly increases the surface area of the epithelium and facilitates the absorption of materials. Brush borders are found in the convoluted tubules of the kidney and in the lining of the small intestine. brush discharge A luminous discharge from a conductor that takes the form of luminous branching threads that penetrate into the surrounding gas. It is a form of *corona and it occurs when the electric Üeld near the surface of the conductor exceeds a certain value but is not sufÜciently high for a spark to appear. brusselator A type of chemical reaction mechanism that leads to an *oscillating reaction. It involves the conversion of reactants A and B into products C and B by a series of four steps:
A→X 2X + Y → 3Y B+X→Y+C X→D Autocatalysis occurs as in the *Lotka– Volterra mechanism and the *oregonator. If the concentrations of A and B are maintained constant, the concentrations of X and Y oscillate with time. A graph of the concentration of X against that of Y is a closed loop (the limit cycle of the reaction). The reaction settles down to this limit cycle whatever the initial concentrations of X and Y, i.e. the limit cycle is an attractor for the system. The reaction mechanism is named after the city of Brussels, where the research group that discovered it is based.
Bryophyta A phylum of simple plants – the mosses – possessing no vascular tissue and rudimentary rootlike organs (rhizoids). They grow in a variety of damp habitats, from fresh water to rock surfaces. Some use other plants for support. Mosses show a marked *alternation of generations between gamete-bearing forms (gametophytes) and spore-bearing forms (sporophytes): they possess erect or prostrate leafy stems (the gametophyte generation, which is *haploid); these give rise to leaÛess stalks bearing capsules (the sporophyte generation, which is *diploid), the latter being dependent on the former for water and nutrients. Spores formed in the capsules are released and grow to produce new plants. Formerly, this phylum also included the liverworts (see H) and the mosses were classiÜed as a class (Musci) of the Bryophyta. The term 'bryophytes' is still used informally to refer to both the mosses and the liverworts.
• A resource devoted to mosses, liverworts, and hornworts from Southern Illinois University, Carbondale
Bryozoa (Ectoprocta) A phylum of aquatic, mainly marine, invertebrates – the moss animals and sea mats. They live in colonies, 50 cm or more across, which are attached to rocks, seaweeds, or shells. The individuals making up the colonies are about 1 mm long and superÜcially resemble cnidarian *polyps, with a mouth surrounded by ciliated tentacles that trap minute particles of organic matter in the water. Some have a horny or calcareous outer skeleton into
budding
which the body can be withdrawn. Bryozoans are placed in the clade Lophotrochozoa.
BSE See   . bubble chamber A device for detecting ionizing radiation. It consists of a chamber containing a liquid, often hydrogen, kept at slightly above its boiling point at a preliminary pressure that is high enough to prevent boiling. Immediately before the passage of the ionizing particles the pressure is reduced, and the particles then act as centres for the formation of bubbles, which can be photographed to obtain a record of the particles' tracks. The device was invented in 1952 by Donald Glaser. Compare  . buccal cavity (oral cavity) The mouth cavity: the beginning of the *alimentary canal, which leads to the pharynx and (in vertebrates) to the oesophagus. In vertebrates it is separated from the nasal cavity by the *palate. In mammals it contains the tongue and teeth, which assist in the mechanical breakdown of food, and the openings of the *salivary gland ducts. Buchner funnel A type of funnel with an internal perforated tray on which a Ûat circular Ülter paper can be placed, used for Ültering by suction. It is named after the German chemist Eduard Buchner (1860–1917). buckminsterfullerene A form of carbon composed of clusters of 60 carbon atoms bonded together in a polyhedral structure composed of pentagons and hexagons (see illustration). Originally it was identiÜed in
Buckminsterfullerene structure.
1985 in products obtained by Üring a highpower laser at a graphite target. It can be made by an electric arc struck between graphite electrodes in an inert atmosphere. The molecule, C60, was named after the US architect Richard Buckminster Fuller (1895–1983) because of the resemblance of the structure to the geodesic dome, which Fuller invented. The molecules are informally called buckyballs; more formally, the substance itself is also called fullerene. The substance is a yellow crystalline solid (fullerite), soluble in benzene. Various fullerene derivatives are known in which organic groups are attached to carbon atoms on the sphere. In addition, it is possible to produce novel enclosure compounds by trapping metal ions within the C60 cage. Some of these have semiconducting properties. The electric-arc method of producing C60 also leads to a smaller number of fullerenes such as C70, which have less symmetrical molecular structures. It is also possible to produce forms of carbon in which the atoms are linked in a cylindrical, rather than spherical, framework with a diameter of a few nanometres. They are known as buckytubes (or nanotubes).
• Information about IUPAC nomenclature and representation of fullerenes and related compounds
buckyball See . buckytube See . bud 1. (in botany) A condensed immature shoot with a short stem bearing small folded or rolled leaves. The outer leaves of a bud are often scalelike and protect the delicate inner leaves. A terminal (or apical) bud exists at the tip of a stem or branch while axillary (or lateral) buds develop in the *axils of leaves. However, in certain circumstances buds can be produced anywhere on the surface of a plant. Some buds remain dormant, but may become active if the terminal bud is removed. It is common gardening practice to remove the terminal buds of some shoots to induce the development of lateral shoots from axillary buds. See also  . 2. (in biology) An outgrowth from a parent organism that breaks away and develops into a new individual in the process of *budding. budding 1. (in biology) A method of asexual reproduction in which a new individual is derived from an outgrowth (bud) that be-
comes detached from the body of the parent. In animals the process is also called gemmation; it is common in cnidarians (e.g. Hydra) and also occurs in some sponges and other invertebrates. Among fungi, budding is characteristic of the yeasts. 2. (in horticulture) A method of grafting in which a bud of the scion is inserted onto the stock, usually beneath the bark.
buffer A solution that resists change in pH when small amounts of an acid or alkali are added over a certain range or when the solution is diluted. Acidic buffers consist of a weak acid with a salt of the acid. The salt provides the negative ion A–, which is the conjugate base of the acid HA. An example is carbonic acid and sodium hydrogencarbonate. Basic buffers have a weak base and a salt of the base (to provide the conjugate acid). An example is ammonia solution with ammonium chloride. In an acidic buffer, for example, molecules HA and ions A– are present. When acid is added most of the extra protons are removed by the base: A– + H+ → HA When base is added, most of the extra hydroxide ions are removed by reaction with undissociated acid: OH– + HA → A– + H2O Thus, the addition of acid or base changes the pH very little. The hydrogen-ion concentration in a buffer is given by the expression Ka = [H+] = [A–]/[HA] i.e. it depends on the ratio of conjugate base to acid. As this is not altered by dilution, the hydrogen-ion concentration for a buffer does not change much during dilution. In the laboratory, buffers are used to prepare solutions of known stable pH. Natural buffers occur in living organisms, where the biochemical reactions are very sensitive to change in pH (see – ). The main natural buffers are H2CO3/HCO3– and H2PO4–/HPO42–. Buffer solutions are also used in medicine (e.g. in intravenous injections), in agriculture, and in many industrial processes (e.g. dyeing, fermentation processes, and the food industry). bugs See H. bulb An underground plant organ that enables a plant to survive from one growing season to the next. It is a modiÜed shoot with a short Ûattened stem. A terminal bud develops at the centre of its upper surface, sur-
116 terminal bud
scale leaf
swollen leaf bases
flattened stem
foliage leaves and flowers develop from terminal bud leaf bases shrivel as food used for growing plant
axillary bud
withered foliage leaves swelling leaf bases forming new bulb
new bud
Development of a bulb. rounded by swollen leaf bases that contain food stored from the previous growing season. Papery brown scale leaves cover the outside of the bulb. The stored food is used in the growing season when the terminal bud produces foliage leaves and Ûowers. The new leaves photosynthesize and some of the manufactured food passes into the leaf bases forming a new bulb (see illustration). If more than one bud develops, then additional bulbs form, resulting in vegetative propagation. Examples of bulb-forming plants are daffodil, onion, and tulip. Compare .
bulbil A small bulblike organ that may develop in place of a Ûower, from an axillary bud, or at the base of a stem in certain plants. If it becomes detached it develops into a new plant. bulk modulus See  . bulla The rounded hollow projection of
bone from the skull that encloses the *middle ear in mammals.
bumping Violent boiling of a liquid caused by superheating so that bubbles form at a pressure above atmospheric pressure. It can be prevented by putting pieces of porous pot in the liquid to enable bubbles of vapour to form at the normal boiling point. buna rubber A type of synthetic rubber based on polymerization of butadiene (buta1,3-diene). The name comes from Bu (for butadiene) and Na (for sodium, which was used as a catalyst in the original polymerization reaction). An improved form, known as Buna-S, was developed by copolymerizing butadiene with styrene. In 1934, Buna-N was invented, in which the styrene was replaced by acrylonitrile, giving a product with better oil resistance (see  ). bunching In quantum optics, the arrival of photons at a detector grouped more closely together than they would be if grouped randomly. Bunching can also be described without quantum effects in terms of Ûuctuations of the classical electromagnetic Üeld. The arrival of photons at a detector less closely together than randomly is called antibunching and is a speciÜcally quantum-mechanical phenomenon. bundle of His The specialized cardiac muscle Übres in the mammalian heart that receive electrical stimuli from the *atrioventricular node and transmit them throughout the network of *Purkyne Übres. This allows the excitation to reach all parts of the ventricles rapidly and initiates a wave of contraction to expel blood into the aorta and pulmonary artery. It is named after Wilhelm His (1831–1904). Bunsen, Robert Wilhelm (1811–99) German chemist, who held professorships at Kassel, Marburg, and Heidelberg. His early researches on arsenic-containing compounds cost him an eye in an explosion. He then turned to gas analysis and spectroscopy, enabling him and *Kirchhoff to discover the elements *caesium (1860) and *rubidium (1861). He also popularized the use of the *Bunsen burner. Bunsen burner A laboratory gas burner having a vertical metal tube into which the gas is led, with a hole in the side of the base of the tube to admit air. The amount of air can be regulated by a sleeve on the tube. When no air is admitted the Ûame is lumi-
bus nous and smoky. With air, it has a faintly visible hot outer part (the oxidizing part) and an inner blue cone where combustion is incomplete (the cooler reducing part of the Ûame). The device is named after Robert Bunsen, who used a similar device (without a regulating sleeve) in 1855.
Bunsen cell A *primary cell consisting of a zinc cathode immersed in dilute sulphuric acid and a carbon anode immersed in concentrated nitric acid. The electrolytes are separated by a porous pot. The cell gives an e.m.f. of about 1.9 volts. buoyancy The upward thrust on a body immersed in a Ûuid. This force is equal to the weight of the Ûuid displaced (see A' ). burette A graduated glass tube with a tap at one end leading to a Üne outlet tube, used for delivering known volumes of a liquid (e.g. in titration). Burgess shale A fossil-rich deposit of shale and slate dating from the midCambrian period (about 520 million years ago) and located in the Burgess Pass in the Rocky Mountains of British Columbia, Canada. First excavated in 1909, it has since yielded one of the world's oldest assemblages of fossilized marine invertebrates and early vertebrate animals, as well as algae and sponges. The species described include brachiopods, crustaceans, trilobites, and other arthropods, plus a remarkably wellpreserved array of fossil worms and other soft-bodied animals. Many forms are of uncertain classiÜcation and have been assigned to new phyla. Such great diversity of fossils in deposits from this period is seen by some biologists as evidence for a burst of rapid evolution (see C ). Burnet, Sir Frank Macfarlane (1899– 1985) Australian virologist, who spent his working life at the Walter and Eliza Hall Institute in Melbourne. In the early 1930s he developed a method of growing inÛuenza virus in chick embryos. He later discovered that immunological tolerance (failure of the immune response) required repeated exposure to the antigen. For this work he shared the 1960 Nobel Prize for physiology or medicine with Sir Peter *Medawar. He also proposed the *clonal selection theory. bus A set of conducting paths – wires or optical Übres – connecting several components of a computer system and allowing the com-
buta-1,3-diene ponents to send signals to each other. The components take it in turns to transmit.
buta-1,3-diene (butadiene) A colourless gaseous hydrocarbon, CH2:CHCH:CH2; m.p. –109°C; b.p. –4.5°C. It is made by catalytic dehydrogenation of butane (from petroleum or natural gas) and polymerized in the production of synthetic rubbers. The compound is a conjugated *diene in which the electrons in the pi orbitals are partially delocalized over the whole molecule. It can have trans and cis forms, the latter taking part in *Diels–Alder reactions. butanal (butyraldehyde) A colourless Ûammable liquid aldehyde, C3H7CHO; r.d. 0.8; m.p. –99°C; b.p. 75.7°C. butane A gaseous hydrocarbon, C4H10; d. 0.58 g cm–3; m.p. –138°C; b.p. 0°C. Butane is obtained from petroleum (from reÜnery gas or by cracking higher hydrocarbons). The fourth member of the *alkane series, it has a straight chain of carbon atoms and is isomeric with 2-methylpropane (CH3CH(CH3)CH3, formerly called isobutane). It can easily be liqueÜed under pressure and is supplied in cylinders for use as a fuel gas. It is also a raw material for making buta-1,3-diene (for synthetic rubber). butanedioic acid (succinic acid) A colourless crystalline fatty acid, (CH2)2(COOH)2; r.d. 1.6; m.p. 185°C; b.p. 235°C. A weak carboxylic acid, it is produced by fermentation of sugar or ammonium tartrate and used as a sequestrant and in making dyes. It occurs in living organisms as an intermediate in metabolism, especially in the *Krebs cycle. butanoic acid (butyric acid) A colourless liquid water-soluble acid, C3H7COOH; r.d. 0.96; b.p. 163°C. It is a weak acid (Ka = 1.5 × 10–5 mol dm–3 at 25°C) with a rancid odour. Its esters are present in butter and in human perspiration. The acid is used to make esters for Ûavourings and perfumery. butanol Either of two aliphatic alcohols with the formula C4H9OH. Butan-1-ol, CH3(CH2)3OH, is a primary alcohol; r.d. 0.81; m.p. –89.5°C; b.p. 117.3°C. Butan-2-ol, CH3CH(OH)C2H5, is a secondary alcohol; r.d. 0.81; m.p. –114.7°C; b.p. 100°C. Both are colourless volatile liquids obtained from butane and are used as solvents. butanone (methyl ethyl ketone) A colourless Ûammable water-soluble liquid,
CH3COC2H5; r.d. 0.8; m.p. –86.4°C; b.p. 79.6°C. It can be made by the catalytic oxidation of butane and is used as a solvent.
butenedioic acid Either of two isomers with the formula HCOOHC:CHCOOH. Both compounds can be regarded as derivatives of ethene in which a hydrogen atom on each carbon has been replaced by a –COOH group. The compounds show cis–trans isomerism. The trans form is fumaric acid (r.d. 1.64; sublimes at 165°C) and the cis form is maleic acid (r.d. 1.59; m.p. 139–140°C). Both are colourless crystalline compounds used in making synthetic resins. The cis form is rather less stable than the trans form and converts to the trans form at 120°C. Unlike the trans form it can eliminate water on heating to form a cyclic anhydride containing a –CO.O.CO– group (maleic anhydride). Fumaric acid is an intermediate in the *Krebs cycle. butterÛies See L. butterÛy effect See . buttress root See  . butyl group The organic group CH3(CH2)3–. butyl rubber A type of synthetic rubber obtained by copolymerizing 2-methylpropene (CH2:C(CH3)CH3; isobutylene) and methylbuta-1,3-diene (CH2:C(CH3)CH:CH2, isoprene). Only small amounts of isoprene (about 2 mole %) are used. The rubber can be vulcanized. Large amounts were once used for tyre inner tubes. butyraldehyde See . butyric acid See  . Buys Ballot's law A law relating to winds stating that observers with their backs to the wind will experience a lower pressure on the left than on the right in the northern hemisphere, and lower on the right than on the left in the southern hemisphere. The law was propounded by the Dutch meteorologist Christoph Buys Ballot (1817–90) in 1857. by-product A compound formed during a chemical reaction at the same time as the main product. Commercially useful byproducts are obtained from a number of industrial processes. For example, calcium chloride is a by-product of the *Solvay process for making sodium carbonate. Propanone is a by-product in the manufacture of *phenol.
byte A subdivision of a *word in a computer, it usually consists of eight *bits. A kilobyte is 1024 bytes (not 1000 bytes). BZ See . B–Z reaction (Belousov–Zhabotinskii reaction) A chemical reaction that shows a periodic colour change between magenta and blue with a period of about one minute. It occurs with a mixture of sulphuric acid, potassium bromate(V), cerium sulphate, and
B–Z reaction propanedioic acid. The colour change is caused by alternating oxidation–reductions in which cerium changes its oxidation state (Ce3+ gives a magenta solution while Ce4+ gives a blue solution). The B–Z reaction is an example of a chemical *oscillating reaction – a reaction in which there is a regular periodic change in the concentration of one or more reactants. The mechanism is highly complicated, involving a large number of steps. See .
C C A high-level general-purpose computer language developed in 1972. It is fast and can be used as an alternative to assembly language, with which it shares some features. It was superseded in the mid-1980s by an object-oriented version known as C++, which is better suited to the design of modular programs.
• The current C standard
cadmium Symbol Cd. A soft bluish metal belonging to *group 12 (formerly IIB) of the periodic table; a.n. 48; r.a.m. 112.41; r.d. 8.65; m.p. 320.9°C; b.p. 765°C. The element's name is derived from the ancient name for calamine, zinc carbonate ZnCO3, and it is usually found associated with zinc ores, such as sphalerite (ZnS), but does occur as the mineral greenockite (CdS). Cadmium is usually produced as an associate product when zinc, copper, and lead ores are reduced. Cadmium is used in low-melting-point alloys to make solders, in Ni–Cd batteries, in bearing alloys, and in electroplating (over 50%). Cadmium compounds are used as phosphorescent coatings in TV tubes. Cadmium and its compounds are extremely toxic at low concentrations; great care is essential where solders are used or where fumes are emitted. It has similar chemical properties to zinc but shows a greater tendency towards complex formation. The element was discovered in 1817 by F. Stromeyer.
cadmium cell See W . cadmium sulphide A water-insoluble compound, CdS; r.d. 4.82. It occurs naturally as the mineral greenockite and is used as a pigment and in semiconductors and Ûuorescent materials. caecum A pouch in the alimentary canal of vertebrates between the *small intestine and *colon. The caecum (and its *appendix) is large and highly developed in herbivorous animals (e.g. rabbits and cows), in which it
contains a large population of bacteria essential for the breakdown of cellulose. In humans the caecum is a *vestigial organ and is poorly developed.
caesium Symbol Cs. A soft silvery-white metallic element belonging to *group 1 (formerly IA) of the periodic table; a.n. 55; r.a.m. 132.905; r.d. 1.88; m.p. 28.4°C; b.p. 678°C. It occurs in small amounts in a number of minerals, the main source being carnallite (KCl.MgCl2.6H2O). It is obtained by electrolysis of molten caesium cyanide. The natural isotope is caesium–133. There are 15 other radioactive isotopes. Caesium–137 (half-life 33 years) is used as a gamma source. As the heaviest alkali metal, caesium has the lowest ionization potential of all elements, hence its use in photoelectric cells, etc.
caesium chloride structure A type of ionic crystal structure in which the anions are at the eight corners of a cubic unit cell with one cation at the centre of the cell. It can equivalently by described as cations at the corners of the cell with an anion at the centre. Each type of ion has a coordination number of 8. Examples of compounds with this structure are CsCl, CsBr, CsI, CsCN, CuZn, and NH4Cl. caesium clock An *atomic clock that depends on the energy difference between two states of the caesium–133 nucleus when it is in a magnetic Üeld. In one type, atoms of caesium–133 are irradiated with *radiofrequency radiation, whose frequency is chosen to correspond to the energy difference between the two states. Some caesium nuclei absorb this radiation and are excited to the higher state. These atoms are deÛected by a further magnetic Üeld, which causes them to hit a detector. A signal from this detector is fed back to the radio-frequency oscillator to prevent it drifting from the resonant frequency of 9 192 631 770 hertz. In this way the device is locked to this frequency with an accuracy better than 1 part in 1013.
The caesium clock is used in the *SI unit deÜnition of the second.
caffeine (1,3,7-trimethylxanthine) An alkaloid, C8H10N4O2; m.p. 235°C; sublimes at 176°C. It is a stimulant and diuretic and is present in coffee, tea, and some soft drinks. See . cage compound See .
processes. In plants it is a constituent (in the form of calcium pectate) of the *middle lamella.
calcium acetylide See  .
Cainozoic See C.
calcium bicarbonate See  .
calciferol See  D.
calcium carbide See  .
calcination The formation of a calcium carbonate deposit from hard water. See   .
calcium carbonate A white solid, CaCO3, which is only sparingly soluble in water. Calcium carbonate decomposes on heating to give *calcium oxide (quicklime) and carbon dioxide. It occurs naturally as the minerals *calcite (rhombohedral; r.d. 2.71) and *aragonite (rhombic; r.d. 2.93). Rocks containing calcium carbonate dissolve slowly in acidiÜed rainwater (containing dissolved CO2) to cause temporary hardness. In the laboratory, calcium carbonate is precipitated from *limewater by carbon dioxide. Calcium carbonate is used in making lime (calcium oxide) and is the main raw material for the *Solvay process.
calcinite A mineral form of *potassium hydrogencarbonate, KHCO3. calcite One of the most common and widespread minerals, consisting of crystalline calcium carbonate, CaCO3. Calcite crystallizes in the rhombohedral system; it is usually colourless or white and has a hardness of 3 on the Mohs' scale. It has the property of double refraction, which is apparent in Iceland spar – the transparent variety of calcite. It is an important rock-forming mineral and is a major constituent in limestones, marbles, and carbonatites. calcitonin (thyrocalcitonin) A peptide hormone in vertebrates that lowers the concentration of calcium (and phosphate) in the blood. It operates in opposition to *parathyroid hormone. Calcitonin is produced by the *C cells, which in mammals are located in the *thyroid gland. calcitriol See  D. calcium Symbol Ca. A soft grey metallic element belonging to *group 2 (formerly IIA) of the periodic table; a.n. 20; r.a.m. 40.08; r.d. 1.54; m.p. 839°C; b.p. 1484°C. Calcium compounds are common in the earth's crust; e.g. limestone and marble (CaCO3), gypsum (CaSO4.2H2O), and Ûuorite (CaF2). The element is extracted by electrolysis of fused calcium chloride and is used as a getter in vacuum systems and a deoxidizer in producing nonferrous alloys. It is also used as a reducing agent in the extraction of such metals as thorium, zirconium, and uranium. Calcium is an *essential element for living organisms, being required for normal growth and development. In animals it is an important constituent of bones and teeth and is present in the blood, being required for muscle contraction and other metabolic
calcium chloride A white deliquescent compound, CaCl2, which is soluble in water; r.d. 2.15; m.p. 782°C; b.p. >1600°C. There are a number of hydrated forms, including the monohydrate, CaCl2.H2O, the dihydrate, CaCl2.2H2O (r.d. 0.84), and the hexahydrate, CaCl2.6H2O (trigonal; r.d. 1.71; the hexahydrate loses 4H2O at 30°C and the remaining 2H2O at 200°C). Large quantities of it are formed as a byproduct of the *Solvay process and it can be prepared by dissolving calcium carbonate or calcium oxide in hydrochloric acid. Crystals of the anhydrous salt can only be obtained if the hydrated salt is heated in a stream of hydrogen chloride. Solid calcium chloride is used in mines and on roads to reduce dust problems, whilst the molten salt is the electrolyte in the extraction of calcium. An aqueous solution of calcium chloride is used in refrigeration plants. calcium cyanamide A colourless solid, CaCN2, which sublimes at 1300°C. It is prepared by heating calcium dicarbide at 800°C in a stream of nitrogen: CaC2(s) + N2(g) → CaCN2(s) + C(s) The reaction has been used as a method of Üxing nitrogen in countries in which cheap electricity is available to make the calcium dicarbide (the cyanamide process). Calcium
calcium dicarbide
cyanamide can be used as a fertilizer because it reacts with water to give ammonia and calcium carbonate: CaCN2(s) + 3H2O(l) → CaCO3(s) + 2NH3(g) It is also used in the production of melamine, urea, and certain cyanide salts.
calcium dicarbide (calcium acetylide; calcium carbide; carbide) A colourless solid compound, CaC2; tetragonal; r.d. 2.22; m.p. 450°C; b.p. 2300°C. In countries in which electricity is cheap it is manufactured by heating calcium oxide with either coke or ethyne at temperatures above 2000°C in an electric arc furnace. The crystals consist of Ca2+ and C2– ions arranged in a similar way to the ions in sodium chloride. When water is added to calcium dicarbide, the important organic raw material ethyne (acetylene) is produced: CaC2(s) + 2H2O(l) → Ca(OH)2(s) + C2H2(g) calcium Ûuoride A white crystalline solid, CaF2; r.d. 3.2; m.p. 1423°C; b.p. 2500°C. It occurs naturally as the mineral *Ûuorite (or Ûuorspar) and is the main source of Ûuorine. The calcium fluoride structure (Ûuorite structure) is a crystal structure in which the calcium ions are each surrounded by eight Ûuoride ions arranged at the corners of a cube. Each Ûuoride ion is surrounded by four calcium ions at the corners of a tetrahedron. calcium hydrogencarbonate (calcium bicarbonate) A compound, Ca(HCO3)2, that is stable only in solution and is formed when water containing carbon dioxide dissolves calcium carbonate: CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq) It is the cause of temporary *hardness of water, because the calcium ions react with soap to give scum. Calcium hydrogencarbonate is unstable when heated and decomposes to give solid calcium carbonate. This explains why temporary hardness is removed by boiling and the formation of 'scale' in kettles and boilers. calcium hydroxide (slaked lime) A white solid, Ca(OH)2, which dissolves sparingly in water (see ); hexagonal; r.d. 2.24. It is manufactured by adding water to calcium oxide, a process that evolves much heat and is known as slaking. It is used as a cheap alkali to neutralize the acidity in certain soils
and in the manufacture of mortar, whitewash, bleaching powder, and glass.
calcium nitrate A white deliquescent compound, Ca(NO3)2, that is very soluble in water; cubic; r.d. 2.50; m.p. 561°C. It can be prepared by neutralizing nitric acid with calcium carbonate and crystallizing it from solution as the tetrahydrate Ca(NO3)2.4H2O, which exists in two monoclinic crystalline forms (α, r.d. 1.9; β, r.d. 1.82). There is also a trihydrate, Ca(NO3)2.3H2O. The anhydrous salt can be obtained from the hydrate by heating but it decomposes on strong heating to give the oxide, nitrogen dioxide, and oxygen. Calcium nitrate is sometimes used as a nitrogenous fertilizer. calcium octadecanoate (calcium stearate) An insoluble white salt, Ca(CH3(CH2)16COO)2, which is formed when soap is mixed with water containing calcium ions and is the scum produced in hard-water regions. calcium oxide (quicklime) A white solid compound, CaO, formed by heating calcium in oxygen or by the thermal decomposition of calcium carbonate; cubic; r.d. 3.35; m.p. 2580°C; b.p. 2850°C. On a large scale, calcium carbonate in the form of limestone is heated in a tall tower (lime kiln) to a temperature above 550°C: CaCO3(s) ˆ CaO(s) + CO2(g) Although the reaction is reversible, the carbon dioxide is carried away by the upward current through the kiln and all the limestone decomposes. Calcium oxide is used to make calcium hydroxide, as a cheap alkali for treating acid soil, and in extractive metallurgy to produce a slag with the impurities (especially sand) present in metal ores. calcium phosphate(V) A white insoluble powder, Ca3(PO4)2; r.d. 3.14. It is found naturally in the mineral *apatite, Ca5(PO4)3(OH,F,Cl), and as rock phosphate. It is also the main constituent of animal bones. Calcium phosphate can be prepared by mixing solutions containing calcium ions and hydrogenphosphate ions in the presence of an alkali: HPO42– + OH– → PO43– + H2O 3Ca2+ + 2PO43– → Ca3(PO4)2 It is used extensively as a fertilizer. The compound was formerly called calcium orthophosphate (see ).
calomel
calcium stearate See  .
to be characteristic features of the landscapes of Mars and Venus.
calcium sulphate A white solid compound, CaSO4; r.d. 2.96; 1450°C. It occurs naturally as the mineral *anhydrite, which has a rhombic structure, transforming to a monoclinic form at 200°C. More commonly, it is found as the dihydrate, *gypsum, CaSO4.2H2O (monoclinic; r.d. 2.32). When heated, gypsum loses water at 128°C to give the hemihydrate, 2CaSO4.H2O, better known as *plaster of Paris. Calcium sulphate is sparingly soluble in water and is a cause of permanent *hardness of water. It is used in the manufacture of certain paints, ceramics, and paper. The naturally occurring forms are used in the manufacture of sulphuric acid.
Calgon Trade name for a water-softening agent. See   .
calculus A series of mathematical techniques developed independently by Isaac Newton and Gottfried Leibniz (1646–1716). Differential calculus treats a continuously varying quantity as if it consisted of an inÜnitely large number of inÜnitely small changes. For example, the velocity v of a body at a particular instant can be regarded as the inÜnitesimal distance, written ds, that it travels in the vanishingly small time interval, dt; the instantaneous velocity v is then ds/dt, which is called the derivative of s with respect to t. If s is a known function of t, v at any instant can be calculated by the process of *differentiation. The differential calculus is a powerful technique for solving many problems concerned with rate processes, maxima and minima, and similar problems. Integral calculus is the opposite technique. For example, if the velocity of a body is a known function of time, the inÜnitesimal distance ds travelled in the brief instant dt is given by ds = vdt. The measurable distance s travelled between two instants t1 and t2 can then be found by a process of summation, called *integration, i.e.
californium Symbol Cf. A radioactive metallic transuranic element belonging to the *actinoids; a.n. 98; mass number of the most stable isotope 251 (half-life about 700 years). Nine isotopes are known; californium–252 is an intense neutron source, which makes it useful in neutron *activation analysis and potentially useful as a radiation source in medicine. The element was Ürst produced by Glenn Seaborg (1912–99) and associates in 1950.
s=
∫t
The technique is used for Ünding areas and volumes and other problems involving the summation of inÜnitesimals.
caldera A crater or large depression at the top of an inactive shield *volcano, which may be 1–20 km in diameter. It forms when magma subsides from the summit, sometimes aided by the explosive ejection of material. Often a caldera Ülls with water, forming a crater lake. Large calderas appear
caliche A mixture of salts found in deposits between gravel beds in the Atacama and Tarapaca regions of Chile. They vary from 4 m to 15 cm thick and were formed by periodic leaching of soluble salts during wet geological epochs, followed by drying out of inland seas in dry periods. They are economically important as a source of nitrates. A typical composition is NaNO3 17.6%, NaCl 16.1%, Na2SO4 6.5%, CaSO4 5.5%, MgSO4 3.0%, KNO3 1.3%, Na2B4O7 0.94%, KClO3 0.23%, NaIO3 0.11%, sand and gravel to 100%.
calixarenes Compounds that have molecules with a cuplike structure (the name comes from the Greek calix, cup). The simplest, has four phenol molecules joined by four –CH2– groups into a ring (forming the base of the 'cup'). The four phenol hexagons point in the same direction to form a cavity that can bind substrate molecules. Interest has been shown in the potential ability of calixarene molecules to mimic enzyme action. callus 1. (in botany) A protective tissue, consisting of parenchyma cells, that develops over a cut or damaged plant surface. Callus tissue can also be induced to form in cell cultures by hormone treatment. 2. (in pathology) A thick hard area of skin that commonly forms on the palms of the hands and soles of the feet as a result of continuous pressure or friction. 3. (in physiology) Hard tissue formed round bone ends following a fracture, which is gradually converted to new bone. calomel See (I) .
calomel half cell
calomel half cell (calomel electrode) A type of half cell in which the electrode is mercury coated with calomel (HgCl) and the electrolyte is a solution of potassium chloride and saturated calomel. The standard electrode potential is –0.2415 volt (25°C). In the calomel half cell the reactions are HgCl(s) ˆ Hg+(aq) + Cl–(aq) Hg+(aq) + e ˆ Hg(s) The overall reaction is HgCl(s) + e ˆ Hg(s) + Cl–(aq) This is equivalent to a Cl2(g)|Cl–(aq) half cell.
caloric theory A former theory concerning the nature of heat, which was regarded as a weightless Ûuid (called caloric). It was unable to account for the fact that friction could produce an unlimited quantity of heat and it was abandoned when Joule showed that heat is a form of energy. calorie The quantity of heat required to raise the temperature of 1 gram of water by 1°C (1 K). The calorie, a c.g.s. unit, is now largely replaced by the *joule, an *SI unit. 1 calorie = 4.186 8 joules. Calorie (kilogram calorie; kilocalorie) 1000 calories. This unit is still in limited use in estimating the energy value of foods, but is obsolescent. caloriÜc value The heat per unit mass produced by complete combustion of a given substance. CaloriÜc values are used to express the energy values of fuels; usually these are expressed in megajoules per kilogram (MJ kg–1). They are also used to measure the energy content of foodstuffs; i.e. the energy produced when the food is oxidized in the body. The units here are kilojoules per gram (kJ g–1), although Calories (kilocalories) are often still used in nontechnical contexts. CaloriÜc values are measured using a *bomb calorimeter. calorimeter Any of various devices used to measure thermal properties, such as *caloriÜc value, speciÜc *heat capacity, speciÜc *latent heat, etc. See  . Calvin, Melvin (1911–97) US biochemist. After World War II, at the Lawrence Radiation Laboratory, Berkeley, he investigated the light-independent reactions of *photosynthesis. Using radioactive carbon-14 to label carbon dioxide, he discovered the
*Calvin cycle, for which he was awarded the 1961 Nobel Prize for chemistry.
Calvin cycle The metabolic pathway of the light-independent stages of *photosynthesis, which occurs in the stroma of the chloroplasts. The pathway was elucidated by Melvin Calvin and his co-workers and involves the Üxation of carbon dioxide and its subsequent reduction to carbohydrate. During the cycle, carbon dioxide combines with *ribulose bisphosphate, through the mediation of the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco), to form an unstable six-carbon compound that breaks down to form two molecules of the three-carbon compound glycerate 3phosphate. This is converted to glyceraldehyde 3-phosphate, which is used to regenerate ribulose bisphosphate and to produce glucose and fructose. calx A metal oxide formed by heating an ore in air. calyptra 1. A layer of cells that covers the developing sporophyte of mosses, liverworts, clubmosses, horsetails, and ferns. In mosses it forms a hood over the *capsule and in liverworts it forms a sheath at the base of the capsule stalk. 2. See  . calyptrogen The region within the root *apical meristem that divides to produce the *root cap (calyptra). calyx The *sepals of a Ûower, collectively, forming the outer whorl of the *perianth. It encloses the petals, stamens, and carpels and protects the Ûower in bud. See also . cambium (lateral meristem) A plant tissue consisting of actively dividing cells (see ) that is responsible for increasing the girth of the plant, i.e. it causes secondary growth. The two most important cambia are the vascular (or fascicular) cambium and the *cork cambium. The vascular cambium occurs in the stem and root; it divides to produce secondary *xylem and secondary *phloem (new food- and water-conducting tissues). In mature stems the vascular cambium is extended laterally to form a complete ring: the sections of this ring between the vascular bundles comprises the interfascicular cambium. Compare  . Cambrian The earliest geological period of the Palaeozoic era. It is estimated to have
begun about 542 million years ago and lasted for some 54 million years. During this period marine animals with mineralized shells made their Ürst appearance and Cambrian rocks are the Ürst to contain an abundance of fossils. Cambrian fossils are chieÛy of marine animals; they include *trilobites, which dominated the Cambrian seas, echinoderms, brachiopods, molluscs, and primitive *graptolites (from the mid Cambrian). Trace *fossils also provide evidence for a variety of worms.
Cambrian explosion A relatively short interval of rapid intense evolution that supposedly occurred in the early to midCambrian period, some 540 to 520 million years ago. The supposition is based on the sudden appearance in the fossil record from this time of many diverse and novel forms, particularly marine animals, among which can be found representatives of all major modern groups. See B . camcorder See  . camera 1. An optical device for obtaining still photographs or for exposing cinematic Ülm. It consists of a light-proof box with a lens at one end and a plate or Ülm at the other. To make an exposure the shutter is opened and an image of the object to be photographed is formed on the lightsensitive Ülm. The length of the exposure is determined by the intensity of light available, the Ülm speed, and the *aperture of the lens. In the simpler cameras the shutter speed and aperture are controlled manually, but in automatic cameras the iris over the lens or the shutter is adjusted on the basis of information provided by a built-in *exposure meter. In ciné cameras the shutter automatically opens as the Ülm comes to rest behind the lens for each frame; the Ülm passes through the camera so that a set number (commonly 16, 18, or 24) of frames are exposed every second. 2. A similar device (a digital camera) in which the Ülm is replaced by a semiconductor array, which records the picture and stores it within the camera in a (usually) replaceable memory module. Moving pictures can be similarly recorded using a *video camera. 3. The part of a television system that converts optical images into electronic signals. It consists of a lens system, which focuses the image to be televised on the photosensitive mosaic of the camera tube, causing localized discharge of those of its elements that are illuminated. This mo-
saic is scanned from behind by an electron beam so that the beam current is varied as it passes over areas of light and shade. The signal so picked up by the scanning beam is preampliÜed in the camera and passed to the transmitter with sound and synchronization signals. In *colour television three separate camera tubes are used, one for each *primary colour.
camouÛage A high degree of similarity between an animal and its visual environment, which enables it to be disguised or concealed. By blending into the background the animal can elude predators or remain invisible to potential prey. See also  ; . Compare  . cAMP See  AMP. camphor A white crystalline cyclic ketone, C10H16O; r.d. 0.99; m.p. 179°C; b.p. 204°C. It was formerly obtained from the wood of the Formosan camphor tree, but can now be synthesized. The compound has a characteristic odour associated with its use in mothballs. It is a plasticizer in celluloid. Canada balsam A yellow-tinted resin used for mounting specimens in optical microscopy. It has similar optical properties to glass. canaliculus A very small channel that occurs between the cells of the liver and bone. In the liver the bile canaliculi carry bile to the bile ducts; in bone, canaliculi connect lacunae, the cavities containing bone cells. canal rays Streams of positive ions produced in a *discharge tube by boring holes (canals) in the cathode. The positive ions attracted to the cathode pass through the holes and emerge on the other side as positive rays. cancer Any disorder of cell growth that results in invasion and destruction of surrounding healthy tissue by abnormal cells. Cancer cells arise from normal cells whose nature is permanently changed. They multiply more rapidly than healthy body cells and do not seem subject to normal control by nerves and hormones. They may spread via the bloodstream or lymphatic system to other parts of the body, where they produce further tissue damage (metastases). Malignant tumour is another name for cancer. A cancer that arises in epithelium is called a carcinoma; one that arises in connective tis-
sue is called a sarcoma. Leukaemia is cancer of white blood cells; lymphoma is cancer of *lymphoid tissue; and myeloma is cancer of *plasma cells of the bone marrow. Causative agents (carcinogens) include various chemicals (including those in tobacco smoke), ionizing radiation, silica and asbestos particles, and *oncogenic viruses (see also ). Hereditary factors and stress may also play a role. Whatever the initiating factor, the mechanism by which cancer arises is mutation of genes in somatic cells.
• Extensive multimedia coverage of cancer biology and different types of cancer, produced by Emory University.
candela Symbol Cd. The *SI unit of luminous intensity equal to the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 Hz and has a radiant intensity in that direction of 1/683 watt per steradian. candle power Luminous intensity as formerly expressed in terms of the international candle but now expressed in candela. cane sugar See . canine tooth A sharp conical *tooth in mammals that is large and highly developed in carnivores (e.g. dogs) for tearing meat. There are two canines in each jaw, each situated between the second *incisor and the Ürst *premolar. In some animals (e.g. herbivores, such as giraffes and rabbits) canine teeth are absent. cannabinoids A large group of structurally related phenolic compounds found in the plant Cannabis sativa. The main one is tetrahydrocannabinol (THC), which is the compound responsible for the effects of cannabis. It effects cannabinoid receptors found in the brain (and also in the spleen). The name 'cannabinoid' is also applied to structurally unrelated compounds found naturally in animal tissue and having an effect on the cannabinoid receptors. These endocannabinoids (endogenous cannabinoids) are believed to act as 'messengers' between cells. cannabis An illegal drug produced from the plant Cannabis sativa. The dried inÛorescences of the plant are known as marijuana and the thick resin produced from the plant is known as hashish. The normal
method of using cannabis is to smoke it, although it can also be taken in certain foods. Cannabis is a class B controlled substance in the UK. It contains a large number of related compounds known as herbal *cannabinoids. The main active component is tetrahydrocannabinol (THC).
Cannizzaro reaction A reaction of aldehydes to give carboxylic acids and alcohols. It occurs in the presence of strong bases with aldehydes that do not have alpha hydrogen atoms. For example, benzenecarbaldehyde gives benzenecarboxylic acid and benzyl alcohol: 2C6H5CHO → C6H5COOH + C6H5CH2OH Aldehydes that have alpha hydrogen atoms undergo the *aldol reaction instead. The Cannizzaro reaction is an example of a *disproportionation. It was discovered in 1853 by the Italian chemist Stanislao Cannizzaro (1826–1910). canonical form One of the possible structures of a molecule that together form a *resonance hybrid. capacitance The property of a conductor or system of conductors that describes its ability to store electric charge. The capacitance (C) is given by Q/V, where Q is stored charge on one conductor and V the potential difference between the two conductors (or between a single conductor and earth); it is measured in farads (or, in practice, microfarads). An isolated sphere has a capacitance of 4πεr, where r is the radius and ε the *permittivity of the medium surrounding it. Capacitance is more commonly applied to systems of conductors (or semiconductors) separated by insulators (see ). capacitation The Ünal stage in the maturation process of a spermatozoon. This takes place inside the genital tract as the sperm penetrates the ovum. capacitor An arrangement of conductors separated by an insulator (dielectric) used to store charge or introduce *reactance into an alternating-current circuit. The earliest form was the *Leyden jar. Capacitors used as circuit elements have two conducting plates separated by the dielectric. The dielectric may be air, paper impregnated with oil or wax, plastic Ülm, or ceramic. The simplest form has two parallel rectangular conducting plates (area A) separated by a dielectric (thickness d, permittivity ε). The capacitance
of such a capacitor is Aε/d. Electrolytic capacitors are devices in which a thin layer of an oxide is deposited on one of the electrodes to function as the dielectric.
capacitor microphone A microphone consisting of a *capacitor with a steady voltage applied across its parallel plates. One plate is Üxed, the other is a thin diaphragm that is moved by the pressure of the sound waves. The movements of the diaphragm cause a variation in the spacing and therefore in the *capacitance of the device. This variation in capacitance is, in turn, reÛected in a similar variation in the charge carried by each plate. The consequent current to and from one plate is carried by a resistor, the varying potential difference across which constitutes the device's output signal. It was formerly known as a condenser microphone. capillarity See  . capillary 1. A tube of small diameter. 2. (blood capillary) The narrowest type of blood vessel in the vertebrate circulatory system. Capillaries conduct blood from *arterioles to all living cells: their walls are only one cell layer thick, so that oxygen and nutrients can pass through them into the surrounding tissues. Capillaries also transport waste material (e.g. urea and carbon dioxide) to venules for ultimate excretion. Capillaries can be constricted or dilated, according to local tissue requirements. capillary electrophoresis (CE) A technique for investigating mixtures of charged species. In capilliary zone electrophoresis (CZE), the sample is introduced into a Üne capillary tube, with each end of the capillary placed in a reservoir containing an electrolyte (e.g. a buffer solution). The source reservoir contains a positive electrode and the destination reservoir contains a negative electrode. A high potential difference is maintained between the electrodes. Components of the sample Ûow through the buffer solution in the capillary under the inÛuence of the electric Üeld. Their mobility depends on their charge, size, and shape, and the components separate as they move through the tube. They are detected close to the end of the capillary, usually by ultraviolet absorption. Capilliary gel electrophoresis (CGE) is used for separating large charged species, such as DNA fragments. The capilliary is Ülled with a gel and separation depends on the size of the species. In CE, a
graph of detector output against time is known as an electropherogram. Individual components can be identiÜed by their retention times in the capillary. The technique can be highly sensitive and is widely used in forensic laboratories.
capillary gel electrophoresis (CGE) See  . capillary zone electrophoresis (CZE) See  . capitulum A type of Ûowering shoot (see  ) characteristic of plants of the family Compositae (Asteraceae), e.g. daisy and dandelion. The tip of the shoot is Ûattened and bears many small stalkless Ûowers (Ûorets) surrounded by an involucre (ring) of bracts. This arrangement gives the appearance of a single Ûower. capric acid See  . caproic acid See  . caprolactam (6-hexanelactam) A white crystalline substance, C6H11NO; r.d. 1.02; m.p. 69–71°C; b.p. 139°C. It is a *lactam containing the –NH.CO– group with Üve CH2 groups making up the rest of the sevenmembered ring. Caprolactam is used in making *nylon. caprylic acid See  . capsid The protein coat of a *virus. The chemical nature of the capsid is important in stimulating the body's *immune response against the invading virus. capsule 1. (in botany) a. A dry fruit that releases its seeds when ripe; it is formed from several fused carpels and contains many seeds. The seeds may be dispersed through pores (as in the poppy), through a lid (as in plantain), or by the splitting and separation of the individual carpels (as in the crocus). Various other forms of capsules include the *silicula and *siliqua. b. The part of the sporophyte of mosses and liverworts in which the haploid spores are produced. It is borne on a long stalk (seta) and sheds its spores when mature (see ). 2. (in microbiology) A thick gelatinous layer completely surrounding the cell wall of certain bacteria. It appears to have a protective function, making ingestion of the bacterial cell by *phagocytes more difÜcult and preventing desiccation. 3. (in animal anatomy) a. The membranous or Übrous envelope that surrounds certain organs, e.g. the kidneys,
capture spleen, and lymph nodes. b. The ligamentous sheath of connective tissue that surrounds various skeletal joints.
capture Any of various processes in which a system of particles absorbs an extra particle. There are several examples in atomic and nuclear physics. For instance, a positive ion may capture an electron to give a neutral atom or molecule. Similarly, a neutral atom or molecule capturing an electron becomes a negative ion. An atomic nucleus may capture a neutron to produce a different (often unstable) nucleus. Another type of nuclear capture is the process in which the nucleus of an atom absorbs an electron from the innermost orbit (the K shell) to transform into a different nucleus. In this process (called K capture) the atom is left in an excited state and generally decays by emission of an X-ray photon. Radiative capture is any such process in which the capture results in an excited state that decays by emission of photons. A common example is neutron capture to yield an excited nucleus, which decays by emission of a gamma ray. carapace 1. The dorsal part of the *exoskeleton of some crustaceans (e.g. crabs), which spreads like a shield over several segments of the head and thorax. 2. The domed dorsal part of the shell of tortoises and turtles, formed of bony plates fused with the ribs and vertebrae and covered by a horny epidermal layer. The ventral part of the shell (plastron) is similar but Ûatter. carat 1. A measure of Üneness (purity) of gold. Pure gold is described as 24-carat gold. 14-carat gold contains 14 parts in 24 of gold, the remainder usually being copper. 2. A unit of mass equal to 0.200 gram, used to measure the masses of diamonds and other gemstones. carbamide See . carbanion An organic ion with a negative charge on a carbon atom; i.e. an ion of the type R3C–. Carbanions are intermediates in certain types of organic reaction (e.g. the *aldol reaction). carbene A species of the type R2C:, in which the carbon atom has two electrons that do not form bonds. Methylene, :CH2, is the simplest example. Carbenes are highly reactive and exist only as transient intermediates in certain organic reactions. They attack double bonds to give cyclopropane
derivatives. They also cause insertion reactions, in which the carbene group is inserted between the carbon and hydrogen atoms of a C–H bond: C–H + :CR2 → C–CR2–H
carbenium ion See . carbide Any of various compounds of carbon with metals or other more electropositive elements. True carbides contain the ion C4– as in Al4C3. These are saltlike compounds giving methane on hydrolysis, and were formerly called methanides. Compounds containing the ion C22– are also saltlike and are known as dicarbides. They yield ethyne (acetylene) on hydrolysis and were formerly called acetylides. The above types of compound are ionic but have partially covalent bond character, but boron and silicon form true covalent carbides, with giant molecular structures. In addition, the transition metals form a range of interstitial carbides in which the carbon atoms occupy interstitial positions in the metal lattice. These substances are generally hard materials with metallic conductivity. Some transition metals (e.g. Cr, Mn, Fe, Co, and Ni) have atomic radii that are too small to allow individual carbon atoms in the interstitial holes. These form carbides in which the metal lattice is distorted and chains of carbon atoms exist (e.g. Cr3C2, Fe3C). Such compounds are intermediate in character between interstitial carbides and ionic carbides. They give mixtures of hydrocarbons on hydrolysis with water or acids. carbocation An ion with a positive charge that is mostly localized on a carbon atom. There are two types: Carbonium ions have Üve bonds to the carbon atom and a complete outer shell of eight electrons. A simple example is the ion CH5 +, which has a trigonal bipyramidal shape. Ions of this type are transient species. They can be produced by electron impact and detected by mass spectroscopy. Carbenium ions have three bonds to the carbon atom and are planar, with six outer electrons and a vacant p-orbital. Ions of this type are intermediates in a number of organic reactions (for example, in the SN1 mechanism of *nucleophilic substitution). Certain carbenium ions are stabilized by delocalization of the charge. An example is the orange-red salt (C6H5)3C+Cl-. Carbenium ions can be produced by superacids.
carbocyclic See . carbohydrate One of a group of organic compounds based on the general formula Cx(H2O)y. The simplest carbohydrates are the *sugars (saccharides), including glucose and sucrose. *Polysaccharides are carbohydrates of much greater molecular weight and complexity; examples are starch, glycogen, and cellulose. Carbohydrates perform many vital roles in living organisms. Sugars, notably glucose, and their derivatives are essential intermediates in the conversion of food to energy. Starch and other polysaccharides serve as energy stores in plants, particularly in seeds, tubers, etc., which provide a major energy source for animals, including man. Cellulose, lignin, and others form the supporting cell walls and woody tissue of plants. Chitin is a structural polysaccharide found in the body shells of many invertebrate animals. Carbohydrates also occur in the surface coat of animal cells and in bacterial cell walls.
'lead' in pencils (hence its alternative name). The third crystalline allotrope is fullerite (see ). There are also several amorphous forms of carbon, such as *carbon black and *charcoal. There are two stable isotopes of carbon (proton numbers 12 and 13) and four radioactive ones (10, 11, 14, 15). Carbon–14 is used in *carbon dating. Carbon occurs in all organic compounds and is therefore fundamental to the structure of all living organisms. It is an *essential element for plants and animals, being ultimately derived from atmospheric carbon dioxide assimilated by plants during photosynthesis (see  ). The ubiquitous nature of carbon in living organisms is due to its unique ability to form stable covalent bonds with other carbon atoms and also with hydrogen, oxygen, nitrogen, and sulphur atoms, resulting in the formation of a variety of compounds containing chains and rings of carbon atoms.
carbolic acid See .
carbon assimilation The incorporation of carbon from atmospheric carbon dioxide into organic molecules. This process occurs during *photosynthesis. See  .
carbon Symbol C. A nonmetallic element belonging to *group 14 (formerly IVB) of the periodic table; a.n. 6; r.a.m. 12.011; m.p. ∼3550°C; b.p. ∼4827°C. Carbon has three main allotropic forms (see ). *Diamond (r.d. 3.52) occurs naturally and can be produced synthetically. It is extremely hard and has highly refractive crystals. The hardness of diamond results from the covalent crystal structure, in which each carbon atom is linked by covalent bonds to four others situated at the corners of a tetrahedron. The C–C bond length is 0.154 nm and the bond angle is 109.5°. Graphite (r.d. 2.25) is a soft black slippery substance (sometimes called black lead or plumbago). It occurs naturally and can also be made by the *Acheson process. In graphite the carbon atoms are arranged in layers, in which each carbon atom is surrounded by three others to which it is bound by single or double bonds. The layers are held together by much weaker van der Waals' forces. The carbon–carbon bond length in the layers is 0.142 nm and the layers are 0.34 nm apart. Graphite is a good conductor of heat and electricity. It has a variety of uses including electrical contacts, high-temperature equipment, and as a solid lubricant. Graphite mixed with clay is the
carbonate A salt of carbonic acid containing the carbonate ion, CO32–. The free ion has a plane triangular structure. Metal carbonates may be ionic or may contain covalent metal–carbonate bonds (complex carbonates) via one or two oxygen atoms. The carbonates of the alkali metals are all soluble but other carbonates are insoluble; they all react with mineral acids to release carbon dioxide. carbonate minerals A group of common rock-forming minerals containing the anion CO32– as the fundamental unit in their structure. The most important carbonate minerals are *calcite, *dolomite, and *magnesite. See also . carbonation The solution of carbon dioxide in a liquid under pressure. carbon bisulphide See  . carbon black A Üne carbon powder made by burning hydrocarbons in insufÜcient air. It is used as a pigment and a Üller (e.g. for rubber).
carbon capture and storage (CCS) The removal of carbon dioxide from a point source of pollution and its subsequent transfer to a repository so that it does not enter the atmosphere. Methods of CCS are being developed as ways of reducing emissions of greenhouse gases (see  ) and mitigating the environmental impact of burning coal, natural gas, or oil. For example, it is feasible to remove carbon dioxide from power station Ûue gases by scrubbing, using activated carbon Ülters. However, this signiÜcantly increases the operating cost of the power station. Once removed from a point source, the carbon dioxide is piped to a suitable storage site, often located in porous underground rock formations, notably depleted oil and gas Üelds. Other possibilities are storage in deep ocean sites and chemical reaction with metal oxides to create stable carbonates. carbon cycle 1. (in biology) One of the major cycles of chemical elements in the environment (see  ). Carbon (as carbon dioxide) is taken up from the atmosphere and incorporated into the tissues of plants in *photosynthesis. It may then pass into the bodies of animals as the plants are eaten (see  ). During the respiration of plants, animals, and organisms that bring about decomposition, carbon dioxide is returned to the atmosphere.
The combustion of fossil fuels (e.g. coal and peat) also releases carbon dioxide into the atmosphere. See illustration. See also  . 2. (in physics) A series of nuclear reactions in which four hydrogen nuclei combine to form a helium nucleus with the liberation of energy, two positrons, and two neutrinos. The process is believed to be the source of energy in many stars and to take place in six stages. In this series carbon–12 acts as if it were a catalyst, being reformed at the end of the series: 12 C 6
+ 11 H →
13 N 7
+ 11H →
+γ
15 O 8
+ e+ + νe
+ e+ + νe 12 C 6
+ 42 He.
See  .
carbon dating (radiocarbon dating) A method of estimating the ages of archaeological specimens of biological origin. As a result of *cosmic radiation a small number of atmospheric nitrogen nuclei are continuously being transformed by neutron
carbon dioxide in the atmosphere
carbon in fossil fuels
respiration in decomposers
fossilization
organic compounds in green plants
The carbon cycle in nature.
organic compounds in animals
carbon compounds in dead organic matter
bombardment into radioactive nuclei of carbon–14: + n → 146 C + p Some of these radiocarbon atoms Ünd their way into living trees and other plants in the form of carbon dioxide, as a result of *photosynthesis. When the tree is cut down photosynthesis stops and the ratio of radiocarbon atoms to stable carbon atoms begins to fall as the radiocarbon decays. The ratio 14C/12C in the specimen can be measured and enables the time that has elapsed since the tree was cut down to be calculated. The method has been shown to give consistent results for specimens up to some 40 000 years old, though its accuracy depends upon assumptions concerning the past intensity of the cosmic radiation. The technique was developed by Willard F. Libby (1908–80) and his coworkers in 1946–47. 14 N 7
carbon dioxide A colourless odourless gas, CO2, soluble in water, ethanol, and acetone; d. 1.977 g dm–3 (0°C); m.p. –56.6°C; b.p. –78.5°C. It occurs in the atmosphere (0.04% by volume) but has a short residence time in this phase as it is both consumed by plants during *photosynthesis and produced by *respiration and by combustion. It is readily prepared in the laboratory by the action of dilute acids on metal carbonates or of heat on heavy-metal carbonates. Carbon dioxide is a by-product from the manufacture of lime and from fermentation processes. Carbon dioxide has a small liquid range and liquid carbon dioxide is produced only at high pressures. The molecule CO2 is linear with each oxygen making a double bond to the carbon. Chemically, it is unreactive and will not support combustion. It dissolves in water to give *carbonic acid. Large quantities of solid carbon dioxide (dry ice) are used in processes requiring large-scale refrigeration. It is also used in Üre extinguishers as a desirable alternative to water for most Üres, and as a constituent of medical gases as it promotes exhalation. It is also used in carbonated drinks. The level of carbon dioxide in the atmosphere has increased by some 30% since the Industrial Revolution, mainly because of extensive burning of fossil fuels and the destruction of large areas of rainforest. This has been postulated as the main cause of the average increase of 0.74°C in global temperatures over the last 100 years, through the *greenhouse effect. Atmospheric CO2 concentration continues to rise, in spite of ten-
carbonic acid tative steps to control emissions, giving the prospect of accelerated *global warming in the foreseeable future.
carbon disulphide (carbon bisulphide) A colourless highly refractive liquid, CS2, slightly soluble in water and soluble in ethanol and ether; r.d. 1.261; m.p. –110°C; b.p. 46.3°C. Pure carbon disulphide has an ethereal odour but the commercial product is contaminated with a variety of other sulphur compounds and has a most unpleasant smell. It was previously manufactured by heating a mixture of wood, sulphur, and charcoal; modern processes use natural gas and sulphur. Carbon disulphide is an excellent solvent for oils, waxes, rubber, sulphur, and phosphorus, but its use is decreasing because of its high toxicity and its Ûammability. It is used for the preparation of xanthates in the manufacture of viscose yarns. carbon Übres Fibres of carbon in which the carbon has an oriented crystal structure. Carbon Übres are made by heating textile Übres and are used in strong composite materials for use at high temperatures. carbon footprint The total amount of greenhouse gases (GHGs) emitted to the atmosphere as a result of the activities of an individual, household, business, or other entity. It is usually measured as the mass of carbon dioxide, or carbon dioxide equivalent (i.e. including methane, nitrogen oxides, and other GHGs), emitted per year, and indicates the environmental impact of those activities through their contribution to the *greenhouse effect and hence global warming. Calculation of a full carbon footprint involves both direct and indirect emissions. Direct emissions include those arising from activities that are controlled directly by an individual or organization, such as the combustion of fuel for heating or transport. Indirect emissions are those arising from all goods and services used by the individual or organization, but not directly controlled by them, such as the energy used in the extraction and processing of raw materials and manufacture of goods. carbonic acid A dibasic acid, H2CO3, formed in solution when carbon dioxide is dissolved in water: CO2(aq) + H2O(l) ˆ H2CO3(aq) The acid is in equilibrium with dissolved carbon dioxide, and also dissociates as follows: H2CO3 ˆ H+ + HCO3–
carbonic anhydrase Ka = 4.5 × 10–7 mol dm–3 HCO3– ˆ CO32– + H+
Ka = 4.8 × 10–11 mol dm–3 The pure acid cannot be isolated, although it can be produced in ether solution at –30°C. Carbonic acid gives rise to two series of salts: the *carbonates and the *hydrogencarbonates.
carbonic anhydrase An enzyme, present in red blood cells and kidney cells, that catalyses the reaction between carbon dioxide with water: CO2 + H2O ˆ H2CO3 H2CO3 ˆ H+ + HCO3–. This reaction is one of the fastest known and controls the elimination of carbon dioxide from the body and the pH of urine. It also facilitates the transfer of carbon dioxide from the tissues to the blood and from the blood to the alveoli (air sacs) of the lungs. See also  .
Carboniferous A geological period in the Palaeozoic era. It began about 360 million years ago, following the Devonian period, and extended until the beginning of the Permian period, about 299 million years ago. In Europe the period is divided into the Lower and Upper Carboniferous, which roughly correspond to the Mississippian and Pennsylvanian periods, respectively, of North America. During the Lower Carboniferous a marine transgression occurred and the characteristic rock of this division – the Carboniferous limestone – was laid down in the shallow seas. Fauna included foraminiferans, corals, bryozoans, brachiopods, blastoids, and other invertebrates. The Upper Carboniferous saw the deposition of the millstone grit, a mixture of shale and sandstone formed in deltaic conditions, followed by the coal measures, alternating beds of coal, sandstone, shale, and clay. The coal was formed from the vast swamp forests composed of seed ferns, lycopsids, and other plants. During the period Üshes continued to diversify and amphibians became more common. carbonium ion See . carbonize (carburize) To change an organic compound into carbon by heating, or to coat something with carbon in this way. carbon monoxide A colourless odourless gas, CO, sparingly soluble in water and solu-
_O C _ O C_ O _ _ _ _ O C C C O
carbon monoxide carbon dioxide tricarbon dioxide (carbon suboxide)
Carbon monoxide. Oxides of carbon. ble in ethanol and benzene; d. 1.25 g dm–3 (0°C); m.p. –199°C; b.p. –191.5°C. It is Ûammable and highly toxic. In the laboratory it can be made by the dehydration of methanoic acid (formic acid) using concentrated sulphuric acid. Industrially it is produced by the oxidation of natural gas (methane) or (formerly) by the water-gas reaction. It is formed by the incomplete combustion of carbon and is present in car-exhaust gases. It is a neutral oxide, which burns in air to give carbon dioxide, and is a good reducing agent, used in a number of metallurgical processes. It has the interesting chemical property of forming a range of transition metal carbonyls, e.g. Ni(CO)4. Carbon monoxide is able to use vacant p-orbitals in bonding with metals; the stabilization of low oxidation states, including the zero state, is a consequence of this. This also accounts for its toxicity, which is due to the binding of the CO to the iron in haemoglobin, thereby blocking the uptake of oxygen.
carbon suboxide See  . carbon tetrachloride See . carbonyl chloride (phosgene) A colourless gas, COCl2, with an odour of freshly cut hay. It is used in organic chemistry as a chlorinating agent, and was formerly used as a war gas. carbonyl compound A compound containing the carbonyl group >C=O. Aldehydes, ketones, and carboxylic acids are examples of organic carbonyl compounds. Inorganic carbonyls are complexes in which carbon monoxide has coordinated to a metal atom or ion, as in *nickel carbonyl, Ni(CO)4. See also . carbonyl group The group >C=O, found in aldehydes, ketones, carboxylic acids, amides, etc., and in inorganic carbonyl complexes (see  ). carboranes Compounds similar to the *boranes, but with one or more boron atoms replaced by carbon atoms.
carborundum See  . carboxyhaemoglobin The highly stable product formed when *haemoglobin combines with carbon monoxide. Carbon monoxide competes with oxygen for haemoglobin, with which it binds strongly: the afÜnity of haemoglobin for carbon monoxide is 250 times greater than that for oxygen. This reduces the availability of haemoglobin for combination with (and transport of) oxygen and accounts for the toxic effects of carbon monoxide on the respiratory system. carboxylate An anion formed from a *carboxylic acid. For example, ethanoic acid gives rise to the ethanoate ion, CH3COO–. carboxyl group The organic group –COOH, present in *carboxylic acids. carboxylic acids Organic compounds containing the group –COOH (the carboxyl group; i.e. a carbonyl group attached to a hydroxyl group). In systematic chemical nomenclature carboxylic-acid names end in the sufÜx -oic, e.g. ethanoic acid, CH3COOH. They are generally weak acids. Many longchain carboxylic acids occur naturally as esters in fats and oils and are therefore also known as *fatty acids. See also .
O R
carboxyl group OH
Carboxylic acid structure. carboxypeptidase An *exopeptidase enzyme in pancreatic juice that is secreted into the duodenum. The enzyme is secreted as an inactive precursor, procarboxypeptidase, which is activated by another pancreatic protease, *trypsin. See also . carburize See . carbylamine reaction See  . carcerulus A dry fruit that is a type of *schizocarp. It consists of a number of oneseeded fragments (mericarps) that adhere to a central axis. It is characteristic of mallow. carcinogen Any agent that produces *can-
cer, e.g. tobacco smoke, certain industrial chemicals, and *ionizing radiation (such as X-rays and ultraviolet rays).
carcinoma See . cardiac 1. Relating to the heart. 2. Relating to the part of the stomach nearest to the oesophagus. cardiac cycle The sequence of events that occurs in the heart during one full heartbeat. These events comprise contraction (see ) and relaxation (see ) of the chambers of the heart, associated with opening and closing of the heart valves. When both the atria and the ventricles are relaxed, pressure in the heart is low and blood Ûows from the vena cava and pulmonary vein into the atria and through to the ventricles. The aortic and pulmonary valves, at the junction between the left ventricle and aorta and the right ventricle and pulmonary artery, respectively, are closed; therefore, blood can enter but not leave the heart, which increases the pressure in the chambers. As the pressure in the heart increases, the atria begin to contract, forcing the blood into the ventricles and closing the *tricuspid valve and the *bicuspid valve. A wave of ventricular contraction follows, expelling the blood into the aorta and pulmonary artery to complete the cardiac cycle. At a resting heart rate, the human cardiac cycle lasts approximately 0.85 second. cardiac muscle A specialized form of *muscle that is peculiar to the vertebrate heart. There are two types of cardiac muscle Übres: contractile Übres, which are striated and contain numerous myoÜbrils; and conducting Übres, or *Purkyne Übres, which branch extensively and conduct electrical signals throughout the muscle. The muscle itself shows spontaneous contraction and does not need nervous stimulation (see ). The vagus nerve to the heart can, however, affect the rate of contraction. cardiac output The volume of blood pumped per minute by each ventricle, which is also the total blood Ûow through the pulmonary circuit. At rest, normal human cardiac output is approximately 5 litres per minute, rising to 22 litres per minute during maximum physical exertion. The cardiac output can be calculated from heart rate (number of beats per minute) and stroke volume (volume of blood expelled from the heart per beat).
cardiovascular centre
cardiovascular centre One of the areas in the brain that are responsible for the modiÜcation of the cardiovascular system based upon the integration of sensory information from the autonomic nervous system. These centres inÛuence the heart rate via the sympathetic and parasympathetic nerves and by the action of certain hormones. Carius method A method of determining the amount of sulphur and halogens in an organic compound, by heating the compound in a sealed tube with silver nitrate in concentrated nitric acid. The compound is decomposed and silver sulphide and halides are precipitated, separated, and weighed. carnallite A mineral consisting of a hydrated mixed chloride of potassium and magnesium, KCl.MgCl2.6H2O. carnassial teeth Molar and premolar teeth modiÜed for shearing Ûesh by having cusps with sharp cutting edges. They are typical of animals of the order *Carnivora (e.g. tigers, wolves), in which they are the Ürst molars in the lower jaw and the last premolars in the upper. Carnivora An order of mainly Ûesh-eating mammals that includes the dogs, wolves, bears, badgers, weasels, and cats. Carnivores typically have very keen sight, smell, and hearing. The hinge joint between the lower jaw and skull is very tight, allowing no lateral movement of the lower jaw. This – together with the arrangement of jaw muscles – enables a very powerful bite. The teeth are specialized for stabbing and tearing Ûesh: canines are large and pointed and some of the cheek teeth are modiÜed for shearing (see  ). carnivore An animal that eats meat, especially a member of the order *Carnivora (e.g. tigers, wolves). Carnivores are specialized by having strong powerful jaws and welldeveloped canine teeth. They may be *predators or carrion eaters. See also . Compare ; . carnivorous plant (insectivorous plant) Any plant that supplements its supply of nitrates in conditions of nitrate deÜciency by digesting small animals, especially insects. Such plants are adapted in various ways to attract and trap the insects and produce proteolytic enzymes to digest them. Venus' Ûy trap (Dionaea), for example, has spiny-margined hinged leaves that snap shut on an alighting insect. Sundews (Drosera) trap and
digest insects by means of glandular leaves that secrete a sticky substance, and pitcher plants (families Nepenthaceae and Sarraceniaceae) have leaves modiÜed as pitchers into which insects fall, drowning in the water and digestive enzymes at the bottom.
Carnot, Nicolas Léonard Sadi (1796–1832) French physicist, who Ürst worked as a military engineer. He then turned to scientiÜc research and in 1824 published his analysis of the efÜciency of heat engines. The key to this analysis is the thermodynamic *Carnot cycle. He died at an early age of cholera. Carnot cycle The most efÜcient cycle of operations for a reversible *heat engine. Published in 1824 by Nicolas Carnot, it consists of four operations on the working substance in the engine (see illustration): a. Isothermal expansion at thermodynamic temperature T1 with heat Q1 taken in. b. Adiabatic expansion with a fall of temperature to T2. c. Isothermal compression at temperature T2 with heat Q2 given out. d. Adiabatic compression with a rise of temperature back to T1. P T1q1
isothermal expansion
adiabatic expansion adiabatic T2q2 compression isothermal compression
Carnot cycle. According to the Carnot principle, the efÜciency of any reversible heat engine depends only on the temperature range through which it works, rather than the properties of the working substances. In any reversible engine, the efÜciency (η) is the ratio of the work done (W) to the heat input (Q1), i.e. η = W/Q1. As, according to the Ürst law of *thermodynamics, W = Q1 – Q2, it follows that η = (Q1 – Q2)/Q1. For the Kelvin temperature scale, Q1/Q2 = T1/T2 and η = (T1 – T2)/T1. For maximum efÜciency T1 should be as high as possible and T2 as low as possible.
carnotite A radioactive mineral consisting of hydrated uranium potassium vanadate, K2(UO2)2(VO4)2.nH2O. It varies in colour from bright yellow to lemon- or greenishyellow. It is a source of uranium, radium, and vanadium. The chief occurrences are in the Colorado Plateau, USA; Radium Hill, Australia; and Katanga, Democratic Republic of Congo. Caro's acid See (VI) . carotene A member of a class of *carotenoid pigments. Examples are βcarotene and lycopene, which colour carrot roots and ripe tomato fruits respectively. αand β-carotene yield vitamin A when they are broken down during animal digestion. carotenoid Any of a group of yellow, orange, red, or brown plant pigments chemically related to terpenes. Carotenoids are responsible for the characteristic colour of many plant organs, such as ripe tomatoes, carrots, and autumn leaves. They also function as accessory *photosynthetic pigments in the light-dependent reactions of *photosynthesis. See ; . Carothers, Wallace Hume (1896–1937) US industrial chemist, who joined the Du Pont company where he worked on polymers. In 1931 he produced *neoprene, a synthetic rubber. His greatest success came in 1935 with the discovery of the polyamide that came to be known as *nylon. Carothers, who suffered from depression, committed suicide. carotid artery The major artery that supplies blood to the head. A pair of common carotid arteries arise from the aorta (on the left) and the innominate artery (on the right) and run up the neck; each branches into an external and an internal carotid artery, which supply the head. carotid body One of a pair of tissue masses adjacent to the *carotid sinus. Each contains receptors that are sensitive to oxygen and pH levels (acidity) in the blood. High levels of carbon dioxide in the blood lower the pH (i.e. increase the acidity). By responding to Ûuctuations in pH, the carotid body coordinates reÛex changes in respiration rate. carotid sinus An enlarged region of the *carotid artery at its major branching point in the neck. Its walls contain many receptors
carrier molecule that are sensitive to changes in pressure and it regulates blood pressure by initiating reÛex changes in heart rate and dilation of blood vessels.
carpal (carpal bone) One of the bones that form the wrist (see ) in terrestrial vertebrates. carpel The female reproductive organ of a Ûower. Typically it consists of a *stigma, *style, and *ovary. It is thought to have evolved by the fusion of the two edges of a Ûattened megasporophyll (see ). Each Ûower may have one carpel (monocarpellary) or many (polycarpellary), either free (apocarpous) or fused together (syncarpous). See also . carpus The wrist (or corresponding part of the forelimb) in terrestrial vertebrates, consisting of a number of small bones (carpals). The number of carpal bones varies with the species. The rabbit, for example, has two rows of carpals, the Ürst (proximal) row containing three bones and the second (distal) row Üve. In humans there are also eight carpals. This large number of bones enables Ûexibility at the wrist joint, between the hand and forelimb. See also  . carrier 1. (in radio) See  . 2. (in physics) See  . 3. (in medicine) An individual who harbours a particular disease-causing microorganism without ill-effects and who can transmit the microorganism to others. Compare . 4. (in genetics) An individual with an *allele for some defective condition that is masked by a normal *dominant allele. Such individuals therefore do not suffer from the condition themselves but they may pass on the defective allele to their offspring. In humans, women may be carriers of such conditions as red–green colour blindness and haemophilia, the alleles for which are carried on the X chromosomes (see  ). 5. (in biochemistry) See  ;  . carrier gas The gas that carries the sample in *gas chromatography. carrier molecule 1. A molecule that plays a role in transporting electrons through the *electron transport chain. Carrier molecules are usually proteins bound to a nonprotein group; they can undergo oxidation and reduction relatively easily, thus allowing electrons to Ûow through the system. There are
four types of carrier: Ûavoproteins (e.g. *FAD), *cytochromes, iron–sulphur proteins (e.g. ferredoxin), and *ubiquinone. 2. A lipid-soluble molecule that can bind to lipidinsoluble molecules and transport them across membranes. Carrier molecules have speciÜc sites that interact with the molecules they transport. Several different molecules may compete for transport by the same carrier. See  .
carrier wave An electromagnetic wave of speciÜed frequency and amplitude that is emitted by a radio transmitter in order to carry information. The information is superimposed onto the carrier by means of *modulation. carrying capacity Symbol K. The maximum population of a particular species that can be supported indeÜnitely by a given habitat or area without damage to the environment. It can be manipulated by human intervention. For example, the carrying capacity for grazing mammals could be increased by boosting the yield of their grassland habitat by the application of fertilizer. See also K . Cartesian coordinates A system used in analytical geometry to locate a point P, with reference to two or three axes (see graphs). In a two-dimensional system the vertical axis is the y-axis and the horizontal axis is the xaxis. The point at which the axes intersect each other is called the origin, O. Values of y φ2 the Ürst metal will acquire a positive surface charge with respect to the other at the area of contact. As a result, a contact potential difference occurs between the two metals or semiconductors. contact process A process for making sulphuric acid from sulphur dioxide (SO2), which is made by burning sulphur or by roasting sulphide ores. A mixture of sulphur dioxide and air is passed over a hot catalyst 2SO2 + O2 → 2SO3 The reaction is exothermic and the conditions are controlled to keep the temperature at an optimum 450°C. Formerly, platinum catalysts were used but vanadium– vanadium oxide catalysts are now mainly employed (although less efÜcient, they are less susceptible to poisoning). The sulphur trioxide is dissolved in sulphuric acid H2SO4 + SO3 → H2S2O7 and the oleum is then diluted. containment 1. The prevention of the escape of radioactive materials from a *nuclear reactor. 2. The process of preventing the plasma in a *thermonuclear reactor from touching the walls of the vessel by means of magnetic Üelds. contig map See  . continent A large landmass that rises above the deep ocean Ûoor. Geologically, the boundary of a continent lies offshore at the edge of the gentle slope of the continental shelf. The British Isles and other offshore islands consequently are parts of the nearby continents. It is generally accepted that there are seven continents – Asia, Africa, North America, South America, Europe, Australia, and Antarctica – occupying about 29% of the earth's surface. continental drift The theory that the earth's continents once formed a single mass and have since moved relative to each other. It was Ürst postulated by A. Snider in 1858 and greatly developed by Alfred Wegener in 1915. He used evidence, such as the Üt of South America into Africa and the distribution of rock types, Ûora, fauna, and geological structures, to suggest that the present
c (a) 200 million years ago
Gondwan
(b) 135 million years ago
Asia Africa India Antarctic
a Australia
(c) 65 million years ago
Continental drift. distribution of the continents results from the breaking up of one or two greater land masses. The original land mass was named Pangaea and it was suggested that this broke up into the northerly Laurasia and the southerly Gondwanaland (see illustration). The theory was not accepted for about 50 years by the majority of geologists but during the early 1960s, the seaÛoor-spreading hypothesis of Harry Hess (1906–69) and the subsequent development of *plate tectonics produced a mechanism to explain the drift of the continents.
• Animation of continental drift during the earth's history
continuous culture A technique used to grow microorganisms or cells continually in a particular phase of growth. For example, if a constant supply of cells is required, a cell culture maintained in the log phase is best; the conditions must therefore be continually monitored and adjusted accordingly so that
continuous function
the cells do not enter the stationary phase (see   ). Growth may also have to be maintained in a particular growth phase if an enzyme or chemical product is produced only during that phase.
continuous function A function f(x) is continuous at x = a if the limit of f(x) as x approaches a is f(a). A function that does not satisfy this condition is said to be a discontinuous function. continuous phase See . continuous spectrum See . continuous variation (quantitative variation) The range of differences that can be observed in many characteristics in a population. Characteristics resulting from polygenic inheritance (see  ) show continuous variation, e.g. the wide range of foot sizes in an adult human population. Compare  . continuous wave A wave that is transmitted continuously rather than in pulses. continuum A system of axes that form a *frame of reference. The three dimensions of space and the dimension of time together can be taken to form a four-dimensional continuum; this was suggested by Minkowski in connection with special *relativity. contour A line drawn on a map or chart that joins points with equal elevation above (or below) a level (usually mean sea level). Contours thus show the relief of the land surface or sea bed (below sea level the line is called a submarine contour). The difference in height between two consecutive contours is the contour interval. contour feathers *Feathers that are arranged in regular rows on a bird's body, giving the body its streamlined shape. Each has a central horny shaft (the rachis) with a Ûattened vane on each side. Each vane is composed of two rows of Ülament-like *barbs, which are connected to each other by means of hooked *barbules to form a smooth surface. There is often a small second vane, the aftershaft, near the base of the feather. contraception See  . contractile root Any of the modiÜed adventitious roots that develop from the base of the stem of a bulb or corm. The new bulb or corm develops at a higher level in the soil
than the old one. The contractile roots shorten and pull it down to a suitable level.
contractile vacuole A membranesurrounded cavity in a cell that periodically expands, Ülling with water, and then suddenly contracts, expelling its contents to the cell's exterior. It is thus an organ of *osmoregulation and excretion. Contractile vacuoles are common in freshwater sponges and typical of freshwater protists, such as Amoeba (which has one spherical vacuole) and Paramecium (in which a number of accessory vacuoles are attached to a main vacuole). contraction (in animal physiology) The shortening of muscle Übres in order to exert a force on a tissue or organ of the body. In striated muscle contraction is brought about by interaction of actin and myosin Ülaments (see ;  ): it provides a force for *locomotion and plays a role in maintaining the balance and posture of the animal. See also  . control 1. The part of an experiment that acts as a standard by which to compare experimental observations. 2. The natural regulation of biological processes. See  . 3. See  ;  . control grid A wire-mesh electrode placed between the cathode and anode in a *thermionic valve or a *cathode-ray tube to control the Ûow of electrons from one to the other. A Ûuctuating potential signal fed to the control grid produces a current signal at the anode with similar but ampliÜed Ûuctuations. It thus forms the basis of the electronic valve ampliÜer. In a cathode-ray tube the grid controls the intensity of the electron beam and hence the brightness of the image on the screen. control mechanism (in biology) Any mechanism that regulates a biological process, such as a metabolic pathway or enzyme-controlled reaction, or that helps to maintain the *internal environment (see ). See also . control rod One of a number of rods of a material, such as boron or cadmium, that absorbs neutrons. Control rods can be moved into or out of the core of a *nuclear reactor to control the rate of the reaction taking place within it. control unit (CU) The part of the central
Cooper, Leon
processor of a *computer that supervises the execution of a computer program.
be divergent even if the individual terms ai tend to zero as i tends to inÜnity.
convection A process by which heat is transferred from one part of a Ûuid to another by movement of the Ûuid itself. In natural convection the movement occurs as a result of gravity; the hot part of the Ûuid expands, becomes less dense, and is displaced by the colder denser part of the Ûuid as this drops below it. This is the process that occurs in most domestic hot-water systems between the boiler and the hot-water cylinder. A natural convection current is set up transferring the hot water from the boiler up to the cylinder (always placed above the boiler) so that the cold water from the cylinder can move down into the boiler to be heated. In some modern systems, where small-bore pipes are used or it is inconvenient to place the cylinder above the boiler, the circulation between boiler and hot-water cylinder relies upon a pump. This is an example of forced convection, where hot Ûuid is transferred from one region to another by a pump or fan.
converging lens or mirror A lens or mirror that can refract or reÛect a parallel beam of light so that it converges at a point (the principal focus). Such a mirror is concave; a converging lens is thicker at its centre than at its edges (i.e. it is biconvex, planoconvex, or convexo-concave). Compare    .
conventional current A 19th-century convention, still in use, that treats any electrical current as a Ûow of positive charge from a region of positive potential to one of negative potential. The real motion, however, in the case of electrons Ûowing through a metal conductor, is in the opposite direction, from negative to positive. In semiconductors *hole conduction is in the direction of the conventional current; electron conduction is in the opposite direction. convergent evolution The development of superÜcially similar structures in unrelated organisms, usually because the organisms live in the same kind of environment. Examples are the wings of insects and birds and the streamlined bodies of whales and Üsh. Compare  . convergent series A series a1 + a2 + … + ai + …, for which a partial sum Sn = a1 + a2 + … + an tends to a Ünite (or zero) limit as n tends to inÜnity. This limit is the sum of the series. For example, the series 1 + 1/2 + 1/4 + 1/8 + … (with the general term ai equal to (1/2)i–1) tends to the limit 2. A series that is not convergent is said to be a divergent series. In such a series the partial sum tends to plus or minus inÜnity or may oscillate. For example, the series 1 + 1/2 + 1/3 + 1/4 + … (with ai equal to 1/i) is divergent. As can be seen from this latter example, a series may
conversion electron See  . converter 1. An electrical machine for converting alternating current into direct current, or less frequently, vice versa. 2. The reaction vessel in the *Bessemer process or some similar steel-making process. 3. A computer device for converting information coded in one form into some other form. converter reactor A *nuclear reactor that converts fertile material (e.g. thorium–232) into *Üssile material (e.g. uranium–233). A converter reactor can also be used to produce electrical power. convex Curving outwards. A convex mirror is one in which the reÛecting surface is formed from the exterior surface of a sphere or paraboloid. A convex lens has at least one face formed from the exterior surface of a sphere. A biconvex lens has both faces convex and is therefore thickest at its centre. The plano-convex lens has one plane face and one convex face. The convexo-concave lens (also called a meniscus) has one convex face and one *concave face. See . convoluted tubule See   ;   ; . coolant A Ûuid used to remove heat from a system by *convection (usually forced), either to control the temperature or to extract energy. In a water-cooled car engine the coolant is water (or water and antifreeze), which is pumped around the engine and cooled in the radiator. In a *nuclear reactor the coolant is used to transfer the heat of the reaction from the core to a heat exchanger or to the steam-raising plant. In gas-cooled reactors the coolant is usually carbon dioxide. Pressurized water or boiling water is used as both coolant and *moderator in several types of reactor. In fast reactors, liquid sodium is used as the coolant. Cooper, Leon See B, J.
cooperative phenomenon
cooperative phenomenon A phenomenon in which the constituents of a system cannot be regarded as acting independently from each other. Cooperative phenomena result from interactions between the constituents. Phenomena that can be described by the *liquid-drop model of nuclei, such as nuclear Üssion, are examples of cooperative phenomena because they involve the *nucleus as a whole rather than individual nucleons. Other examples of cooperative phenomena occur when a substance undergoes a *phase transition, as in the phenomena of ferromagnetism (see ) or *superconductivity. Cooper pairs See . coordinate See C ;  . coordinate bond See  . coordinate geometry See  . coordinate system A system that uniquely speciÜes points in a plane or in three-dimensional space. The simplest coordinate system is the *Cartesian coordinate system. In a plane two coordinates are necessary to specify a point. In threedimensional space three coordinates are required. Many coordinate systems can be used to specify a point; however, sometimes one particular coordinate system is more convenient than others; indeed, certain problems can be solved in one coordinate system but not in others. For example, the Schrödinger equation for the hydrogen atom can be solved using spherical *polar coordinates but not using Cartesian coordinates. coordination (in animal physiology) The processes involved in the reception of sensory information, the integration of that information, and the subsequent response of the organism. Coordination is controlled by regions of the brain that deal with speciÜc functions, such as locomotion and breathing, and is carried out by the nervous system. coordination compound A compound in which coordinate bonds are formed (see  ). The term is used especially for inorganic *complexes.
coordination number The number of groups, molecules, atoms, or ions surround-
ing a given atom or ion in a complex or crystal. For instance, in a square-planar complex the central ion has a coordination number of four. In a close-packed crystal (see  ) the coordination number is twelve.
Copepoda A class of crustaceans occurring in marine and freshwater habitats. Copepods are usually 0.5–2 mm long and lack both a carapace and compound eyes. Copepods are important members of plankton: some are free-living, feeding on microscopic organisms; others are parasitic. A familiar freshwater genus is Cyclops, so named because the members have a single median eye. Copernican astronomy The system of astronomy that was proposed by the Polish astronomer Nicolaus Copernicus in his book De revolutionibus orbium coelestium, which was published in the month of his death (in 1543) and Ürst seen by him on his deathbed. It used some elements of *Ptolemaic astronomy, but rejected the notion, then current, that the earth was a stationary body at the centre of the universe. Instead, Copernicus proposed the apparently unlikely concept that the sun was at the centre of the universe and that the earth travelled in a circular orbit about it. In addition Copernicus revived the idea that the movement of the sun and the Üxed stars was due to the daily axial rotation of the earth. Galileo's attempts, some 70 years later, to convince the Catholic church that in spite of scriptural authority to the contrary, the Copernican system was correct, met with such stern resistance that De revolutionibus was placed on the church's list of forbidden books, where it remained until 1835. Copernicus, Nicolaus (Mikolaj Kopernik; 1473–1543) Polish astronomer, who studied mathematics and optics. By 1514 he had formulated his proposal that the planets, including the earth, orbit the sun in circular paths, although it was not formally published until the year he died. This refutation of an earth-centred universe raised hostile opposition from the church as well as from other astronomers. copolymer See . copper Symbol Cu. A red-brown *transition element; a.n. 29; r.a.m. 63.546; r.d. 8.92; m.p. 1083.4°C; b.p. 2567°C. Copper has been extracted for thousands of years; it was known to the Romans as cuprum, a name
linked with the island of Cyprus. The metal is malleable and ductile and an excellent conductor of heat and electricity. Coppercontaining minerals include cuprite (Cu2O) as well as azurite (2CuCO3.Cu(OH)2), chalcopyrite (CuFeS2), and malachite (CuCO3. Cu(OH)2). Native copper appears in isolated pockets in some parts of the world. The large mines in the USA, Chile, Canada, Zambia, Democratic Republic of Congo, and Peru extract ores containing sulphides, oxides, and carbonates. They are usually worked by smelting, leaching, and electrolysis. Copper metal is used to make electric cables and wires. Its alloys, brass (copper– zinc) and bronze (copper–tin), are used extensively. Water does not attack copper but in moist atmospheres it slowly forms a characteristic green surface layer (patina). The metal will not react with dilute sulphuric or hydrochloric acids, but with nitric acid oxides of nitrogen are formed. Copper compounds contain the element in the +1 and +2 oxidation states. Copper(I) compounds are mostly white (the oxide is red). Copper(II) salts are blue in solution. The metal also forms a large number of coordination complexes.
copperas See (II) . copper(I) chloride A white solid compound, CuCl; cubic; r.d. 4.14; m.p. 430°C; b.p. 1490°C. It is obtained by boiling a solution containing copper(II) chloride, excess copper turnings, and hydrochloric acid. Copper(I) is present as the [CuCl2]– complex ion. On pouring the solution into air-free distilled water copper(I) chloride precipitates. It must be kept free of air and moisture as it oxidizes to copper(II) chloride under those conditions. Copper(I) chloride is essentially covalent and its structure is similar to that of diamond; i.e. each copper atom is surrounded tetrahedrally by four chlorine atoms and vice versa. In the vapour phase, dimeric and trimeric species are present. Copper(I) chloride is used in conjunction with ammonium chloride as a catalyst in the dimerization of ethyne to but-1-ene-3-yne (vinyl acetylene), which is used in the production of synthetic rubber. In the laboratory a mixture of copper(I) chloride and hydrochloric acid is used for converting benzene diazonium chloride to chlorobenzene – the Sandmeyer reaction.
copper(I) oxide copper(II) chloride A brown-yellow powder, CuCl2; r.d. 3.386; m.p. 620°C. It exists as a blue-green dihydrate (rhombic; r.d. 2.54; loses H2O at 100°C). The anhydrous solid is obtained by passing chlorine over heated copper. It is predominantly covalent and adopts a layer structure in which each copper atom is surrounded by four chlorine atoms at a distance of 0.23 nm and two more at a distance of 0.295 nm. A concentrated aqueous solution is dark brown in colour due to the presence of complex ions such as [CuCl4]2–. On dilution the colour changes to green and then blue because of successive replacement of chloride ions by water molecules, the Ünal colour being that of the [Cu(H2O)6]2+ ion. The dihydrate can be obtained by crystallizing the solution. copper glance A mineral form of copper(I) sulphide, Cu2S. copper(II) nitrate A blue deliquescent solid, Cu(NO3)2.3H2O; r.d. 2.32; m.p. 114.5°C. It may be obtained by reacting either copper(II) oxide or copper(II) carbonate with dilute nitric acid and crystallizing the resulting solution. Other hydrates containing 6 or 9 molecules of water are known. On heating it readily decomposes to give copper(II) oxide, nitrogen dioxide, and oxygen. The anhydrous form can be obtained by reacting copper with a solution of nitrogen dioxide in ethyl ethanoate. It sublimes on heating suggesting that it is appreciably covalent. copper(I) oxide A red insoluble solid, Cu2O; r.d. 6.0; m.p. 1235°C. It is obtained by reduction of an alkaline solution of copper(II) sulphate. Since the addition of alkalis to a solution of copper(II) salt results in the precipitation of copper(II) hydroxide the copper(II) ions are complexed with tartrate ions; under such conditions the concentration of copper(II) ions is so low that the solubility product of copper(II) hydroxide is not exceeded. When copper(I) oxide reacts with dilute sulphuric acid a solution of copper(II) sulphate and a deposit of copper results, i.e. disproportionation occurs. Cu2O + 2H+ → Cu2+ + Cu + H2O When dissolved in concentrated hydrochloric acid the [CuCl2]– complex ion is formed. Copper(I) oxide is used in the manufacture of rectiÜers and the production of red glass.
copper(II) oxide A black insoluble solid, CuO; monoclinic; r.d. 6.3; m.p. 1326°C. It is obtained by heating either copper(II) carbonate or copper(II) nitrate. It decomposes on heating above 800°C to copper(I) oxide and oxygen. Copper(II) oxide reacts readily with mineral acids on warming, with the formation of copper(II) salts; it is also readily reduced to copper on heating in a stream of hydrogen. Copper(II) oxide is soluble in dilute acids forming blue solutions of cupric salts. copper pyrites See . copper(II) sulphate A blue crystalline solid, CuSO4.5H2O; triclinic; r.d. 2.284. The pentahydrate loses 4H2O at 110°C and the Üfth H2O at 150°C to form the white anhydrous compound (rhombic; r.d. 3.6; decomposes above 200°C). The pentahydrate is prepared either by reacting copper(II) oxide or copper(II) carbonate with dilute sulphuric acid; the solution is heated to saturation and the blue pentahydrate crystallizes out on cooling (a few drops of dilute sulphuric acid are generally added to prevent hydrolysis). It is obtained on an industrial scale by forcing air through a hot mixture of copper and dilute sulphuric acid. In the pentahydrate each copper(II) ion is surrounded by four water molecules at the corner of a square, the Üfth and sixth octahedral positions are occupied by oxygen atoms from the sulphate anions, and the Üfth water molecule is held in place by hydrogen bonding. Copper(II) sulphate has many industrial uses, including the preparation of the Bordeaux mixture (a fungicide) and the preparation of other copper compounds. It is also used in electroplating and textile dying and as a timber preservative. The anhydrous form is used in the detection of traces of moisture. Copper(II) sulphate pentahydrate is also known as blue vitriol. coprecipitation The removal of a substance from solution by its association with a precipitate of some other substance. For example, if A and B are present in solution and a reagent is added such that A forms an insoluble precipitate, then B may be carried down with the precipitate of A, even though it is soluble under the conditions. This can occur by occlusion or absorption. copulation See  . coral Any of a group of sedentary colonial marine invertebrates belonging to the class
Anthozoa of the phylum *Cnidaria. A coral colony consists of individual *polyps within a protective skeleton that they secrete: this skeleton may be soft and jelly-like, horny, or stony. The horny skeleton secreted by corals of the genus Corallium, especially C. rubrum, constitutes the red, or precious, coral used as a gemstone. The skeleton of stony, or true, corals consists of almost pure calcium carbonate and forms the coral reefs common in tropical seas.
cordite An explosive mixture of cellulose nitrate and nitroglycerin, with added plasticizers and stabilizers, used as a propellant for guns. core 1. A rod or frame of magnetic material that increases the inductance of a coil through which it passes. Cores are used in transformers, electromagnets, and the rotors and stators of electrical machines. It may consist of laminated metal, ferrite, or compressed ferromagnetic particles in a matrix of an insulating binder (dust core). 2. The inner part of a *nuclear reactor in which the nuclear reaction takes place. 3. The devices that make up the memory in certain types of computer. 4. The central region of a star or planet. Coriolis force A Üctitious force sometimes used to simplify calculations involving rotating systems, such as the movement of air, water, and projectiles over the surface of the rotating earth. The concept was Ürst used in 1835 by Gaspard de Coriolis (1792–1843), a French physicist. The daily rotation of the earth means that in 24 hours a point on its equator moves a distance of some 40 000 kilometres, giving it a tangential velocity of about 1670 kilometres per hour. A point at the latitude of, say, Rome, travels a shorter distance in the same time and therefore has a lower tangential velocity – about 1340 km/hr. Air over the equator has the full tangential velocity of 1670 km/hr and as it travels north, say, it will retain this velocity; to an observer outside the earth this would be clear. However, to an observer in Rome it appears to be moving eastwards, because the earth at that point is moving eastwards more slowly than the air. The Coriolis force (which is quite Üctitious) is the force that a naive observer thinks is needed to push the air eastwards. cork (phellem) A protective waterproof plant tissue produced by the *cork cambium. It develops in plants undergoing *secondary
coronal mass ejection
growth and replaces the epidermis. Its cells, whose walls are impregnated with *suberin, are arranged in radial rows and Üt closely together except where the cork is interrupted by *lenticels. Some cork cells become airÜlled while others contain deposits of lignin, tannins, and fatty acids, which give the cork a particular colour. The cork oak (Quercus suber) produces cork that can be used commercially.
cork cambium (phellogen) A type of *cambium arising within the outer layers of the stems of woody plants, usually as a complete ring surrounding the inner tissues. The cells of the cork cambium divide to produce an outer corky tissue (*cork or phellem) and an inner secondary cortex (phelloderm). Cork, cork cambium, and phelloderm together make up the periderm, an impermeable outer layer that protects the inner stem tissues if the outer tissues split as the stem girth increases with age. It thus takes over the functions of the epidermis. corm An underground organ formed by certain plants, e.g. crocus and gladiolus, that enables them to survive from one growing season to the next (see illustration). It consists of a short swollen food-storing stem surrounded by protective scale leaves. One or more buds in the axils of scale leaves produce new foliage leaves and Ûowers in the subsequent season, using up the food stored in the stem. Compare . cornea A transparent layer of tissue, continuous with the *sclerotic, that forms the front part of the vertebrate eye, over the iris and lens. The cornea refracts light entering the eye onto the lens, thus assisting in the focusing of images onto the *retina. See also . corniÜcation See . CORN rule See  . corolla The *petals of a Ûower, collectively, forming the inner whorl of the *perianth. It encircles the stamens and carpels. The form of the corolla is very variable. The petals may either be free (polypetalous) or united to form a tube (gamopetalous or sympetalous). corona 1. The outer part of the sun's atmosphere. Its two main components are the K-corona (or inner corona), with a temperature of about 2 × 106 K at a height of some 75 000 km, and the F-corona (or outer
corona), which is considerably cooler and extends for several million kilometres into space. 2. A glowing region of the air surrounding a conductor when the potential gradient near it exceeds a critical value. It is caused by ionization of the air and may be accompanied by hissing sounds. Corona discharge (or point discharge) occurs at sharp points where the surface charge density is high by the attraction, charging, and consequent repulsion of air molecules. bud
scale leaves swollen stem
flower and foliage leaves
corm shrivels as food used for growing plant
withered foliage leaves
swelling stem forming new corm
Corm development. coronal mass ejection (CME; coronal transient) A huge bubble of energetic gas interlaced with magnetic Üeld lines that is hurled out of the sun's corona into interplanetary space, reaching speeds of between
200 and 1000 km s–1 as it expands over several hours. CMEs are often connected with *solar Ûares or solar *prominences but may occur alone. The frequency of these events reaches its peak at *sunspot maximum (see  ). On earth CMEs can produce magnetic storms and spectacular aurorae, disrupt radio communications and electricity supplies, and damage power transmission lines and satellites.
coronary vessels Two pairs of blood vessels (the coronary arteries and coronary veins) that supply the muscles of the heart itself. The coronary arteries arise from the aorta and divide into branches that encircle the heart. A blood clot in a coronary artery (coronary thrombosis) is one of the causes of a 'heart attack'. corpus callosum The sweeping band (commissure) of *white matter that provides a connection between the two halves of the cerebrum in the brain. It enables the transfer of information and learning from one cerebral hemisphere to the other. corpuscular theory See . corpus luteum (yellow body) The yellowish mass of tissue that forms in the cavity of a *GraaÜan follicle in the ovary of a mammal after the release of the egg cell. It secretes the hormone *progesterone. Some species of sharks, reptiles, and birds have similar structures in their ovaries but the function of these is less well understood. corrosion Chemical or electrochemical attack on the surface of a metal. See also  ; . cortex 1. (in botany) The tissue between the epidermis and the vascular system in plant stems and roots. It is composed of *parenchyma cells and shows little or no structural differentiation. Cortex is produced by activity of the *apical meristem. See also . 2. (in zoology) The outermost layer of tissue of various organs, including the adrenal glands (adrenal cortex), kidneys (renal cortex), and cerebral hemispheres (see  ). corticosteroid Any of several hormones produced by the cortex of the *adrenal glands. Glucocorticoids regulate the use of carbohydrates, proteins, and fats in the body and include *cortisol and *cortisone. Mineralocorticoids regulate salt and water balance (see ).
corticotrophin See ACTH. cortisol (hydrocortisone) A hormone (see ), produced by the adrenal glands, that promotes the synthesis and storage of glucose and is therefore important in the normal response to stress, suppresses or prevents inÛammation, and regulates deposition of fat in the body. It is used as treatment for various allergies and for rheumatic fever, certain skin conditions, and adrenal failure (Addison's disease). cortisone A biologically inactive *corticosteroid produced in the adrenal glands from the active hormone *cortisol, which is structurally very similar to it. Cortisone is reconverted to cortisol in the liver and other organs. Cortisone may be administered therapeutically as an inactive precursor (prodrug) of cortisol. corundum A mineral form of aluminium oxide, Al2O3. It crystallizes in the trigonal system and occurs as well-developed hexagonal crystals. It is colourless and transparent when pure but the presence of other elements gives rise to a variety of colours. *Ruby is a red variety containing chromium; *sapphire is a blue variety containing iron and titanium. Corundum occurs as a rockforming mineral in both metamorphic and igneous rocks. It is chemically resistant to weathering processes and so also occurs in alluvial (placer) deposits. The second hardest mineral after diamond (it has a hardness of 9 on the Mohs' scale), it is used as an abrasive. corymb A type of Ûowering shoot (see  ) in which the lower Ûower stalks are longer than the higher ones, resulting in a Ûat-topped cluster of Ûowers. Examples are candytuft and wallÛower. cos See  . cosecant See  . cosech See  . cosh See  . cosine See  . cosine rule In any triangle, with sides of length a, b, and c, c2 = a2 + b2 – 2abcosθ, where θ is the angle between sides a and b. cosmic censorship A hypothesis concerning singularities and *black holes in the general theory of *relativity. It was suggested
in 1969 by the British physicist Roger Penrose (1931– ). The cosmic censorship conjecture asserts that all singularities in general relativity are hidden behind an event horizon. The conjecture has never been proved mathematically, although there is some evidence for it in many situations. Even if cosmic censorship is not correct, singularities would not be seen experimentally if the singularities are removed by *quantum gravity. It may be that in classical general relativity the cosmic censorship hypothesis is true for 'reasonable' physical situations but that it is possible to construct counter-examples to it for various special situations.
cosmological constant A term that can be added to Einstein's Üeld equation for general *relativity theory. The cosmological constant is independent of space and time. It was put forward by Einstein in 1917 to allow for the possibility of a static universe. Although the discovery of the *expansion of the universe removed the original motivation for the cosmological constant, the discovery that the expansion of the universe is accelerating suggests that the constant has a non-zero value, albeit by a factor of 10120 smaller than expected theoretically. Explaining this small non-zero value is one of the main challenges for theoretical physics at the present time.
cosmic neutrino background (CNB) The neutrino analogue of the cosmic *microwave background radiation, i.e. a background radiation consisting of neutrinos. The CNB has not yet been detected; it is thought that it would have a temperature of about 1.9 kelvin.
cosmological principle The claim that on extremely large scales, i.e. much greater scales than those associated with *largescale structure, the universe is homogeneous and isotropic. There is some evidence that the cosmological principle is valid, notably from the cosmic microwave background radiation, but it cannot be said to have been demonstrated conclusively.
cosmic radiation High-energy particles that fall on the earth from space. Primary cosmic rays consist of nuclei of the most abundant elements, with *protons (hydrogen nuclei) forming by far the highest proportion; electrons, positrons, neutrinos, and gamma-ray photons are also present. The particle energies range from 10–11 J to 10 J (108 to 1020 eV) and as they enter the earth's atmosphere they collide with oxygen and nitrogen nuclei producing secondary cosmic rays. The secondary rays consist of elementary particles and gamma-ray photons. A single high-energy primary particle can produce a large shower of secondary particles. The sources of the primary radiation are not all known, although the sun is believed to be the principal source of particles with energies up to about 1010 eV. It is believed that all particles with energies of less than 1018 eV originate within the Galaxy. cosmic string See . cosmid A hybrid *vector, used in *gene cloning, that includes the cos gene (from the lambda bacteriophage). It also contains drug resistance *marker genes and other plasmid genes. Cosmids can incorporate larger DNA fragments than either phage or plasmid vectors alone and are especially suitable for cloning large mammalian genes or multigene fragments. cosmoid scale See .
cosmology The study of the nature, origin, and evolution of the universe. Various theories concerning the origin and evolution of the universe exist. See Chronology. See also - ; - ;  .
• The home page of the Caltech Observational Cosmology Group • A cosmology website run by the University of Cambridge
cotangent See  . coth See  . Cottrell precipitator An electrostatic precipitator used to remove dust particles from industrial waste gases, by attracting them to charged grids or wires. cotyledon (seed leaf) A part of the embryo in a seed plant. The number of cotyledons is an important feature in classifying plants. Among the Ûowering plants, the class known as *Monocotyledoneae have a single cotyledon and *Dicotyledoneae have two. Conifers have either two cotyledons, as in Taxus (yews), or Üve to ten, as in Pinus (pines). In seeds without an *endosperm, e.g. garden pea and broad bean, the cotyledons store food, which is used in germination. In seeds showing *epigeal germination,
coudé system
c.150 AD 1543 1576 1584 1633 1854 1917 1922 1927 1929 1948
Greek astronomer Aristarchus of Samos (c. 320–230 BC) proposes a suncentred universe. Greek-Egyptian astronomer Ptolemy (2nd century AD) proposes an earthcentred universe. Copernicus publishes his sun-centred theory of the universe (solar system). English mathematician Thomas Digges (c. 1546–95) proposes that the universe is infinite (because stars are at varying distances). Italian philosopher Giordano Bruno (1548–1600) states that the universe is infinite. Galileo champions Copernicus's sun-centred universe, but is forced by the Roman Catholic Inquisition to recant. Helmholtz predicts the heat death of the universe, based on thermodynamics. Einstein proposes a static universe theory. Russian astronomer Alexander Friedmann proposes the expanding universe theory. George Lemaître proposes the big-bang theory of the universe. Edwin Hubble demonstrates the expansion of the universe. US physicists George Gamow (1904–68), Ralph Alpher (1921–2007), and Hans Bethe (1906–2005) develop the big-bang theory, and the α-β-γ theory of the origin of the elements; Alpher also predicts that the big bang would have produced a microwave background. British astronomers Herman Bondi (1919–2005), Thomas Gold (1920– 2004), and Fred Hoyle (1915–2001) propose a steady-state theory of the universe in which matter is continuously being formed. US astrophysicists Arno Penzias (1933– ) and Robert Wilson (1936– ) discover the microwave background radiation. US physicist Allan Guth (1947– ) proposes the inflationary theory of the universe. US COBE (Cosmic Background Explorer) astronomical satellite detects ripples in residual cosmic radiation (cited as evidence of the big bang). US WMAP (Wilkinson Microwave Anisotropy Probe) launched. The ekpyrotic universe model is proposed as an alternative to the inflationary model. WMAP results give a detailed map of the cosmic microwave background and support the inflationary theory.
e.g. runner bean, they emerge above the soil surface and become the Ürst photosynthetic leaves.
coudé system See . coulomb Symbol C. The *SI unit of electric charge. It is equal to the charge trans-
ferred by a current of one ampere in one second. The unit is named after Charles de Coulomb.
Coulomb, Charles Augustin de (1736– 1806) French physicist, who served as an army engineer in Martinique before return-
ing to France. He is best known for his 1785 proposal of *inverse-square laws to describe the interaction between electrical charges and between magnets (see C' ), which he proved experimentally using a *torsion balance.
Coulomb explosion The sudden disruption of a molecule from which the electrons have been stripped to leave only the nuclei, which repel each other because of their electric charge. The technique of coulomb explosion imaging uses this effect to investigate the shape of molecules. A beam of highenergy neutral molecules is produced by Ürst adding electrons, accelerating the ions in an electric Üeld, and then removing the electrons. The beam collides with a thin metal foil having a thickness of about 30 atoms. As the molecules pass through this foil their electrons are scattered and only the nuclei of the molecules emerge. The process occurs within a very short period of time, shorter than the time required for a complete molecular vibration, and consequently the nuclei retain the molecular shape until they are suddenly repulsed by the like charges. The nuclei then impinge on a detector that records their velocity and direction, thus enabling the spatial arrangement of the original molecule to be derived. Coulomb Üeld See C' . Coulomb force See C' . Coulomb's law The force (sometimes called the Coulomb force) between two charged particles, regarded as point charges Q1 and Q2 a distance d apart, is proportional to the product of the charges and inversely proportional to the square of the distance between them. The law is now usually stated in the form F = Q1Q2/4πεd2, where ε is the absolute *permittivity of the intervening medium. ε = εrε0, where εr is the relative permittivity (the dielectric constant) and ε0 is the electric constant. The electric Üeld surrounding a point charge is called the Coulomb field and the scattering of charged particles by the Coulomb Üeld surrounding an atomic nucleus is called Coulomb scattering. The law was Ürst published by Charles de Coulomb in 1785. counter Any device for detecting and counting objects or events, often incident charged particles or photons. The latter devices usually work by allowing the particle to cause ionization, which creates a current or
coupling voltage pulse. The pulses are then counted electronically. See C ;  ; G ;  ;  ;  . These names are often applied merely to the actual detectors; the ancillary counting mechanism is then called a *scaler.
countercurrent Ûow Flow of two Ûuids in opposite directions with transfer of heat or matter between them. Compare  . countercurrent heat exchange A *counterÛow mechanism that enables Ûuids at different temperatures Ûowing in channels in opposite directions to exchange their heat content without mixing. An example of countercurrent heat exchange occurs in the feet of penguins, in which heat from blood in the arteries supplying the feet is transferred to blood returning to the body's core in veins that lie close to these arteries. This helps to maintain the core temperature in freezing conditions. counterÛow The Ûow of two Ûuids in apposed vessels in opposite directions. In biological systems such an arrangement enables the efÜcient transfer of heat, ions, molecules, etc., from Ûuids that are rich in these resources to Ûuids that are deÜcient in them. counter ion An ion of opposite charge to a given ion. For example, in a crystal of sodium chloride, the chloride ions can be regarded as counter ions to the sodium ions. In certain colloids, the charge on the surface of colloidal particles is neutralized by oppositely charged counter ions in the surrounding solution. country rock (host rock) Older rock that surrounds veins of minerals or an igneous magma *intrusion, such as a *batholith. The extreme heat of the intrusion may cause changes (contact metamorphism) in the composition of the adjacent country rock. couple Two equal and opposite parallel forces applied to the same body that do not act in the same line. The forces create a torque, the *moment of which is equal to the product of the force and the perpendicular distance between them. coupling 1. (in physics) An interaction between two different parts of a system or between two or more systems. Examples of coupling in the *spectra of atoms and nuclei
coupling constant
are *Russell–Saunders coupling, *j-j coupling, and spin–orbit coupling. In the spectra of molecules there are Üve idealized ways (called the Hund coupling cases) in which the different types of angular momentum in a molecule (the electron orbital angular momentum L, the electron spin angular momentum S, and the angular momentum of nuclear rotation N) couple to form a resultant angular momentum J. (In practice, the coupling for many molecules is intermediate between Hund's cases due to interactions, which are ignored in the idealized cases.) In *solid-state physics an example of coupling is electron–phonon coupling, the analysis of which gives the theories of electrical *conductivity and *superconductivity. See also  . 2. (in chemistry) A type of chemical reaction in which two molecules join together; for example, the formation of an *azo compound by coupling of a diazonium ion with a benzene ring.
coupling constant A physical constant that is a measure of the strength of interaction between two parts of a system or two or more systems. In the case of a *Üeld theory, the coupling constant is a measure of the magnitude of the force exerted on a particle by a Üeld. In the case of a *quantum Üeld theory, a coupling constant is not constant but is a function of energy, the dependence on energy being described by the *renormalization group. See also ;  . courtship Behaviour in animals that plays a part in the initial attraction of a mate or as a prelude to copulation. Courtship often takes the form of *displays that have evolved through *ritualization; some are derived from other contexts (e.g. food begging in some birds). Chemical stimuli (see ) are also important in many mammals and insects. As well as ensuring that the prospective mate is of the same species, the male's courtship performance allows females to choose between different males. The later stages of courtship may involve both partners in an alternating series of displays that inhibit *aggression and fear responses and ensure synchrony of sexual arousal. COV See  . covalent bond See  . covalent crystal A crystal in which the atoms are held together by covalent bonds.
Covalent crystals are sometimes called macromolecular or giant-molecular crystals. They are hard high-melting substances. Examples are diamond and boron nitride.
covalent radius An effective radius assigned to an atom in a covalent compound. In the case of a simple diatomic molecule, the covalent radius is half the distance between the nuclei. Thus, in Cl2 the internuclear distance is 0.198 nm so the covalent radius is taken to be 0.099 nm. Covalent radii can also be calculated for multiple bonds; for instance, in the case of carbon the values are 0.077 nm for single bonds, 0.0665 nm for double bonds, and 0.0605 nm for triple bonds. The values of different covalent radii can sometimes be added to give internuclear distances. For example, the length of the bond in interhalogens (e.g. ClBr) is nearly equal to the sum of the covalent radii of the halogens involved. This, however, is not always true because of other effects (e.g. ionic contributions to the bonding). covariance In *statistics, a measure of the association between a pair of random variables. It equals the expected value of the product of their deviations (from the mean value). For two sets of observations _ (x1,y1),…,(x n,yn), where x is the mean of _ xi and y is the mean of yi, it is given by n
(x – x ) (y i – y ) (1/n) i Σ =1 i See also .
Cowan, Clyde See P, W E. Cowper's glands (bulbourethral glands) A pair of pea-sized glands that lie beneath the prostate gland. Cowper's glands secrete an alkaline Ûuid that forms part of the *semen. This Ûuid neutralizes the acidic environment of the urethra, thereby protecting the sperm. The glands are named after William Cowper (1666–1709). See also  . coxa The Ürst segment, attached to the thorax, of an insect's leg. See also ; . CP invariance The symmetry generated by the combined operation of changing *charge conjugation (C) and *parity (P). CP violation occurs in weak interactions in kaon decay and in *B-mesons. See also CPT ;  . CPT theorem The theorem that the com-
bined operation of changing *charge conjugation C, *parity P, and *time reversal T, denoted CPT, is a fundamental *symmetry of relativistic *quantum Üeld theory. No violation of the CPT theorem is known experimentally. When C, P, and T (or any two of them) are violated, the principles of relativistic quantum Üeld theory are not affected; however, violation of CPT invariance would drastically alter the fundamentals of relativistic quantum Üeld theory. It is not known whether *superstrings obey versions of the CPT theorem.
CPU (central processing unit) The main operating part of a *computer; it includes the control unit (CU) and the arithmetic/logic unit (see ALU). Its function is to fetch instructions from memory, decode them, and execute the program. It also provides timing signals. An *integrated circuit that has a complete CPU on a single silicon chip is called a microprocessor. crack cocaine See . cracking The process of breaking down chemical compounds by heat. The term is applied particularly to the cracking of hydrocarbons in the kerosine fraction obtained from *petroleum reÜning to give smaller hydrocarbon molecules and alkenes. It is an important process, both as a source of branched-chain hydrocarbons suitable for gasoline (for motor fuel) and as a source of ethene and other alkenes. Catalytic cracking is a similar process in which a catalyst is used to lower the temperature required and to modify the products obtained. cranial nerves Ten to twelve pairs of nerves in vertebrates that emerge directly from the brain. They supply the sense organs and muscles of the head, neck, and viscera. Examples of cranial nerves include the *optic nerve (II) and the *vagus nerve (X). With the *spinal nerves, the cranial nerves form an important part of the *peripheral nervous system. cranial reÛex See . Craniata A clade or subphylum of chordate animals characterized by a cartilaginous or bony skull protecting the brain and major sense organs. Craniates comprise two major clades: the Myxini (hagÜshes) and the Vertebrata (lampreys and jawed vertebrates). HagÜshes have long eel-like bodies lacking Üns, vertebrae, and a lower jaw; the notochord is retained into adulthood. HagÜshes
scavenge for food on the seabed, often feeding on dead Üsh and whales. In vertebrates the notochord is present only in the embryo or larva and becomes replaced by the *vertebral column (backbone) before birth or metamorphosis. This has permitted the vertebrates a greater degree of movement and subsequent improvement in the sense organs and enlargement of the brain. Lampreys (clade Hyperoartia; order Petromyzontiformes) are slender wormlike animals that lack jaws but possess rudimentary vertebrae in adult life. They chieÛy feed as parasites on Üsh using their toothed circular mouth as a sucker to bore into their host and suck its blood. The jawed vertebrates comprise the *Gnathostomata.
cranium (brain case) The part of the vertebrate *skull that encloses and protects the brain. It is formed by the fusion of several Ûattened bones, which have immovable joints (sutures) between them. C-reactive protein (CRP) A protein secreted into blood plasma by the liver in response to inÛammation and infection. It binds to the cell walls of certain bacteria and fungi, thereby increasing susceptibility of the target cell to ingestion by phagocytes; it can also activate the *complement cascade, hence triggering destruction of pathogens by this means. cream of tartar See  . creatine A compound, synthesized from the amino acids arginine, glycine, and methionine, that occurs in muscle. In the form of creatine phosphate (or phosphocreatine), it is an important reserve of energy for muscle contraction, which is released when creatine phosphate loses its phosphate and is converted to creatinine, which is excreted in the urine (at a rate of 1.2–1.5 g/day in humans). See also . creatinine See . creationist A proponent of the theory of *special creation. creep The continuous deformation of a solid material, usually a metal, under a constant stress that is well below its yield point. It usually only occurs at high temperatures and the creep characteristics of any material destined to be used under conditions of high stress at high temperatures must be investigated.
cremocarp
cremocarp A dry fruit that is a type of *schizocarp formed from two one-seeded carpels. The carpels remain separate and form indehiscent mericarps that are attached to a central supporting strand (carpophore) for some time before dispersal. It is characteristic of the Umbelliferae (Apiaceae; carrot family). crenation The shrinkage of cells that occurs when the surrounding solution is *hypertonic to the cellular cytoplasm. Water leaves the cells by *osmosis, which causes the plasma membrane to wrinkle and the cellular contents to condense. creosote 1. (wood creosote) An almost colourless liquid mixture of phenols obtained by distilling tar obtained by the destructive distillation of wood. It is used medically as an antiseptic and expectorant. 2. (coal-tar creosote) A dark liquid mixture of phenols and cresols obtained by distilling coal tar. It is used for preserving timber. cresols See . Cretaceous The Ünal geological period of the Mesozoic era. It extended from about 144 million years ago, following the Jurassic, to about 65 million years ago, when it was succeeded by the Palaeogene period. The name of the period is derived from creta (Latin: chalk) and the Cretaceous was characterized by the deposition of large amounts of *chalk in western Europe. The Cretaceous was the time of greatest Ûooding in the Mesozoic. Angiosperm plants made their Ürst appearance on land and in the early Cretaceous Mesozoic reptiles reached their peak. At the end of the period there was a *mass extinction of the dinosaurs, Ûying reptiles, and ammonites, the cause of which may be related to environmental changes resulting from collisions of the earth with large meteorites (see A ;  ). Creutzfeldt–Jakob disease (CJD) A disease of humans characterized by dementia and destruction of brain tissue, Ürst described by the German psychiatrists H. G. Creutzfeldt (1885–1964) and A. M. Jakob (1884–1931). It is now known to be caused by an abnormal *prion protein and is transmissible, although there is also an inherited familial form. This rare disease typically affects middle-aged and elderly people and leads to rapid mental deterioration and death. The abnormal prion interferes with the structure
of normal prion protein in brain tissue, resulting in accumulations of the protein and consequent tissue damage. In most cases the source of infection is unknown. However, it is well established that infection can result, for example, via injections of growth hormone derived from infected human cadavers. During the 1990s a novel form of the disease emerged, called variant CJD, which typically affects young healthy individuals. This is thought to be caused by consumption of beef products derived from cattle infected with *bovine spongiform encephalopathy.
CRG process (catalytic rich gas process) An industrial process for producing fuel gas from naphtha and other hydrocarbon sources. It involves a nickel-based catalyst, pressures of up to 70 bar, and temperatures between 250°C and 650°C depending on the feedstock. The reactions are: CnH2n+2 + nH2O → nCO + (2n +1)H2 CO + 3H2 → CH4 + H2O CO + H2O → CO2 + H2 The result is a mixture of methane, carbon monoxide, carbon dioxide, and trace amounts of ethane and other hydrocarbons. With partial carbon dioxide removal it is possible to produce town gas with medium caloriÜc value containing about 30% CH4, 30% H2, and 2% CO. The process can be used to produce SNG. In this case there are multiple methanation stages and complete removal of CO2 to give a product containing about 98.5% CH4, 0.9% H2, and 0.1% CO.
Crick, Francis Harry Compton (1916–2004) British molecular biologist, who in 1951 teamed up with James *Watson at Cambridge University to try to Ünd the structure of *DNA. This they achieved in 1953, using the X-ray diffraction data of Rosalind Franklin (1920–58) and Maurice Wilkins (1916–2004). Crick went on to investigate *codons and the role of transfer *RNA. Crick, Watson, and Wilkins shared a Nobel Prize in 1962. crista 1. See  . 2. See . cristobalite A mineral form of *silicon(IV) oxide, SiO2. critical angle See   . critical damping See . critical density In astronomy, the mean
density of the universe below which value it is an expanding and continuously open system. The luminous material in the universe (galaxies, etc.) is estimated to account for about 10% of this Ügure. The remainder is thought to consist mainly of dark matter (see  ).
critical group A large group of related organisms that, although variations exist between them, cannot be divided into smaller groups of equivalent taxonomic rank to the parent group. Critical groups are found among plants that reproduce by *apomixis; for example, the 400 or so species of Rubus (brambles, etc.) are regarded as a critical group. critical mass The minimum mass of Üssile material that will sustain a nuclear *chain reaction. For example, when a nucleus of uranium–235 disintegrates two or three neutrons are released in the process, each of which is capable of causing another nucleus to disintegrate, so creating a chain reaction. However, in a mass of U–235 less than the critical mass, too many neutrons escape from the surface of the material for the chain reaction to proceed. In the atom bomb, therefore, two or more subcritical masses have to be brought together to make a mass in excess of the critical mass before the bomb will explode. critical pressure The pressure of a Ûuid in its *critical state; i.e. when it is at its critical temperature and critical volume. critical reaction A nuclear *chain reaction in which, on average, one transformation causes exactly one other transformation so that the chain reaction is self-sustaining. If the average number of transformations caused by one transformation falls below one, the reaction is subcritical and the chain reaction ceases; if it exceeds one the reaction is supercritical and proceeds explosively. critical state The state of a Ûuid in which the liquid and gas phases both have the same density. The Ûuid is then at its *critical temperature, *critical pressure, and *critical volume. critical temperature 1. The temperature above which a gas cannot be liqueÜed by an increase of pressure. See also  . 2. See  . critical volume The volume of a Üxed mass of a Ûuid in its *critical state; i.e. when
it is at its critical temperature and critical pressure. The critical specific volume is its volume per unit mass in this state: in the past this has often been called the critical volume.
CRO See - . Cromagnon man The earliest form of modern humans (Homo sapiens), which is believed to have appeared in Europe about 35 000 years ago and possibly at least 70 000 years ago in Africa and Asia. Fossils indicate that these hominids were taller and more delicate than *Neanderthal man, which they replaced. They used intricately worked tools of stone and bone and left the famous cave drawings at Lascaux in the Dordogne. The name is derived from the site at Cromagnon, France, where the Ürst fossils were found in 1868. Crookes, Sir William (1832–1919) British chemist and physicist, who in 1861 used *spectroscopy to discover *thallium and in 1875 invented the radiometer. He also developed an improved vacuum tube (Crookes' tube) for studying gas discharges. crop 1. A plant that is cultivated for the purpose of harvesting its seeds, roots, leaves, or other parts that are useful to humans. See . 2. An enlarged portion of the anterior section of the alimentary canal in some animals, in which food may be stored and/or undergo preliminary digestion. The term is most commonly applied to the thinwalled sac in birds between the oesophagus and the *proventriculus. In female pigeons the crop contains glands that secrete crop milk, used to feed nestlings. crop rotation An agricultural practice in which different crops are cultivated in succession on the same area of land over a period of time so as to maintain soil fertility and reduce the adverse effects of pests. Legumes are important in the rotation as they are a source of nitrogen for the soil (see  ;  ). In the UK, other crops that may be included in a typical four-stage rotation are wheat, barley, and root crops. However, the use of pesticides enables the monoculture of crops in modern farming systems (see ). cross 1. A mating between two selected individuals. Controlled crosses are made for many reasons, e.g. to investigate the inheritance of a particular characteristic or to improve a livestock or crop variety. See also
cross-fertilization
 ;  ;  . 2. An organism resulting from such a mating.
cross linkage A short side chain of atoms linking two longer chains in a polymeric material.
cross-fertilization See .
crossover value (COV) The percentage of linked genes (see ) that are exchanged during the process of *crossing over during the Ürst prophase of *meiosis. The COV can be calculated by the percentage of offspring that show *recombination and is used to map the genes on a chromosome (see  ). A small COV for a given pair of genes indicates that the genes are situated close together on the chromosome.
chiasma
paired homologous chromosomes
cross-pollination See .
direction of movement
cross product See  .
recombinant chromatids
Crossing over. At two chiasmata in a pair of homologous chromosomes. crossing over An exchange of portions of chromatids between *homologous chromosomes. As the chromosomes begin to move apart at the end of the Ürst prophase of *meiosis, they remain in contact at a number of points (see ). At these points the chromatids break and rejoin in such a way that sections are exchanged (see illustration). Crossing over thus alters the pattern of genes in the chromosomes. See .
cross section 1. A plane surface formed by cutting a solid, especially by cutting at right angles to its longest axis. 2. The area of such a surface. 3. A measure of the probability that a collision will occur between a beam of radiation and a particular particle, expressed as the effective area presented by the particle in that particular process. It is measured in square metres or *barns. The elastic cross section amounts for all elastic scattering in which the radiation loses no energy to the particle. The inelastic cross section accounts for all other collisions. It is further subdivided to account for speciÜc interactions, such as the absorption cross section, Üssion cross section, ionization cross section, etc. crown ethers Organic compounds with molecules containing large rings of carbon and oxygen atoms. The crown ethers are
oxygen CH2 group metal ion
Crown ethers.
dicyclohexyl-18-crown-6 complex
macrocyclic polyethers. The Ürst to be synthesized was the compound 18-crown-6, which consists of a ring of six –CH2–CH2–O– units (i.e. C12H24O6). The general method of naming crown ethers is to use the form ncrown-m, where n is the number of atoms in the ring and m is the number of oxygen atoms. Substituted crown ethers can also be made. The crown ethers are able to form strongly bound complexes with metal ions by coordination through the oxygen atoms. The stability of these complexes depends on the size of the ion relative to the cavity available in the ring of the particular crown ether. Crown ethers also form complexes with ammonium ions (NH4+) and alkyl ammonium ions (RNH3+). They can be used for increasing the solubility of ionic salts in nonpolar solvents. For example, dicyclohexyl-18crown-6 complexes with the potassium ion of potassium permanganate and allows it to dissolve in benzene, giving a purple neutral solution that can oxidize many organic compounds. They also act as catalysts in certain reactions involving organic salts by complexing with the positive metal cation and thereby increasing its separation from the organic anion, which shows a consequent increase in activity. Some of the uses of crown ethers depend on their selectivity for speciÜc sizes of anions. Thus they can be used to extract speciÜc ions from mixtures and enrich isotope mixtures. Their selectivity also makes them useful analytical reagents. See also .
crucible A dish or other vessel in which substances can be heated to a high temperature. crude oil See . Crustacea A subphylum of *arthropods containing over 35 000 species distributed worldwide, mainly in freshwater and marine habitats, where they constitute a major component of plankton. Crustaceans include shrimps, crabs, lobsters, etc. (see D) and the terrestrial woodlice, all of which belong to the class Malacostraca; the barnacles (class Cirripedia); the water Ûeas (see D), fairy shrimps, and tadpole shrimps (class Branchiopoda); and the copepods (see C). The segmented body usually has a distinct head (bearing *compound eyes, two pairs of *antennae, and various mouthparts), thorax, and abdomen, and is protected by a shell-like carapace. Each body segment may bear a pair of branched (bira-
mous) appendages used for locomotion, as gills, and for Ültering food particles from the water. Appendages in the head region are modiÜed to form jaws and in the abdominal region are often reduced or absent. Typically, the eggs hatch to produce a freeswimming nauplius larva. This develops either by a series of moults or undergoes metamorphosis to the adult form.
• Overview of crustacean phylogeny at the Tree of Life web project
cryobiology The study of the effects of very low temperatures on organisms, tissues, and cells. The ability of some animal tissues to remain viable in a frozen state (cryopreservation) enables them to be preserved by freezing for future use as *grafts. cryogenic pump A *vacuum pump in which pressure is reduced by condensing gases on surfaces maintained at about 20 K by means of liquid hydrogen or at 4 K by means of liquid helium. Pressures down to 10–8 mmHg (10–6 Pa) can be maintained; if they are used in conjunction with a *diffusion pump, pressures as low as 10–15 mmHg (10–13 Pa) can be reached. cryogenics The study of very low temperatures and the techniques for producing them. Objects are most simply cooled by placing them in a bath containing liqueÜed gas maintained at a constant pressure. In general, a liqueÜed gas can provide a constant bath temperature from its triple point to its critical temperature and the bath temperature can be varied by changing the pressure above the liquid. The lowest practical temperature for a liquid bath is 0.3 K. Refrigerators (see ) consist essentially of devices operating on a repeated cycle, in which a low-temperature reservoir is a continuously replenished liquid bath. Above 1 K they work by compressing and expanding suitable gases. Below this temperature liquids or solids are used and by *adiabatic demagnetization it is possible to reach 10–6 K. cryohydrate A eutectic mixture of ice and some other substance (e.g. an ionic salt) obtained by freezing a solution. cryolite A rare mineral form of sodium aluminoÛuoride, Na3AlF6, which crystallizes in the monoclinic system. It is usually white but may also be colourless. The only important occurrence of the mineral is in Green-
cryometer
O N
(2,2,2) cryptand
spherical cryptand
Cryptands. land. It is used chieÛy to lower the melting point of alumina in the production of aluminium.
wire, switching its resistance from zero to a Ünite value. Cryotron switches can be made very small and take very little current.
cryometer A thermometer designed to measure low temperatures. *Thermocouples can be used down to about 1 K and *resistance thermometers can be used at 0.01 K. Below this magnetic thermometers (0.001 K) and nuclear-resonance thermometers (3 × 10–7 K) are required.
cryptands Compounds with large threedimensional molecular structures containing ether chains linked by three-coordinate nitrogen atoms. Thus cryptands are macropolycyclic polyaza-polyethers. For example, the compound (2,2,2)-cryptand has three chains of the form –CH2CH2OCH2CH2OCH2CH2–. These chains are linked at each end by a nitrogen atom. Cryptands, like the *crown ethers, can form coordination complexes with ions that can Üt into the cavity formed by the open three-dimensional structure, i.e. they can 'cryptate' the ion. Various types of cryptand have been produced having both spherical and cylindrical cavities. The cryptands have the same kind of properties as the crown ethers and the same uses. In general, they form much more strongly bound complexes and can be used to stabilize unusual ionic species. For example, it is possible to produce the negative Na– ion in the compound [(2,2,2)-cryptand-Na]+Na–, which is a gold-coloured crystalline substance stable at room temperature. Cluster ions, such as Pb52–, can be similarly stabilized.
cryophyte An organism that can live in ice and snow. Most cryophytes are algae, including the green alga Chlamydomonas nivalis and some diatoms, but they also include certain dinoÛagellates, mosses, bacteria, and fungi. cryoscopic constant See    . cryoscopy The use of *depression of freezing point to determine relative molecular masses. cryostat A vessel enabling a sample to be maintained at a very low temperature. The *Dewar Ûask is the most satisfactory vessel for controlling heat leaking in by radiation, conduction, or convection. Cryostats usually consist of two or more Dewar Ûasks nesting in each other. For example, a liquid nitrogen bath is often used to cool a Dewar Ûask containing a liquid helium bath. cryotron A switch that relies on *superconductivity. It consists of a coil of wire of one superconducting material surrounding a straight wire of another superconducting material; both are immersed in a liquidhelium bath. A current passed through the coil creates a magnetic Üeld, which alters the superconducting properties of the central
cryptic coloration The type of colouring or marking of an animal that helps to camouÛage it in its natural environment. It may enable the animal to blend with its background or, like the stripes of zebras and tigers, help to break up the outline of its body. crypts of Lieberkühn (intestinal glands) Tubular glands that lie between the Ünger-
crystal pick-up
like projections (see ) of the inner surface of the small intestine. The cells of these glands (called Paneth cells) secrete *intestinal juice as they gradually migrate along the side of the crypt and the villus; they are eventually shed into the lumen of the intestine.
crystal A solid with a regular polyhedral shape. All crystals of the same substance grow so that they have the same angles between their faces. However, they may not have the same external appearance because different faces can grow at different rates, depending on the conditions. The external form of the crystal is referred to as the crystal habit. The atoms, ions, or molecules forming the crystal have a regular arrangement and this is the crystal structure. crystal counter A type of solid-state *counter in which a potential difference is applied across a crystal; when the crystal is struck by an elementary particle or photon, the electron–ion pairs created cause a transient increase in conductivity. The resulting current pulses are counted electronically. crystal defect An imperfection in the regular lattice pattern of a crystal. See Feature (pp 210–211). crystal-Üeld theory A theory of the electronic structures of inorganic *complexes, in which the complex is assumed to consist of a central metal atom or ion surrounded by ligands that are ions. For example, the complex [PtCl4]2– is thought of as a Pt2+ ion surrounded by four Cl– ions at the corners of a square. The presence of these ions affects the energies of the d-orbitals, causing a splitting of energy levels. The theory can be used to explain the spectra of complexes and their magnetic properties. Ligand-Üeld theory is a development of crystal-Üeld theory in which the overlap of orbitals is taken into account. Crystal-Üeld theory was initiated in 1929 by the German-born US physicist Hans Albrecht Bethe (1906–2005) and extensively developed in the 1930s. crystal habit See . crystal lattice The regular pattern of atoms, ions, or molecules in a crystalline substance. A crystal lattice can be regarded as produced by repeated translations of a unit cell of the lattice. See also  .
• An interactive version of the structures of a range of crystals from a US Navy website • Crystal models from the University of Calgary
crystalline Having the regular internal arrangement of atoms, ions, or molecules characteristic of crystals. Crystalline materials need not necessarily exist as crystals; all metals, for example, are crystalline although they are not usually seen as regular geometric crystals. crystallite A small crystal, e.g. one of the small crystals forming part of a microcrystalline substance. crystallization The process of forming crystals from a liquid or gas. crystallography The study of crystal form and structure. See also X- . crystalloids See . crystal meth See . crystal microphone A microphone in which the sound waves fall on a plate of Rochelle salt or similar material with piezoelectric properties, the variation in pressure being converted into a varying electric Üeld by the *piezoelectric effect. Crystal microphones have a good high-frequency response and are nondirectional; they are now rarely used except when their cheapness is important. crystal oscillator (piezoelectric oscillator) An oscillator in which a piezoelectric crystal is used to determine the frequency. An alternating electric Üeld applied to two metallic Ülms sputtered onto the parallel faces of a crystal, usually of quartz, causes it to vibrate at its natural frequency; this frequency can be in the kilohertz or megahertz range, depending on how the crystal is cut. The mechanical vibrations in turn create an alternating electric Üeld across the crystal that does not suffer from frequency drift. The device can be used to replace the tuned circuit in an oscillator by providing the resonant frequency or it can be coupled to the oscillator circuit, which is tuned approximately to the crystal frequency. In this type, the crystal prevents frequency drift. The device is widely used in *quartz clocks and watches. crystal pick-up A pick-up in a record player in which the mechanical vibrations
CRYSTAL DEFECTS
A crystal *lattice is formed by a repeated arrangement of atoms, ions, or molecules. Within one cubic centimetre of material one can expect to find up to 1022 atoms and it is extremely unlikely that all of these will be arranged in perfect order. Some atoms will not be exactly in the right place with the result that the lattice will contain *defects. The presence of defects within the crystal structure has profound consequences for certain bulk properties of the solid, such as the electrical resistance and the mechanical strength.
Point defects Local crystal defects called point defects, appear as either impurity atoms or gaps in the lattice. Impurity atoms can occur in the lattice either at interstitial sites (between atoms in a non-lattice site) or at substitutional sites (replacing an atom in the host lattice). Lattice gaps are called vacancies and arise when an atom is missing from its site in the lattice. Vacancies are sometimes called Schottky defects. A vacancy in which the missing atom has moved to an interstitial position is known as a Frenkel defect. Colour centres In ionic crystals, the ions and vacancies always arrange themselves so that there is no build-up of one type of charge in any small volume of the crystal. If ions or charges are introduced into or removed from the lattice, there will, in general, be an accompanying rearrangement of the ions and their outer valence electrons. This rearrangement is called charge compensation and is most dramatically observed in colour centres. If certain crystals are irradiated with X-rays, gamma rays, neutrons, or electrons a colour change is observed. For example, diamond may be coloured blue by electron bombardment and quartz may be coloured brown by irradiation with neutrons. The high-energy radiation produces defects in the lattice and, in an attempt to maintain charge neutrality, the crystal undergoes some measure of charge compensation. Just as electrons around an atom have a series of discrete permitted energy levels, so charges residing at point defects exhibit sets of discrete levels, which are separated from one another by energies corresponding to wavelengths in the visible region of the spectrum. Thus light of certain wavelengths can be absorbed at the defect sites, and the material appears to be coloured. Heating the irradiated crystal can, in many cases, repair the irradiation damage and the crystal loses its coloration. Dislocations Non-local defects may involve entire planes of atoms. The most important of these is called a dislocation. Dislocations are essentially line-defects; that is,
Formation of a Schottky defect
Point defects in a two-dimensional crystal
Formation of a Frenkel defect
there is an incomplete plane of atoms in the crystal lattice. In 1934, Taylor, Orowan, and Polanyi independently proposed the concept of the dislocation to account for the mechanical strength of metal crystals. Their microscopic studies revealed that when a metal crystal is plastically deformed, the deformation does not occur by a separation of individual atoms but rather by a slip of one plane of atoms over another plane. Dislocations provide a mechanism for this slipping of planes that does not require the bulk movement of crystal material. The passage of a dislocation in a crystal is similar to the movement of a ruck in a carpet. A relatively large force is required to slide the carpet as a whole. However, moving a ruck over the carpet can inch it forward without needing such large forces. This movement of dislocations is called plastic flow.
Strength of materials In practice most metal samples are polycrystalline; that is they consist of many small crystals or grains at different angles to each other. The boundary between two such grains is called a grain boundary. The plastic flow of dislocations may be hindered by the presence of grain boundaries, impurity atoms, and other dislocations. Pure metals produced commercially are generally too weak to be of much mechanical use. The weakness of these samples can be attributed to the ease with which the dislocations are able to move within the sample. Slip, and therefore deformation, can then occur under relatively low stresses. Impurity atoms, other dislocations, and grain boundaries can all act as obstructions to the slip of atomic planes. Traditionally, methods of making metals stronger involved introducing defects that provide regions of disorder in the material. For example, in an alloy, such as steel, impurity atoms (e.g. carbon) are introduced into the lattice during the forging process. The perfection of the iron lattice structure is disturbed and the impurities oppose the dislocation motion. This makes for greater strength and stiffness. The complete elimination of dislocations may seem an obvious way to strengthen materials. However, this has only proved possible for hair-like single crystal specimens called whiskers. These whiskers are only a few micrometers thick and are seldom more than a few millimetres long; nevertheless their strength approaches the theoretical value. A
Dislocation in a two-dimensional crystal. The extra plane of atoms AB causes strain at bond CD. On breaking, the bond flips across to form CB. This incremental movement shifts the dislocation across so that the overall effect is to slide the two planes BDG and CF over each other.
produced by undulations in the record groove are transmitted to a piezoelectric crystal, which produces a varying electric Üeld of the same frequency as the sound. This signal is ampliÜed and fed to loudspeakers in order to recreate the sound.
crystal structure See . crystal system A method of classifying crystalline substances on the basis of their unit cell. There are seven crystal systems. If the cell is a parallelopiped with sides a, b, and c and if α is the angle between b and c, β the angle between a and c, and γ the angle between a and b, the systems are: (1) cubic a=b=c and α=β=γ=90° (2) tetragonal a=b≠c and α=β=γ=90° (3) rhombic (or orthorhombic) a≠b≠c and α=β=γ=90° (4) hexagonal a=b≠c and α=β=γ=90° (5) trigonal a=b≠c and α=β=γ≠90° (6) monoclinic a≠b≠c and α=γ=90°≠β (7) triclinic a=b=c and α≠β≠γ crystal test A type of *presumptive test in which a substance is identiÜed by the formation of characteristic crystals when a certain reagent is added. Usually, such tests are conducted using a microscope (microcrystal test). An example is the *acetone–chlor– haemin test for blood. CSF 1. See  . 2. See - . CS gas The vapour from a white solid, C6H4(Cl)CH:C(CN)2, causing tears and choking, used in 'crowd control'.
r.d. 1.29; m.p. 131°C. It has a novel structure with eight carbon atoms at the corners of a cube, each attached to a hydrogen. Cubane was Ürst synthesized in 1964 by Philip Eaton. The C–C–C bond angle of 90° is highly strained and cubane and its derivatives have been investigated as high-energy fuels and explosives. In particular, octanitrocubane, in which the hydrogen atoms are replaced by –NO2 groups, is possibly the most powerful chemical explosive known, although, so far, only small amounts have been synthesized. It decomposes to carbon dioxide and nitrogen: C8(NO2)8 → 8CO2 + 4N2
cubewano A *Kuiper belt object (KBO) that is in direct orbit around the sun but is not held in orbital resonance with Neptune or any other planet. Cubewanos take their name from QB1 – now (15760) QB1 – the Ürst known KBO, which was discovered in 1992. Cubewanos orbit the sun at mean distances of between 42 and 48 astronomical units. The dwarf planet (136472) Makemake (at an estimated 1500 km in diameter) is the largest cubewano so far known. cubic close packing See  . cubic crystal A crystal in which the unit cell is a cube (see  ). There are three possible packings for cubic crystals: simple cubic, face-centred cubic, and bodycentred cubic. See illustration.
CU See  .
• An interactive version of a simple cubic crystal • An interactive version of a body-centred cubic crystal • An interactive version of a face-centred cubic crystal
cubane A crystalline hydrocarbon, C8H8;
cubic equation An equation in which the
CT scanner (computerized tomography scanner) See .
body-centred
Cubic crystal structures.
simple cubic
face-centred
highest power of the variable is three. It has the general form ax3 + bx2 + cx + d = 0 and, in general, is satisÜed by three values of x.
cubic expansivity See . cubic zircona (CZ) A crystal form of zircon(IV) oxide (zircon dioxide, ZrO2) made by fusing ZrO2 and allowing it to cool under controlled conditions. It is used as an inexpensive diamond substitute in jewellery. Often it is erroneously called *zircon. cultivar A plant that has been developed and maintained by cultivation as a result of agricultural or horticultural practices. The term is derived from cultivated variety. cultivation The planting and breeding of crop plants in *agriculture and horticulture. It involves the investigation of new means of increasing crop yield and quality. culture A batch of cells, which can be microorganisms or of animal or plant origin, that are grown under speciÜc conditions of nutrient levels, temperature, pH, oxygen levels, osmotic factors, light, pressure, and water content. Cultures of cells are prepared in the laboratory for a wide spectrum of scientiÜc research. A *culture medium provides the appropriate conditions for growth. See also  ;  . culture medium A nutrient material, either solid or liquid, used to support the growth and reproduction of microorganisms or to maintain tissue or organ cultures. See also . cumene process An industrial process for making phenol from benzene. A mixture of benzene vapour and propene is passed over a phosphoric acid catalyst at 250°C and high pressure C6H6 + CH3CH:CH2 → C6H5CH(CH3)2 The product is called cumene, and it can be oxidized in air to a peroxide, C6H5C(CH3)2O2H. This reacts with dilute acid to give phenol (C6H5OH) and propanone (acetone, CH3OCH3), which is a valuable by-product. cupellation A method of separating noble metals (e.g. gold or silver) from base metals (e.g. lead) by melting the mixture with a blast of hot air in a shallow porous dish (the cupel). The base metals are oxidized, the oxide being carried away by the blast of air or absorbed by the porous container. cuprammonium ion The tetraam-
Curie, Marie minecopper(II) ion [Cu(NH3)4]2+. See .
cupric compounds Compounds containing copper in its higher (+2) oxidation state; e.g. cupric chloride is copper(II) chloride (CuCl2). cuprite A red mineral cubic form of copper(I) oxide, Cu2O; an important ore of copper. It occurs where deposits of copper have been subjected to oxidation. The mineral has been mined as a copper ore in Chile, Democratic Republic of Congo, Bolivia, Australia, Russia, and the USA. cupronickel A type of corrosion-resistant alloy of copper and nickel containing up to 45% nickel. cuprous compounds Compounds containing copper in its lower (+1) oxidation state; e.g. cuprous chloride is copper(I) chloride (CuCl). cupule 1. A hard or membranous cupshaped structure formed from bracts and enclosing various fruits, such as the hazelnut and acorn. 2. A structure in club mosses (Lycopodium species) that protects the gemma (resting bud) during its development. It is composed of six leaÛike structures. 3. The bright red tissue around the seed of yew (Taxus), forming the yew 'berry'. curare A resin obtained from the bark of South American trees of the genera Strychnos and Chondrodendron that causes paralysis of voluntary muscle. It acts by blocking the action of the neurotransmitter *acetylcholine at *neuromuscular junctions. Curare is used as an arrow poison by South American Indians and was formerly used as a muscle relaxant in surgery. curd The solid component produced by the coagulation of milk during the manufacture of cheese. After being pasteurized, milk is cooled down and a culture of lactic acid bacteria is added to ferment the milk sugar, lactose, to lactic acid. The resulting decrease in pH causes casein, a milk protein, to coagulate, a process known as curdling. The solid curds are then separated from the liquid component, known as whey, and inoculated with different types of microbes to produce different cheeses. curie The former unit of *activity (see  ). It is named after Pierre Curie. Curie, Marie (Marya Sklodowska;
Curie point
1867–1934) Polish-born French chemist, who went to Paris in 1891. She married the physicist Pierre Curie (1859–1906) in 1895 and soon began work on seeking radioactive elements other than uranium in pitchblende (to account for its unexpectedly high radioactivity). By 1898 she had discovered *radium and *polonium, although it took her four years to purify them. In 1903 the Curies shared the Nobel Prize for physics with Henri *Becquerel, who had discovered radioactivity. In 1911 Marie Curie was awarded the Nobel Prize for chemistry.
• A comprehensive website about her life and work run by The American Institute of Physics Center for History of Physics
Curie point (Curie temperature) The temperature at which a ferromagnetic substance loses its ferromagnetism and becomes only paramagnetic. For iron the Curie point is 760°C and for nickel 356°C. Curie's law The susceptibility (χ) of a paramagnetic substance is proportional to the thermodynamic temperature (T), i.e. χ = C/T, where C is the Curie constant. A modiÜcation of this law, the Curie–Weiss law, is more generally applicable. It states that χ = C/(T – θ), where θ is the Weiss constant, a characteristic of the material. The law was Ürst proposed by Pierre Curie and modiÜed by another French physicist, Pierre-Ernest Weiss (1865–1940). curium Symbol Cm. A radioactive metallic transuranic element belonging to the *actinoids; a.n. 96; mass number of the most stable isotope 247 (half-life 1.64 × 107 years); r.d. (calculated) 13.51; m.p. 1340±40°C. There are nine known isotopes. The element was Ürst identiÜed by Glenn Seaborg (1912–99) and associates in 1944 and Ürst produced by L. B. Werner and I. Perlman in 1947 by bombarding americium–241 with neutrons.
curl (rot) The *vector product of the *gradient operator with a vector. For a vector u that has components u1, u2, and u3 in the x, y, and z directions (with respective unit vectors i, j, and k), and is a function of x, y, and z, the curl is given by: curl u = ∇ × u = (∂u3/∂y – ∂u2/∂z)i +
(∂u1/∂z – ∂u3/∂x)j + (∂u2/∂x – ∂u1/∂y)k. See also .
current Symbol I. A Ûow of electric charge through a conductor. The current at a particular cross section is the rate of Ûow of charge. The charge may be carried by electrons, ions, or positive holes (see  ). The unit of current is the ampere. See also  . current balance An instrument used to measure a current absolutely, on the basis of the deÜnition of the ampere. An accurate form consists of a beam balance with similar coils attached to the ends of the balance arms. Fixed coils are situated above and below these two coils. The six coils are then connected in series so that a current passing through them creates a torque on the beam, which is restored to the horizontal by means of a rider. From the position and weight of the rider, and the geometry of the system, the current can be calculated. current density 1. The current Ûowing through a conductor per unit crosssectional area, measured in amperes per square metre. 2. The current Ûowing through an electrolyte per unit area of electrode. cusp 1. (in dentistry) A sharp raised protuberance on the surface of a *molar tooth. The cusps of opposing molars (i.e. on opposite jaws) are complementary to each other, which increases the efÜciency of grinding food during chewing. 2. (in anatomy) A Ûap forming part of a *valve. 3. (in mathematics) A point at which two arcs of a curve intersect. cuticle 1. (in botany) The continuous waxy layer that covers the aerial parts of a plant. Composed of *cutin, it is secreted by the *epidermis and its primary function is to prevent water loss. 2. (in zoology) A layer of horny noncellular material covering, and secreted by, the epidermis of many invertebrates. It is usually made of a collagen-like protein or of *chitin and its main function is protection. In arthropods it is also strong enough to act as a skeleton (see ) and in insects it reduces water loss. Growth is allowed by moulting of the cuticle (see ). cuticularization The secretion by the outer (epidermal) layer of cells of plants and
many invertebrates of substances that then harden to form a *cuticle.
cutin A polymer of long-chain fatty acids that forms the main constituent of the *cuticle of mature epidermal plant cells. Cutin polymers are cross-linked to form a network, which is embedded in a matrix of waxes. The deposition of cutin (cutinization) reduces water loss by the plant and helps prevent the entry of pathogens. See also . cutinization The deposition of *cutin in plant cell walls, principally in the outermost layers of leaves and young stems. cutis See . cutting A part of a plant, such as a bud, leaf, or a portion of a root or shoot, that, when detached from the plant and inserted in soil, can take root and give rise to a new daughter plant. Taking or striking cuttings is a horticultural method for propagating plants. See also  . cutting agent (adulterant) A substance used to dilute illegal drugs such as heroin and cocaine. Examples include Ûour, starch, sugar, and caffeine. Cuvier, George Léopold Chrétien Frédéric Dagobert (1769–1832) French anatomist, who became professor at the Collège de France in 1799, moving in 1802 to the Jardin de Plantes. Cuvier extended the classiÜcation system of *Linnaeus, adding the category *phylum and concentrating on the taxonomy of Üshes. He also initiated the classiÜcation of fossils and established the science of palaeontology. cyanamide 1. An inorganic salt containing the ion CN22–. See  . 2. A colourless crystalline solid, H2NCN, made by the action of carbon dioxide on hot sodamide. It is a weakly acidic compound (the parent acid of cyanamide salts) that is soluble in water and ethanol. It is hydrolysed to urea in acidic solutions. cyanamide process See  . cyanate See  . cyanic acid An unstable explosive acid, HOCN. The compound has the structure H–O–C≡N, and is also called fulminic acid. Its salts and esters are cyanates (or fulminates). The compound is a volatile liquid, which readily polymerizes. In water it hydrolyses to ammonia and carbon dioxide. It
cyanogen is isomeric with another acid, H–N=C=O, which is known as isocyanic acid. Its salts and esters are isocyanates.
cyanide 1. An inorganic salt containing the cyanide ion CN–. Cyanides are extremely poisonous because of the ability of the CN– ion to coordinate with the iron in haemoglobin, thereby blocking the uptake of oxygen by the blood. 2. A metal coordination complex formed with cyanide ions. cyanide process A method of extracting gold by dissolving it in potassium cyanide (to form the complex ion [Au(CN)2]–). The ion can be reduced back to gold with zinc. cyanine dyes A class of dyes that contain a –CH= group linking two nitrogencontaining heterocyclic rings. They are used as sensitizers in photography. Cyanobacteria A phylum consisting of two groups of photosynthetic eubacteria. The blue-green bacteria (formerly known as blue-green algae, or Cyanophyta), which comprise the vast majority of members, contain the photosynthetic pigment chlorophyll a plus accessory pigments: phycocyanins, responsible for their blue colour, and (in some) red pigments (phycoerythrins). Bluegreen bacteria are unicellular but sometimes become joined in colonies or Ülaments by a sheath of mucilage. They occur in all aquatic habitats. A few species Üx atmospheric nitrogen and thus contribute to soil fertility (see  ). Others exhibit symbiosis (see ). The chloroxybacteria (grass-green bacteria or prochlorophytes) have been found in marine and freshwater habitats. They differ from the blue-green bacteria in containing chlorophyll a and chlorophyll b but no blue or red pigments – a combination like that found in plant chloroplasts, which they resemble. cyanocobalamin See  B . cyanogen A colourless gas, (CN)2, with a pungent odour; soluble in water, ethanol, and ether; d. 2.335 g dm–3; m.p. –27.9°C; b.p. –20.7°C. The compound is very toxic. It may be prepared in the laboratory by heating mercury(II) cyanide; industrially it is made by gas-phase oxidation of hydrogen cyanide using air over a silver catalyst, chlorine over activated silicon(IV) oxide, or nitrogen dioxide over a copper(II) salt. Cyanogen is an important intermediate in the preparation of various fertilizers and is also used as a stabi-
cyano group lizer in making nitrocellulose. It is an example of a *pseudohalogen.
cyano group The group –CN in a chemical compound. See .
cyanohydrins Organic compounds formed by the addition of hydrogen cyanide to aldehydes or ketones (in the presence of a base). The Ürst step is attack by a CN– ion on the carbonyl carbon atom. The Ünal product is a compound in which a –CN and –OH group are attached to the same carbon atom. For example, ethanal reacts as follows CH3CHO + HCN → CH3CH(OH)(CN) The product is 2-hydroxypropanonitrile. Cyanohydrins of this type can be oxidized to α-hydroxy carboxylic acids. cyanuric acid A white crystalline watersoluble trimer of cyanic acid, (HNCO)3. It is a cyclic compound having a six-membered ring made of alternating imide (NH) and carbonyl (CO) groups (i.e. three –NH–C(O)– units). It can also exist in a phenolic form (three –N=C(OH)– units). CycadoÜlicales (Pteridospermales; seed ferns) An extinct order of gymnosperms that Ûourished in the Carboniferous period. They possessed characteristics of both the ferns and the seed plants in reproducing by means of seeds and yet retaining fernlike leaves. Their internal anatomy combined both fern and seed-plant characteristics. Cycadophyta A phylum of seed plants (see ) that contains many extinct species; the few modern representatives of the group include Cycas and Zamia. Cycads inhabit tropical and subtropical regions, sometimes growing to a height of 20 m. The stem bears a crown of fernlike leaves. These species are among the most primitive of living seed plants. cyclamates Salts of the acid, C6H11.NH.SO3H, where C6H11– is a cyclohexyl group. Sodium and calcium cyclamates were formerly used as sweetening agents in soft drinks, etc., until their use was banned when they were suspected of causing cancer. cycle A regularly repeated set of changes to a system that brings back all its parameters to their original values once in every set of changes. The duration of one cycle is called its *period and the rate of repetition of cycle, called the *frequency, is measured in *hertz. See   .
cyclic Describing a compound that has a ring of atoms in its molecules. In homocyclic compounds all the atoms in the ring are the same type, e.g. benzene (C6H6) and cyclohexane (C6H12). These two examples are also examples of carbocyclic compounds; i.e. the rings are of carbon atoms. If different atoms occur in the ring, as in pyridine (C5H5N), the compound is said to be heterocyclic. cyclic AMP (cAMP; cyclic adenosine monophosphate) A derivative of *ATP that is widespread in cells as a *second messenger in many biochemical reactions induced by hormones. Binding of the hormone to its receptor on the cell surface activates *G proteins, which in turn activate *adenylate cyclase, the enzyme that catalyses cyclic AMP production. Cyclic AMP controls activity of protein kinase A, enabling it to activate intracellular proteins that mediate the ultimate effects of the hormone on the cell. Cyclic AMP is also involved in controlling gene expression and cell division, in immune responses, and in nervous transmission. cyclic model (of the universe) See  . cyclic phosphorylation (cyclic photophosphorylation) See . cyclization The formation of a cyclic compound from an open-chain compound. See . cyclo- PreÜx designating a cyclic compound, e.g. a cycloalkane or a cyclosilicate. cycloalkanes Cyclic saturated hydrocarbons containing a ring of carbon atoms joined by single bonds. They have the general formula CnH2n, for example cyclohexane, C6H12, etc. In general they behave like the *alkanes but are rather less reactive.
cyclohexadiene-1,4-dione (benzoquinone; quinone) A yellow solid, C6H4O2; r.d. 1.3; m.p. 116°C. It has a six-membered ring of carbon atoms with two opposite carbon atoms linked to oxygen atoms (C=O) and the other two pairs of carbon atoms linked by double bonds (HC=CH). The compound is used in making dyes. See also  . cyclohexane A colourless liquid *cycloalkane, C6H12; r.d. 0.78; m.p. 6.5°C; b.p.
cylinder gas
81°C. It occurs in petroleum and is made by passing benzene and hydrogen under pressure over a heated Raney nickel catalyst at 150°C, or by the reduction of cyclohexanone. It is used as a solvent and paint remover and can be oxidized using hot concentrated nitric acid to hexanedioic acid (adipic acid). The cyclohexane ring is not planar and can adopt boat and chair *conformations; in formulae it is represented by a single hexagon.
cycloid The curve traced by a point on the circumference of a circle as it rolls without slipping along a straight line. The length of the arc formed by one revolution of the circle is 8r, where r is the radius of the circle. The horizontal distance between cusps is 2πr. cyclone An area of low pressure in the atmosphere. Winds rotate about the lowpressure centre in an anti-clockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere. In the mid- and high-latitudes these low-pressure systems are now commonly referred to as *depressions, or lows, and the term cyclone is avoided. See also  . cyclonite A highly explosive nitro compound, (CH2N.NO2)3. It has a cyclic structure with a six-membered ring of alternating CH2 groups and nitrogen atoms, with each nitrogen being attached to a NO2 group. It is made by nitrating hexamine, C6H12N4, which is obtained from ammonia and methanal. Cyclonite is a very powerful explosive used mainly for military purposes. It is also called RDX. The abbreviation is for 'Research Department composition X', used at the Chemical Research and Development Department, Woolwich. cyclopentadiene A colourless liquid cyclic *alkene, C5H6; r.d. 0.8021; m.p. –97.2°C; b.p. 40.0°C. It is prepared as a byproduct during the fractional distillation of crude benzene from coal tar. It undergoes condensation reactions with ketones to give highly coloured compounds (fulvenes) and readily undergoes polymerization at room temperature to give the dimer, dicyclopentadiene. The compound itself is not aromatic because it does not have the required number of pi electrons. However, removal of a hydrogen atom produces the stable cyclopentadienyl ion, C5H5–, which does have aromatic properties. In particular, the ring can coordinate to positive ions in such compounds as *ferrocene.
cyclopentadienyl ion See . cyclophane A compound consisting of one or more aromatic rings forming part of a larger ring system in which aliphatic chains of the CH2 groups link the aromatic rings. Compounds of this type have the sufÜx phane in their names. Depending on the sizes of the (CH2)n chains, the aromatic rings may not be planar.
cyclopropane A colourless gas, C3H6, b.p. –34.5°C, whose molecules contain a triangular ring of carbon atoms. It is made by treating 1,3-dibromopropane with zinc metal, and is used as a general anaesthetic. cyclosarin A highly toxic colourless liquid, C7H14FO2P; r.d. 1.13; m.p. –30°C; b.p. 239°C. it is a Ûuorinated organophosphorus compound, (Ûuoromethylphosphoryl)oxycyclohexane. Cyclosarin was discovered in 1949 and belongs to the G-series of *nerve agents (GF). cyclosis See  . cyclotetramethylenetetranitramine See HMX. cyclotron A cyclic particle *accelerator in which charged particles fed into the centre of the device are accelerated in an outward spiral path inside two hollow D-shaped conductors placed to form a split circle. A magnetic Üeld is applied at right-angles to the plane of the dees and an alternating potential difference is applied between them. The frequency of the alternating p.d. is arranged so that the particles are accelerated each time they reach the evacuated gap between the dees. The magnetic Üeld makes them follow curved paths. After several thousand revolutions inside the dees the particles reach the perimeter of the dees, where a deÛecting Üeld directs them onto the target. In this device protons can achieve an energy of 10–12 J (10 MeV). The Ürst working cyclotron was produced in 1931 by the US physicist Ernest Lawrence (1901–58). See also .
• The US National Superconducting Cyclotron Society's You Tube channel
cylinder gas See  .
cylindrical polar coordinates
cylindrical polar coordinates See  . cyme See  .
cysteine See  .
2 1 buttercup
monochasial cymes
cypsela A dry single-seeded fruit that does not split open during seed dispersal and is formed from a double ovary in which only one ovule develops into a seed. It is similar to an *achene and characteristic of members of the family Compositae (Asteraceae), such as the dandelion. See also .
stitchwort
dichasial cyme 1 = oldest flower
Cymose inflorescence. Different types. cymose inÛorescence (cyme; deÜnite inÛorescence) A type of Ûowering shoot (see ) in which the Ürst-formed Ûower develops from the growing region at the top of the Ûower stalk (see illustration). Thus no new Ûower buds can be produced at the tip and other Ûowers are produced from lateral buds beneath. In a monochasial cyme (or monochasium), the development of the Ûower at the tip is followed by a new Ûower axis growing from a single lateral bud. Subsequent new Ûowers may develop from the same side of the lateral shoots, as in the buttercup, or alternately on opposite sides, as in forget-me-not. In a dichasial cyme (or dichasium), the development of the Ûower at the apex is followed by two new Ûower axes developing from buds opposite one another, as in plants of the family Caryophyllaceae (such as stitchwort). Compare  .
cystine A molecule resulting from the oxidation reaction between the sulphydryl (–SH) groups of two cysteine molecules (see  ). This often occurs between adjacent cysteine residues in polypeptides. The resultant *disulphide bridges (–S–S–) are important in stabilizing the structure of protein molecules. cytidine A nucleoside comprising one cytosine molecule linked to a D-ribose sugar molecule. The derived nucleotides, cytidine mono-, di-, and triphosphate (CMP, CDP, and CTP respectively), participate in various biochemical reactions, notably in phospholipid synthesis. cytochrome Any of a group of proteins, each with an iron-containing *haem group, that form part of the *electron transport chain in mitochondria and chloroplasts. Electrons are transferred by reversible changes in the iron atom between the reduced Fe(II) and oxidized Fe(III) states. See also  . cytochrome oxidase An enzyme complex comprising the terminal two cytochromes of the respiratory chain in the mitochondria (see   ). It is responsible for the reduction of oxygen to form water. cytogenetics The study of inheritance in relation to the structure and function of cells. For example, the results of breeding experiments can be explained in terms of the behaviour of chromosomes during the formation of the reproductive cells. cytokine Any of numerous small proteins released from a variety of cell types that affect cell behaviour. Cytokines can inÛuence the cells releasing them or nearby cells; in some cases they can enter the bloodstream to inÛuence distant cells. Cytokines are crucial to many aspects of cell proliferation, differentiation, migration, and function, and play a central role in immune responses and inÛammation. Cytokines produced by lymphocytes are termed lymphokines, and
those affecting cell migration, particularly of immune cells, form a large group of chemokines. Cytokines fall into two main families: the haemopoietins, which include some of the interleukins, erythropoietin, and the *colony-stimulating factors; and the tumour necrosis factor (TNF) family.
cytokinesis See  ; . cytokinin (kinin) Any of a group of plant hormones chemically related to the purine adenine. Among other roles, cytokinins stimulate cell division in the presence of *auxin and have also been found to delay senescence, overcome *apical dominance, and promote cell expansion. Zeatin is a naturally occurring cytokinin. cytology The study of the structure and function of cells. The development of the light and electron microscopes has enabled the detailed structure of the nucleus (including the chromosomes) and other organelles to be elucidated. Microscopic examination of cells, either live or as stained sections on a slide, is also used in the detection and diagnosis of various diseases, especially *cancer. cytolysis The breakdown of cells, usually as a result of destruction or dissolution of their outer membranes. Certain drugs (cytotoxic drugs) have this effect and are used in the treatment of some forms of cancer. cytomegalovirus A virus belonging to the herpes group (see ). In humans it normally causes symptoms that are milder than the common cold, but it can produce more serious symptoms in those whose *immune response is disturbed (e.g. cancer patients and people who are HIV-positive). Infection in pregnant women may cause congenital handicap in their children. cytoplasm The material surrounding the nucleus of a *cell. It can be differentiated into dense outer ectoplasm, which is concerned primarily with cell movement, and less dense endoplasm, which contains most of the cell's structures. cytoplasmic inheritance The inheritance of genes contained in the cytoplasm of a cell, rather than the nucleus. Only a very small number of genes are inherited in this way. The phenomenon occurs because certain organelles, the *mitochondria and (in plants) the *chloroplasts, contain their own genes and can reproduce independently.
The female reproductive cell (the egg) has a large amount of cytoplasm containing many such organelles, which are consequently incorporated into the cytoplasm of all the cells of the embryo. The male reproductive cells (sperm or pollen), however, consist almost solely of a nucleus. Cytoplasmic organelles are thus not inherited from the male parent. In plants, male sterility can be inherited via the cytoplasm. The inheritance of any such factors does not follow Mendelian laws.
cytoplasmic streaming (cyclosis) The directional movement of cytoplasm in certain cells, which allows movement of substances through the cell, especially around the cell's periphery. It has been observed most clearly in large cells, such as plant sieve elements and unicellular algae, in which simple diffusion is ineffective as a means of local transport in the cell. The mechanism involves the interaction of *myosin proteins (attached to organelles) with *actin microÜlaments parallel to the direction of Ûow and requires energy from ATP. A similar streaming of cytoplasm is responsible for *amoeboid movement. cytosine A *pyrimidine derivative. It is one of the principal component bases of *nucleotides and the nucleic acids *DNA and *RNA. cytoskeleton A network of Übres permeating the matrix of eukaryotic cells that provides a supporting framework for organelles, anchors the cell membrane, facilitates cellular movement, and provides a suitable surface for chemical reactions to take place. The Übres are composed of *microtubules and *actin microÜlaments. cytosol The semiÛuid soluble part of the cytoplasm of cells, which contains the components of the *cytoskeleton. The cell's organelles are suspended in the cytosol. cytotaxonomy See . cytotoxic Destructive to living cells. The term is applied particularly to a class of drugs that inhibit cell division and are therefore used in chemotherapy to destroy cancer cells and to a group of *T cells that destroy virus-infected cells. CZ See  . CZE Capillary zone electrophoresis. See  .
D 2,4-D 2,4-dichlorophenoxyacetic acid (2,4dichlorophenoxyethanoic acid): a synthetic *auxin used as a weedkiller of broad-leaved weeds. See . D'Alembertian Symbol (sometimes printed 2). An operator that is the analogue of the Laplace operator in four-dimensional Minkowski space–time, i.e. = ∂2/∂x2 + ∂2/∂y2 + ∂2/∂z2 – (1/c2) ∂2/∂t2, where c is the speed of light. The D'Alembertian is very useful in the special theory of relativity. D'Alembert's principle A formulation of Newtonian mechanics that reduces dynamics to an equilibrium condition like statics by extending Newton's third law to the case of forces acting on bodies rather than just bodies in static equilibrium, i.e. it postulates that bodies are always in equilibrium even when they are acted on by a force because the force applied to the body minus the rate of change of the momentum with respect to time is always zero. It was proposed by the French mathematician Jean le Rond D'Alembert (1717–83) in 1743. dalton See   . Dalton, John (1766–1844) British chemist and physicist. In 1801 he formulated his law of partial pressures (see D' ), but he is best remembered for *Dalton's atomic theory, which he announced in 1803. Dalton also studied colour blindness (a condition, once called Daltonism, that he shared with his brother). Dalton's atomic theory A theory of *chemical combination, Ürst stated by John Dalton in 1803. It involves the following postulates: (1) Elements consist of indivisible small particles (atoms). (2) All atoms of the same element are identical; different elements have different types of atom. (3) Atoms can neither be created nor destroyed. (4) 'Compound elements' (i.e. compounds) are formed when atoms of different el-
ements join in simple ratios to form 'compound atoms' (i.e. molecules). Dalton also proposed symbols for atoms of different elements (later replaced by the present notation using letters).
Dalton's law The total pressure of a mixture of gases or vapours is equal to the sum of the partial pressures of its components, i.e. the sum of the pressures that each component would exert if it were present alone and occupied the same volume as the mixture of gases. Strictly speaking, the principle is true only for ideal gases. dam A structure built across a river to impound or divert the Ûow of water. A dam may be constructed for one or a number of purposes. It may raise the water level to increase the depth for navigation purposes; divert water; provide a head of water for the generation of hydroelectric power; or store water for industrial or domestic use, irrigation, Ûood control, or power production. In addition, water stored by dams is often used for recreational purposes. Most dams are constructed of either concrete or of earth and rock. Gravity dams depend on the weight of their bulk for stability and usually have a Ûat vertical face upstream. Arch dams are curved concrete structures with a convex curve upstream. Pressure is transmitted to the sides of the dam. damping A decrease in the amplitude of an oscillation as a result of energy being drained from the oscillating system to overcome frictional or other resistive forces. For example, a pendulum soon comes to rest unless it is supplied with energy from an outside source; in a pendulum clock, energy is supplied through an *escapement from a wound spring or a falling mass to compensate for the energy lost through friction. Damping is introduced intentionally in measuring instruments of various kinds to overcome the problem of taking a reading from an oscillating needle. A measuring instrument is said to be critically damped if the system just fails to oscillate and the system comes to rest in the shortest possible time. If it is underdamped it will oscillate re-
peatedly before coming to rest; if it is overdamped it will not oscillate but it will take longer to come to rest than it would if it was critically damped. An instrument, such as a galvanometer, that is critically damped is often called a deadbeat instrument.
largely of dark matter. There is some evidence for the existence of such galaxies. It is thought that they should be very common, particularly since theories of large-scale structure work much better if the existence of plentiful dark galaxies is assumed.
dance of the bees A celebrated example of *communication in animals, Ürst investigated by Karl von Frisch (1886–1982). Honeybee workers on returning to the hive after a successful foraging expedition perform a 'dance' on the comb that contains coded information about the distance and direction of the food source. For example the waggle dance, characterized by tail-wagging movements, indicates the direction of a food source at a distance of more than 100 metres. Other workers, sensing vibrations from the dance, follow the instructions to Ünd the food source.
dark matter See  .
Daniell cell A type of primary *voltaic cell with a copper positive electrode and a negative electrode of a zinc amalgam. The zincamalgam electrode is placed in an electrolyte of dilute sulphuric acid or zinc sulphate solution in a porous pot, which stands in a solution of copper sulphate in which the copper electrode is immersed. While the reaction takes place ions move through the porous pot, but when it is not in use the cell should be dismantled to prevent the diffusion of one electrolyte into the other. The e.m.f. of the cell is 1.08 volts with sulphuric acid and 1.10 volts with zinc sulphate. It was invented in 1836 by the British chemist John Daniell (1790–1845).
darmstadtium Symbol Ds. A radioactive transactinide element; a.n. 110. It has several isotopes; the most stable being 281Ds, with a half-life of about 1.6 minutes. It can be produced by bombarding a plutonium target with sulphur nuclei or by bombarding a lead target with nickel nuclei. Darmstadtium was named after the German city of Darmstadt, the location of the Institute for Heavy Ion Research where it was Ürst produced.
Daphnia A genus of crustaceans belonging to the class Branchiopoda and order Cladocera (water Ûeas). Daphnia species have a transparent carapace and a protruding head with a pair of highly branched antennae for swimming and a single median compound eye. The Üve pairs of thoracic appendages form an efÜcient Ülter-feeding mechanism. Reproduction can take place without mating, i.e. by *parthenogenesis. dark energy Energy in the universe associated with the fact that the expansion of the universe is accelerating and the *cosmological constant could have a non-zero value. Analysis of data from *WMAP indicates that about 70% of the energy of the universe is in the form of dark energy. The nature of dark energy is not known. dark galaxy A galaxy that is composed
• The home page of the Cryogenic Dark Matter Search Experiment
dark period (in botany) The period considered to be critical in the responses of plants to changes in day length (see ). It is believed that such responses, which include the onset of Ûowering, are determined by the length of the period of darkness that occurs between two periods of light. dark reaction See .
Darwin, Charles (1809–82) British naturalist, who studied medicine in Edinburgh followed by theology at Cambridge University, intending a career in the Church. However, his interest in natural history led him to accept an invitation in 1831 to join HMS Beagle as naturalist on a round-the-world voyage. After his return Üve years later he published works on the geology he had observed. He was also formulating his theory of *evolution by means of *natural selection, but it was to be 20 years before he published On the Origin of Species (1859), prompted by similar views expressed by Alfred Russel *Wallace. Among his later works was The Descent of Man (1871). See also D.
• The complete works of Charles Darwin online
Darwinism The theory of *evolution proposed by Charles Darwin in On the Origin of Species (1859), which postulated that present-day species have evolved from simpler ancestral types by the process of *natural se-
Darwin's Ünches
lection acting on the variability found within populations. On the Origin of Species caused a furore when it was Ürst published because it suggested that species are not immutable nor were they specially created – a view directly opposed to the doctrine of *special creation. However the wealth of evidence presented by Darwin gradually convinced most people and the only major unresolved problem was to explain how the variations in populations arose and were maintained from one generation to the next. This became clear with the rediscovery of Mendel's work on classical genetics in the 1900s and led to the present theory known as *neoDarwinism.
Darwin's Ünches (Galapagos Ünches) The 14 species of Ünch, unique to the Galapagos Islands, that Charles Darwin studied during his journey on HMS Beagle. Each is adapted to exploit a different food source. They are not found on the mainland because competition there for these food sources from other birds is Üercer. Darwin believed all the Galapagos Ünches – basically similar but differing in bill shape – to be descendants of a few that strayed from the mainland, and this provided important evidence for his theory of evolution. See also  . DAT (digital audio tape) A type of magnetic tape originally designed for audio recording but now adapted for computer storage and backup use. The recording method allows a capacity of about 1 gigabyte. database A large collection of information that has been coded and stored in a computer in such a way that it can be extracted under a number of different category headings. date-rape drugs Drugs that are used to render the victim unable to resist a sexual assault. Particular drugs used for this purpose are Ûunitrazepam and gammahydroxybutyric acid (GHB). dating techniques Methods of estimating the age of rocks, palaeontological specimens, archaeological sites, etc. Relative dating techniques date specimens in relation to one another; for example, *stratigraphy is used to establish the succession of fossils. Absolute (or chronometric) techniques give an absolute estimate of the age and fall into two main groups. The Ürst de-
pends on the existence of something that develops at a seasonally varying rate, as in *dendrochronology and *varve dating. The other uses some measurable change that occurs at a known rate, as in *chemical dating, radioactive (or radiometric) dating (see  ; - ; – ; – ; – ), and *thermoluminescence.
dative bond See  . daughter 1. A nuclide produced by radioactive *decay of some other nuclide (the parent). 2. An ion or free radical produced by dissociation or reaction of some other (parent) ion or radical. Davy, Sir Humphry (1778–1829) British chemist, who studied gases at the Pneumatic Institute in Bristol, where he discovered the anaesthetic properties of *dinitrogen oxide (nitrous oxide). He moved to the Royal Institution, London, in 1801 and Üve years later isolated potassium and sodium by electrolysis. He also prepared barium, boron, calcium, and strontium as well as proving that chlorine and iodine are elements. In 1816 he invented the *Davy lamp. Davy lamp An oil-burning miner's safety lamp invented by Sir Humphry Davy in 1816 when investigating Üredamp (methane) explosions in coal mines. The lamp has a metal gauze surrounding the Ûame, which cools the hot gases by conduction and prevents ignition of gas outside the gauze. If Üredamp is present it burns within the gauze cage, and lamps of this type are still used for testing for gas. day The time taken for the earth to complete one rotation on its axis. The solar day is the interval between two successive returns of the sun to the *meridian. The mean solar day of 24 hours is the average value of the solar day for one year. The sidereal day is measured with respect to the Üxed stars and is 4.09 minutes shorter than the mean solar day as a result of the imposition of the earth's orbital motion on its rotational motion. day-neutral plant A plant in which Ûowering can occur irrespective of the day length. Examples are cucumber and maize. See . Compare - ; - . d-block elements The block of elements
in the *periodic table consisting of scandium, yttrium, and lanthanum together with the three periods of transition elements: titanium to zinc, zirconium to cadmium, and hafnium to mercury. These elements all have two outer s-electrons and have d-electrons in their penultimate shell; i.e. an outer electron conÜguration of the form (n–1)dxns2, where x is 1 to 10. See also  .
d.c. See  . DDT Dichlorodiphenyltrichloroethane; a colourless organic crystalline compound, (ClC6H4)2CH(CCl3), made by the reaction of trichloromethanal with chlorobenzene. DDT is the best known of a number of chlorinecontaining pesticides used extensively in agriculture in the 1940s and 1950s. The compound is stable, accumulates in the soil, and concentrates in fatty tissue, reaching dangerous levels in carnivores high in the food chain. Restrictions are now placed on the use of DDT and similar pesticides. Deacon process A former process for making chlorine by oxidizing hydrogen chloride in air at 450°C using a copper chloride catalyst. It was patented in 1870 by Henry Deacon (1822–76). deactivation A partial or complete reduction in the reactivity of a substance, as in the poisoning of a catalyst. deadbeat See . deamination The removal of an amino group (–NH2) from a compound. Enzymatic deamination occurs in the liver and is important in amino-acid metabolism, especially in their degradation and subsequent oxidation. The amino group is removed as ammonia and excreted, either unchanged or as urea or uric acid. death The point at which the processes that maintain an organism alive no longer function. In humans it is diagnosed by permanent cessation of the heartbeat; however, the heart can continue beating after a large part of the brain ceases to function (see  ). The death of a cell due to external damage or the action of toxic substances is known as necrosis. This must be distinguished from programmed cell death (see ), which is a normal part of the developmental process. death phase See   .
Debye–Scherrer method death rate (mortality) The rate at which a particular species or population dies, whatever the cause. The death rate is an important factor in controlling the size of a population. Compare  . de Broglie, Louis-Victor Pierre Raymond (1892–1987) French physicist, who taught at the Sorbonne in Paris for 34 years. He is best known for his 1923 theory of *wave–particle duality (see also  B ), which reconciled the corpuscular and wave theories of *light and proved important in *quantum theory. For this work he was awarded the 1929 Nobel Prize. de Broglie wavelength The wavelength of the wave associated with a moving particle. The wavelength (λ) is given by λ = h/mv, where h is the Planck constant, m is the mass of the particle, and v its velocity. The de Broglie wave was Ürst suggested by Louis de Broglie in 1923 on the grounds that electromagnetic waves can be treated as particles (*photons) and one could therefore expect particles to behave in some circumstances like waves (see ). The subsequent observation of *electron diffraction substantiated this argument and the de Broglie wave became the basis of *wave mechanics. debye A unit of electric *dipole moment in the electrostatic system, used to express dipole moments of molecules. It is the dipole moment produced by two charges of opposite sign, each of 1 statcoulomb and placed 10–18 cm apart, and has the value 3.335 64 × 10–30 coulomb metre. Debye–Hückel theory A theory to explain the nonideal behaviour of electrolytes, published in 1923 by Peter Debye (1884–1966) and Erich Hückel (1896– ). It assumes that electrolytes in solution are fully dissociated and that nonideal behaviour arises because of electrostatic interactions between the ions. The theory shows how to calculate the extra free energy per ion resulting from such interactions, and consequently the activity coefÜcient. It gives a good description of nonideal electrolyte behaviour for very dilute solutions, but cannot be used for more concentrated electrolytes. Debye–Scherrer method A method of Xray diffraction in which a beam of X-rays is diffracted by material in the form of powder. Since the powder consists of very small crystals of the material in all possible orienta-
decations, the diffraction pattern is a series of concentric circles. This type of pattern allows the unit cell to be found with great precision. This method was Ürst used by Peter Debye and Paul Scherrer in 1916 and independently by Albert Hull in 1917.
deca- Symbol da. A preÜx used in the metric system to denote ten times. For example, 10 coulombs = 1 decacoulomb (daC). decahydrate A crystalline hydrate containing ten molecules of water per molecule of compound. decalescence See . decanedioic acid (sebacic acid) A white crystalline dicarboxylic acid, HOOC(CH2)8COOH; r.d. 1.12; m.p. 131–134.5°C; b.p. 294.4°C (100 mmHG). Obtained from castor oil, it is used in plasticizers, lubricants, and cosmetics and in the production of other organic chemicals. decanoic acid (capric acid) A white crystalline straight-chain saturated *carboxylic acid, CH3(CH2)8COOH; m.p. 31.5°C. Its esters are used in perfumes and Ûavourings. decantation The process of separating a liquid from a settled solid suspension or from a heavier immiscible liquid by carefully pouring it into a different container. Decapoda An order of crustaceans of the class Malacostraca that are distributed worldwide, mainly in marine habitats. Decapods comprise swimming forms (shrimps and prawns) and crawling forms (crabs, lobsters, and crayÜsh). All are characterized by Üve pairs of walking legs, the Ürst pair of which are highly modiÜed in crawling forms to form powerful grasping pincers. The carapace is fused with the thorax and head forming a *cephalothorax. The antennae are especially long in shrimps and prawns, which also possess several pairs of welldeveloped swimming appendages (pleopods) posterior to the walking legs. Following fertilization by the male, females usually carry the eggs until they hatch. The larvae undergo several transformations before attaining adult form. decarboxylation The removal of carbon dioxide from a molecule. Decarboxylation is an important reaction in many biochemical processes, such as the *Krebs cycle and the synthesis of *fatty acids. See also  .
decay 1. See . 2. The spontaneous transformation of one radioactive nuclide into a daughter nuclide, which may be radioactive or may not, with the emission of one or more particles or photons. The decay of N0 nuclides to give N nuclides after time t is given by N = N0exp(–γt), where γ is called the decay constant or the disintegration constant. The reciprocal of the decay constant is the mean life. The time required for half the original nuclides to decay (i.e. N = ½N0) is called the half-life of the nuclide. The same terms are applied to elementary particles that spontaneously transform into other particles. For example, a free neutron decays into a proton and an electron (see  ). See also  .
• Values of radionuclide half-lives at the NIST website
deci- Symbol d. A preÜx used in the metric system to denote one tenth. For example, 0.1 coulomb = 1 decicoulomb (dC); 0.1 metre = 1 decimetre (dm). decibel A unit used to compare two power levels, usually applied to sound or electrical signals. Although the decibel is one tenth of a bel, it is the decibel, not the bel, that is invariably used. Two power levels P and P0 differ by n decibels when n = 10log10P/P0. If P is the level of sound intensity to be measured, P0 is a reference level, usually the intensity of a note of the same frequency at the threshold of audibility. The logarithmic scale is convenient as human audibility has a range of 1 (just audible) to 1012 (just causing pain) and one decibel, representing an increase of some 26%, is about the smallest change the ear can detect. deciduous Describing plants in which all the leaves are shed at the end of each growing season, usually the autumn in temperate regions or at the beginning of a dry season in the tropics. This seasonal leaf fall helps the plant retain water that would otherwise be lost by transpiration from the leaves. Examples of deciduous plants are rose and horse chestnut. Compare . deciduous teeth (milk teeth) The Ürst of two sets of teeth of a mammal. These teeth are smaller than those that replace them (the *permanent teeth) and fewer in number, since there are no deciduous *molars. See also . decimal system A number system based
on the number 10; the number system in common use. All rational numbers can be written as a Ünite decimal (e.g. ¼ = 0.25) or a repeating decimal (e.g. 5/27 = 0.185 185 185…). An *irrational number can be written to any number of decimal places, but can never be given exactly (e.g. √3 = 1.732 050 8…).
declination 1. The angle between the magnetic meridian and the geographic meridian at a point on the surface of the earth. See . 2. The angular distance of a celestial body north (positive) or south (negative) of the celestial *equator. decoction A solution made by boiling material (e.g. plant substances) in water, followed by Ültration. decoherence A process in which a quantum mechanical state of a system is altered by the interaction between the system and its environment. The process of decoherence has been detected experimentally. Decoherence was postulated in the 1980s and has been used to clarify discussions of the foundations of quantum mechanics. decomposer An organism that obtains energy from the chemical breakdown of dead organisms or animal or plant wastes. Decomposers, most of which are bacteria and fungi, secrete enzymes onto dead matter and then absorb the breakdown products (see ). Many decomposers (e.g. nitrifying bacteria) are specialized to break down organic materials that are difÜcult for other organisms to digest. Decomposers fulÜl a vital role in the *ecosystem, returning the constituents of organic matter to the environment in inorganic form so that they can again be assimilated by plants. Compare . See also  ;  . decomposition 1. (decay) The chemical breakdown of organic matter into its constituents by the action of *decomposers. 2. A chemical reaction in which a compound breaks down into simpler compounds or into elements. deconÜnement temperature See  . decrepitation A crackling noise produced when certain crystals are heated, caused by changes in structure resulting from loss of water of crystallization. deep-sky object Any object located out-
deforestation side our solar system that is of interest to an astronomical observer. The term usually applies to a *star cluster, *nebula, *galaxy, or *galaxy cluster, but not a star.
defecation The expulsion of faeces from the rectum due to contractions of muscles in the rectal wall. A sphincter muscle, which is under voluntary control, is situated at the end of the rectum (the anus); relaxation of this muscle allows defecation to occur. In babies control of the anal sphincter muscle has not been developed and defecation occurs automatically as a reÛex response to the presence of faeces in the rectum. defect 1. See  . 2. See  . 3. See  . deÜciency disease Any disease caused by an inadequate intake of an essential nutrient in the diet, primarily vitamins, minerals, and amino acids. Examples are scurvy (lack of vitamin C), rickets (lack of vitamin D), and iron-deÜciency anaemia. deÜnite inÛorescence See  . deÜnite integral See . deÜnite proportions See  . deÛagration A type of explosion in which the shock wave arrives before the reaction is complete (because the reaction front moves more slowly than the speed of sound in the medium). deforestation The extensive cutting down of forests for the purpose of extracting timber or fuel wood or to clear the land for mining or agriculture. Forests are often situated in upland areas and are important in trapping rainwater. Deforestation in these areas, particularly in India and Bangladesh, has resulted in the Ûooding of low-lying plains; it has also led to an increase in soil erosion and hence desert formation (see ), resulting in crop loss and economic problems for local communities. The felling and burning of trees releases large amounts of carbon dioxide, thereby increasing global carbon dioxide levels and contributing to the *greenhouse effect. Rainforests, particularly those of South America, are rich in both fauna and Ûora; their removal leads to an overall decrease in *biodiversity and the loss of plant species that have potentially beneÜcial pharmaceutical effects. Despite movements to reduce deforestation,
degassing economic pressures ensure that the process still continues.
degassing The removal of dissolved, absorbed, or adsorbed gases from a liquid or solid. Degassing is important in vacuum systems, where gas absorbed in the walls of the vacuum vessel starts to desorb as the pressure is lowered. degaussing The process of neutralizing the magnetization in an object that has inadvertently become magnetized. For example, ferromagnetic components of TV sets may become magnetized and misdirect the electron beams. A degaussing coil is often provided and fed with a diminishing alternating current each time the set is switched on. Ships can be degaussed by surrounding them with current-carrying cables that set up an equal and opposite Üeld. This prevents the ships from detonating magnetic mines. Degaussing is used to protect scientiÜc and other electronic devices from strong magnetic Üelds; usually a system of coils is designed to neutralize such Üelds over the important region or the equipment is surrounded by a shield of suitable alloy (e.g. Mumetal). degeneracy pressure The pressure in a *degenerate gas of fermions caused by the Pauli exclusion principle and the Heisenberg uncertainty principle. Because of the exclusion principle, fermions at a high density, with small interparticle spacing, must have different momenta. Because of the uncertainty principle, the momentum difference must be inversely proportional to the spacing. Consequently, in a high-density gas (small spacing) the particles have high relative momenta, which leads to a degeneracy pressure much greater than the thermal pressure. *White dwarfs and *neutron stars are supported against collapse under their own gravitational Üelds by the degeneracy pressure of electrons and neutrons respectively. degenerate 1. Having quantum states with the same energy. For example, the Üve d-orbitals in an isolated transition-metal atom have the same energy (although they have different spatial arrangements) and are thus degenerate levels. The application of a magnetic or electric Üeld may cause the quantum states to have different energies (see - ). In this case, the degeneracy is said to be 'lifted'. 2. Having a high particle concentration such that the
*Maxwell–Boltzmann distribution does not apply and the behaviour of the particles is governed by *quantum statistics. Systems in which the particles are degenerate are called degenerate gases; examples are the conduction electrons in a metal (or *degenerate semiconductor), the electrons in a *white dwarf, and the neutrons in a *neutron star. See also  .
degenerate semiconductor A heavily doped *semiconductor in which the *Fermi level is located in either the valence band or the conduction band (see  ) causing the material to behave as a metal. degenerate states *Quantum states of a system that have the same energy. degeneration 1. Changes in cells, tissues, or organs due to disease, etc., that result in an impairment or loss of function and possibly death and breakdown of the affected part. 2. The reduction in size or complete loss of organs during evolution. The human appendix has undergone this process and performs no obvious function in man. Degeneration of external organs may cause animals to appear to be more primitive than they really are; for example, early zoologists believed whales were Üsh rather than mammals because of the degeneration of their limbs. See also  . deglutition (swallowing) A reÛex action initiated by the presence of food in the pharynx. During deglutition, the soft *palate is raised, which prevents food from entering the nasal cavity; the *epiglottis closes, which blocks the entrance to the windpipe; and the oesophagus starts to contract (see ), which ensures that food is conveyed to the stomach. degradation A type of organic chemical reaction in which a compound is converted into a simpler compound. An example is the *Barbier–Wieland degradation. degree 1. A unit of plane angle equal to 1/360th of a complete revolution. 2. A division on a *temperature scale. 3. The power to which a variable is raised. If one expression contains several variables the overall degree of the expression is the sum of the powers. For example, the expression p2q3r4 has a degree of 9 overall (it is a seconddegree expression in p). The degree of a polynomial is the degree of the variable with the highest power, e.g. ax5 + bx4 + c has a degree of 5. 4. The highest power to which the
derivative of the highest order is raised in a *differential equation. For example, (d2y/dx2)3 + dy/dx = c is a differential equation of the third degree (but second order).
Examples are the splitting of laburnum pods and primrose capsules; such structures are described as dehiscent (compare ).
degrees of freedom 1. The number of independent parameters required to specify the conÜguration of a system. This concept is applied in the *kinetic theory to specify the number of independent ways in which an atom or molecule can take up energy. There are however various sets of parameters that may be chosen, and the details of the consequent theory vary with the choice. For example, in a monatomic gas each atom may be allotted three degrees of freedom, corresponding to the three coordinates in space required to specify its position. The mean energy per atom for each degree of freedom is the same, according to the principle of the *equipartition of energy, and is equal to kT/2 for each degree of freedom (where k is the *Boltzmann constant and T is the thermodynamic temperature). Thus for a monatomic gas the total molar energy is 3LkT/2, where L is the Avogadro constant (the number of atoms per mole). As k = R/L, where R is the molar gas constant, the total molar energy is 3RT/2. In a diatomic gas the two atoms require six coordinates between them, giving six degrees of freedom. Commonly these are interpreted as six independent ways of storing energy: on this basis the molecule has three degrees of freedom for different directions of translational motion, and in addition there are two degrees of freedom for rotation of the molecular axis and one vibrational degree of freedom along the bond between the atoms. The rotational degrees of freedom each contribute kT/2, to the total energy; similarly the vibrational degree of freedom has an equal share of kinetic energy and must on average have as much potential energy (see   ). The total energy per molecule for a diatomic gas is therefore 3kT/2 (for translational energy of the whole molecule) plus 2kT/2 (for rotational energy of each atom) plus 2kT/2 (for vibrational energy), i.e. a total of 7kT/2. 2. The least number of independent variables required to deÜne the state of a system in the *phase rule. In this sense a gas has two degrees of freedom (e.g. temperature and pressure).
dehydration 1. Removal of water from a substance. 2. A chemical reaction in which a compound loses hydrogen and oxygen in the ratio 2:1. For instance, ethanol passed over hot pumice undergoes dehydration to ethene: C2H5OH – H2O → CH2:CH2 Substances such as concentrated sulphuric acid, which can remove H2O in this way, are known as dehydrating agents. For example, with sulphuric acid, methanoic acid gives carbon monoxide: HCOOH – H2O → CO
dehiscence The spontaneous and often violent opening of a fruit, seed pod, or anther to release and disperse the seeds or pollen.
dehydrogenase Any enzyme that catalyses the removal of hydrogen atoms (dehydrogenation) in biological reactions. Dehydrogenases occur in many biochemical pathways but are particularly important in driving the reactions of the *electrontransport chain in cell respiration. They work in conjunction with the hydrogenaccepting coenzymes *NAD and *FAD. dehydrogenation A chemical reaction in which hydrogen is removed from a compound. Dehydrogenation of organic compounds converts single carbon–carbon bonds into double bonds. It is usually effected by means of a metal catalyst. dehydrohalogenation A type of chemical reaction in which a hydrogen halide is removed from a molecule with formation of a double bond. A simple example is the formation of ethene from chloroethane using alcoholic potassium hydroxide: CH3CH2Cl + KOH → CH2=CH2 + KCl + H2O. deionized water Water from which dissolved ionic salts have been removed by *ion-exchange techniques. It is used for many purposes as an alternative to distilled water. dekatron A neon-Ülled tube with a central anode surrounded by ten cathodes and associated transfer electrodes. As voltage pulses are received by the tube a glow discharge moves from one set of electrodes to the next, enabling the device to be used as a visual counting tube in the decimal system. The tube can also be used for switching.
delayed neutrons delayed neutrons The small proportion of neutrons that are emitted with a measurable time delay in a nuclear Üssion process. Compare  .
delay line A component in an electronic circuit that is introduced to provide a speciÜed delay in transmitting the signal. Coaxial cable or inductor-capacitor networks can be used to provide a short delay but for longer delays an acoustic delay line is required. In this device the signal is converted by the *piezoelectric effect into an acoustic wave, which is passed through a liquid or solid medium, before reconversion to an electronic signal. deletion (in genetics) 1. A *point mutation involving the removal of one or more base pairs in the DNA sequence. 2. A frequently lethal *chromosome mutation that arises from an inequality in *crossing over during meiosis such that one of the chromatids loses more genetic information than it receives. deliquescence The absorption of water from the atmosphere by a hygroscopic solid to such an extent that a concentrated solution of the solid eventually forms. delocalization In certain chemical compounds the valence electrons cannot be regarded as restricted to deÜnite bonds between the atoms but are 'spread' over several atoms in the molecule. Such electrons are said to be delocalized. Delocalization occurs particularly when the compound contains alternating (conjugated) double or triple bonds, the delocalized electrons being those in the pi *orbitals. The molecule is then more stable than it would be if the electrons were localized, an effect accounting for the properties of benzene and other aromatic compounds. Another example is in the ions of carboxylic acids, containing the carboxylate group –COO–. In terms of a simple model of chemical bonding, this group would have the carbon joined to one oxygen by a double bond (i.e. C=O) and the other joined to O– by a single bond (C–O–). In fact, the two C–O bonds are identical because the extra electron on the O– and the electrons in the pi bond of C=O are delocalized over the three atoms. delta A fan-shaped area of sediment deposited at the mouth of a river, where it enters a lake or the sea. Usually the river divides and subdivides into many smaller
channels (distributaries), sometimes depositing bars and building up levees. Marshes may border the delta. The sediment may range in particle size, consisting of gravel, sand, silt, or clay. The larger material is deposited Ürst as the load-carrying capacity of the river diminishes, with the clay being deposited last.
delta-brass A strong hard type of *brass that contains, in addition to copper and zinc, a small percentage of iron. It is mainly used for making cartridge cases. deltahedron A polyhedron that has triangular faces. See W' . delta-iron See . demagnetization The removal of the ferromagnetic properties of a body by disordering the domain structure (see ). One method of achieving this is to insert the body within a coil through which an alternating current is Ûowing; as the magnitude of the current is reduced to zero, the domains are left with no predominant direction of magnetization. deme A group of organisms in the same *taxon. The term is used with various preÜxes that denote how the group differs from other groups. For example, an ecodeme occurs in a particular ecological habitat, cytodemes differ from each other cytologically, and genodemes differ genetically. demodulation The process of extracting the information from a modulated carrier wave (see ; ). The device used is called a demodulator or a detector. de Moivre's theorem For *complex numbers in polar form, the equation (cosθ + i sinθ)n = cosnθ + i sinnθ. It is true for all integer values of n. There is at least one value of the left-hand side of the equation that makes it also true for noninteger values of n. It was named after the French mathematician Abraham de Moivre (1667–1754). denature 1. To add a poisonous or unpleasant substance to ethanol to make it unsuitable for human consumption (see  ). 2. To produce a structural change in a protein or nucleic acid that results in the reduction or loss of its biological properties. Denaturation is caused by heat, chemicals, and extremes of pH. The
dental formula
differences between raw and boiled eggs are largely a result of denaturation. 3. To add another isotope to a Üssile material to make it unsuitable for use in a nuclear weapon.
dendrimer (dendritic polymer) A type of macromolecule in which a number of chains radiate out from a central atom or cluster of atoms. Dendritic polymers have a number of possible applications. See also  . dendrite 1. (in chemistry) A crystal that has branched in growth into two parts. Crystals that grow in this way (dendritic growth) have a branching treelike appearance. 2. (in neurology) Any of the slender branching processes that arise from the *dendrons of the cell body of a motor *neuron. It forms connections (see ) with the axons of other neurons and transmits nerve impulses from these to the cell body. dendrochronology An absolute *dating technique using the *growth rings of trees. It depends on the fact that trees in the same locality show a characteristic pattern of growth rings resulting from climatic conditions. Thus it is possible to assign a deÜnite date for each growth ring in living trees, and to use the ring patterns to date fossil trees or specimens of wood (e.g. used for buildings or objects on archaeological sites) with lifespans that overlap those of living trees. The bristlecone pine (Pinus aristata), which lives for up to 5000 years, has been used to date specimens over 8000 years old. Fossil specimens accurately dated by dendrochronology have been used to make corrections to the *carbon-dating technique. Dendrochronology is also helpful in studying past climatic conditions. Analysis of trace elements in sections of rings can also provide information on past atmospheric pollution. dendron Any of the major cytoplasmic processes that arise from the cell body of a motor neuron. A dendron usually branches into *dendrites. denitriÜcation A chemical process in which nitrates in the soil are reduced to molecular nitrogen, which is released into the atmosphere. This process is effected by denitrifying bacteria (e.g. Pseudomonas denitriÜcans), which use nitrates as a source of energy for other chemical reactions in a manner similar to respiration in other organisms. Compare . See  .
densitometer An instrument used to measure the *photographic density of an image on a Ülm or photographic print. Densitometers work by letting the specimen transmit or reÛect a beam of light and monitoring the transmitted or reÛected intensity. They originally consisted of visual *photometers but most instruments are now photoelectric. The simplest transmission densitometer consists of a light source, a photosensitive cell, and a microammeter: the density is measured in terms of the meter readings with and without the sample in place. They have a variety of uses, including detecting the sound track on a cinematic Ülm, measuring intensities in spectrographic records, and checking photographic prints. density 1. The mass of a substance per unit of volume. In *SI units it is measured in kg m–3. See also  ;  . 2. See  . 3. See  .
• Values of densities of commonly used materials at the NPL website
density functional theory A theory used to describe many-fermion systems in which the energy is a *functional of the density of fermions. Density functional theory has been used extensively in the theory of electrons in atoms, molecules, and solids, and the theory of nucleons in nuclei. dental caries Tooth decay, which involves the destruction of the enamel layer of the tooth by acids produced by the action of bacteria on sugar. Bacteria can bind to teeth on dextran, a sticky substance derived from sucrose. The bacterial cells and other waste attached to dextran gives rise to *plaque. If dental caries is not treated it can spread to the dentine and pulp of the tooth, which leads to infection and death of the tooth. dental formula A representation of the dentition of an animal. A dental formula consists of eight numbers, four above and four below a horizontal line. The numbers represent (from left to right) the numbers of incisors, canines, premolars, and molars in either half of the upper and lower jaws. The total number of teeth in both jaws is therefore obtained by adding up all the numbers in the dental formula and multiplying by 2. Representative dental formulas are shown in the illustration overleaf. See also  .
denticle
(32 teeth)
Representive dental formulas. denticle (placoid scale) See .
dentine The bony material that forms the bulk of a *tooth. Dentine is similar in composition to bone but is perforated with many tiny canals for nerve Übres, blood capillaries, and processes of the dentine-forming cells (*odontoblasts). Ivory, the material that forms elephant tusks, is made of dentine. dentition The type, number, and arrangement of teeth in a species. This can be represented concisely by a *dental formula. See also  ; ; ; ; ; . deoxyribonuclease See DN. deoxyribonucleic acid See DNA. deoxyribose (2-deoxyribose) A pentose (Üve-carbon) sugar, a derivative of *ribose, that is a component of the nucleotides (deoxyribonucleotides) that form the building blocks of *DNA. depleted Denoting a material that contains less of a particular isotope than it normally contains, especially a residue from a nuclear reactor or isotope-separation plant. depleted uranium Uranium mostly consisting of uranium–238, obtained as a byproduct of enriching natural uranium or obtained from reprocessing plants. It has a high density and is used in armour-piercing shells. depletion layer A region in a *semiconductor that has a lower-than-usual number of mobile charge carriers. A depletion layer forms at the interface between two dissimilar regions of conductivity (e.g. a p–n junction). See . depolarization 1. (in physics) The prevention of *polarization in a *primary cell. For example, maganese(IV) oxide (the depolarizer) is placed around the positive electrode of a *Leclanché cell to oxidize the hydrogen released at this electrode. 2. (in neurophysiology) A reduction in the difference of electrical potential that exists across the plasma membrane of a nerve or muscle cell. Depolarization of a nerve-cell membrane occurs during the passage of an *ac-
tion potential along the axon when the nerve is transmitting an impulse.
deposition The laying down of material on the earth's surface. This includes materials that have been eroded elsewhere and transported by natural agents, such as rivers, wind, ice (e.g. glaciers), and the tides and currents in the sea. It also includes the deposition of materials resulting from chemical precipitation, the formation of crusts on the earth's surface through evaporation, and the growth, accumulation, and decay of natural organisms. depression (low; disturbance) An area of low atmospheric pressure in the mid- and high-latitudes that is shown on a synoptic chart (weather map) surrounded by several closed isobars. Depressions generally move towards the northeast in the northern hemisphere and towards the southeast in the southern hemisphere. Unsettled weather conditions are associated with depressions and they form the main source of precipitation in most lowland parts of the midlatitudes. depression of freezing point The reduction in the freezing point of a pure liquid when another substance is dissolved in it. It is a *colligative property – i.e. the lowering of the freezing point is proportional to the number of dissolved particles (molecules or ions), and does not depend on their nature. It is given by ∆t = KfCm, where Cm is the molar concentration of dissolved solute and Kf is a constant (the cryoscopic constant) for the solvent used. Measurements of freezingpoint depression (using a Beckmann thermometer) can be used for Ünding relative molecular masses of unknown substances.
• Raoult's original paper
depsides A class of compounds formed by condensation of a phenolic carboxylic acid (such as gallic acid) with a similar compound, the reaction being between the carboxylic acid group on one molecule and a phenolic –OH group on the other. Depsides are similar to esters, except that the –OH group is linked directly to an aromatic ring. In such compounds, the –O–CO– group is
called a depside linkage. Depsides are found in tannins and other natural products.
depth of Üeld The range of distance in front of and behind an object that is being focused by an optical instrument, such as a microscope or camera, within which other objects will be in focus. The depth of focus is the amount by which the distance between the camera and the Ülm can be changed without upsetting the sharpness of the image. derivative 1. (in chemistry) A compound that is derived from some other compound and usually maintains its general structure, e.g. trichloromethane (chloroform) is a derivative of methane. 2. (in mathematics) See ; . derived unit See  . dermal bone See  . Dermaptera An order of insects comprising the earwigs. Earwigs typically have long thin cylindrical bodies with biting mouthparts and a stout pair of curved forceps at the tip of the abdomen, used for catching prey and in courtship. Some species have a single pair of wings, which at rest are folded back over the abdomen like a fan; others are wingless. Most earwigs are nocturnal and omnivorous. dermis (corium; cutis) The thicker and innermost layer of the *skin of vertebrates, the other layer being the *epidermis. The dermis consists of Übrous connective tissue in which are embedded blood vessels, sensory nerve endings, and (in mammals) hair follicles, sebaceous glands, and sweat ducts. Beneath the dermis lies the *subcutaneous tissue. desalination The removal of salt from sea water for irrigation of the land or to provide drinking water. The process is normally only economic if a cheap source of energy, such as the waste heat from a nuclear power station, can be used. Desalination using solar energy has the greatest economic potential since shortage of fresh water is most acute in hot regions. The methods employed include evaporation, often under reduced pressure (Ûash evaporation); freezing (pure ice forms from freezing brine); *reverse osmosis; *electrodialysis; and *ion exchange. desert A major terrestrial *biome characterized by low rainfall. Hot deserts, such as the Sahara and Kalahari deserts of Africa,
desmids have a rainfall of less than 25 cm a year and extremely high daytime temperatures (up to 36°C). Vegetation is sparse, and desert plants are adapted to conserve water and take advantage of the rain when it falls. The perennials include xerophytic trees and shrubs (see ) and *succulents, such as cacti. Annual plants are *ephemerals, lying dormant as seeds for most of the year and completing their life cycle in the brief rainy periods. Desert animals are typically nocturnal or active at dawn and dusk, thus avoiding the extreme daytime temperatures.
desertiÜcation The gradual conversion of fertile land into desert, usually as a result of human activities. Loss of topsoil leads to further soil erosion until the land can no longer be used to grow crops or support livestock. A major factor contributing to desertiÜcation is bad management of farmland. Overgrazing of livestock removes the plant cover and exposes the soil, making it vulnerable to erosion. Overintensive cultivation of crop plants, especially monoculture (see ), depletes the soil of nutrients and organic matter, resulting in loss of fertility and increasing its susceptibility to erosion. In many Third World countries it is difÜcult to control the process of desertiÜcation as the livelihood of the people often depends on practices that contribute to soil erosion. Another major cause of desertiÜcation is *deforestation. Loss of vegetation also results from reductions in rainfall resulting from climate change. desiccation A method of preserving organic material by the removal of its water content. Cells and tissues can be preserved by desiccation after lowering the samples to freezing temperatures; thereafter they can be stored at room temperature. desiccator A container for drying substances or for keeping them free from moisture. Simple laboratory desiccators are glass vessels containing a drying agent, such as silica gel. They can be evacuated through a tap in the lid. desmids Unicellular mainly freshwater green algae that belong to the class Desmidioideae. Like Spirogyra, they have an elaborate chloroplast. The cells of desmids are characteristically split into two halves joined by a narrow neck, each half being a mirror image of the other. The outer wall of the cell is patterned with various protuberances and
desmotubule covered with a mucilaginous sheath, which may function in the cell's gliding movement.
desmotubule See . desorption The removal of adsorbed atoms, molecules, or ions from a surface.
destructive distillation The process of heating complex organic substances in the absence of air so that they break down into a mixture of volatile products, which are condensed and collected. At one time the destructive distillation of coal (to give coke, coal tar, and coal gas) was the principal source of industrial organic chemicals. detailed balance The cancellation of the effect of one process by another process that operates at the same time with the opposite effect. An example of detailed balance is provided by a chemical reaction between two molecular species A and B, which results in the formation of the molecular species C and D. Detailed balance for this chemical reaction occurs if the rate at which the reaction A + B → C + D occurs is equal to the rate at which the reaction C + D → A + B occurs. The equilibrium state in thermodynamics is characterized by detailed balance. When there is detailed balance in a system, the *self-organization far from equilibrium associated with *non-equilibrium statistical mechanics cannot occur. detector 1. See . 2. See . detergent A substance added to water to improve its cleaning properties. Although water is a powerful solvent for many compounds, it will not dissolve grease and natural oils. Detergents are compounds that cause such nonpolar substances to go into solution in water. *Soap is the original example, owing its action to the presence of ions formed from long-chain fatty acids (e.g. the octadecanoate (stearate) ion, CH3(CH2)16COO–). These have two parts: a nonpolar part (the hydrocarbon chain), which attaches to the grease; and a polar part (the –COO– group), which is attracted to the water. A disadvantage of soap is that it forms a scum with hard water (see   ) and is relatively expensive to make. Various synthetic ('soapless') detergents have been developed from petrochemicals. The commonest, used in washing powders, is sodium dodecylbenzenesulphonate, which contains CH3(CH2)11C6H4SO2O– ions. This, like soap,
is an example of an anionic detergent, i.e. one in which the active part is a negative ion. Cationic detergents have a long hydrocarbon chain connected to a positive ion. Usually they are amine salts, as in CH3(CH2)15N(CH3)3+Br–, in which the polar part is the –N(CH3)3+ group. Nonionic detergents have nonionic polar groups of the type –C2H4–O–C2H4–OH, which form hydrogen bonds with the water. Synthetic detergents are also used as wetting agents, emulsiÜers, and stabilizers for foam.
determinant A *scalar quantity written |A| or det A. It represents a particular deÜned sum of products of a square *matrix. The determinant |A| expands as | ac bd| = ad – bc. determined Describing embryonic tissue at a stage when it can develop only as a certain kind of tissue (rather than as any kind). detonating gas A mixture of hydrogen and oxygen in the ratio two parts hydrogen to one part oxygen by volume, produced by electrolysis of water. When sparked or ignited it explodes to produce water. detoxiÜcation (detoxication) The process by which harmful compounds, such as drugs and poisons, are converted to less toxic compounds in the body. DetoxiÜcation is an important function of the *liver. See also  I ;  II . detritivore An animal that feeds on *detritus. Examples of detritivores are earthworms, blowÛies, maggots, and woodlice. Detritivores play an important role in the breakdown of organic matter from decomposing animals and plants (see ). detritus Particles of organic material derived from dead and decomposing organisms, resulting from the activities of the *decomposers. Detritus is the source of food for *detritivores, which can themselves be eaten by carnivores in a detritus food chain: detritus → detritivore → carnivore. deuterated compound A compound in which some or all of the hydrogen–1 atoms have been replaced by deuterium atoms. deuterium (heavy hydrogen) Symbol D. The isotope of hydrogen that has a mass number 2 (r.a.m. 2.0144). Its nucleus contains one proton and one neutron. The abundance of deuterium in natural hydro-
Dewar Ûask
gen is about 0.015%. It is present in water as the oxide HDO (see also  ), from which it is usually obtained by electrolysis or fractional distillation. Its chemical behaviour is almost identical to hydrogen although deuterium compounds tend to react rather more slowly than the corresponding hydrogen compounds. Its physical properties are slightly different from those of hydrogen, e.g. b.p. 23.6 K (hydrogen 20.4 K).
deuterium oxide See  . Deuteromycota A phylum formerly used in some classiÜcations to include all fungi in which sexual reproduction is absent. Also known as Fungi Imperfecti ('imperfect fungi'). They are now regarded as ascomycetes or basidiomycetes that have lost the ability to produce asci or basidia, respectively. Examples of these fungi are the Penicillium moulds. deuteron A nucleus of a deuterium atom, consisting of a proton and a neutron bound together. Devarda's alloy An alloy of copper (50%), aluminium (45%) and zinc (5%), used in chemical tests for the nitrate ion (in alkaline solutions it reduces a nitrate to ammonia). development (in biology) The complex process of growth and maturation that occurs in living organisms. Cell division and differentiation are important processes in development. In vertebrate animals there are three developmental stages: (1) *cleavage, in which the zygote divides to form a ball of cells, the *blastula; (2) gastrulation, in which the cells become arranged in three primary *germ layers (see ); (3) organogenesis (or organogeny), in which further cell division and differentiation results in the formation of organs. The development of many invertebrates (e.g. insects) and amphibians involves the process of *metamorphosis. See also ;  . deviation 1. (angle of deviation) The angle formed between a ray of light falling on a surface or transparent body and the ray leaving it. 2. The difference between one of an observed set of values and the true value, usually represented by the mean of all the observed values. The mean deviation is the mean of all the individual deviations of the set. See  .
devitriÜcation Loss of the amorphous nature of glass as a result of crystallization. Devonian A geological period in the Palaeozoic era that extended from the end of the Silurian (about 416 million years ago) to the beginning of the Carboniferous (about 360 million years ago). It was named by Adam Sedgwick (1785–1873) and Roderick Murchison (1792–1871) in 1839. The Devonian is divided into seven stages based on invertebrate fossil remains, such as corals, brachiopods, ammonoids, and crinoids, found in marine deposits. There were also extensive continental deposits consisting of conglomerates, red silts, and sandstones, forming the Old Red Sandstone facies. Fossils in the Old Red Sandstone include Üshes and the earliest land plants. Graptolites became extinct early in the Devonian and the trilobites declined. de Vries, Hugo See M, J G. dew See . Dewar, Sir James (1842–1923) British chemist and physicist, born in Scotland. In 1875 he became a professor at Cambridge University, while carrying out much of his experimental work at the Royal Institution in London. He began studying gases at low temperatures and in 1872 invented the *Dewar Ûask. In 1891, together with Frederick Abel (1827–1902), he developed the smokeless propellant explosive *cordite, and in 1898 was the Ürst to liquefy hydrogen. Dewar benzene An isomer of benzene, C6H6. Dewar benzene has the systematic name bicyclo[2,2,0]hexa-2,5-diene, and is a nonplanar bicyclic molecule. It has a high bond strain and will spontaneously slowly convert to benzene. The unsubstituted compound was Ürst synthesized in 1963. Dewar Ûask A vessel for storing hot or cold liquids so that they maintain their temperature independently of the surroundings. Heat transfer to the surroundings is reduced to a minimum: the walls of the vessel consist of two thin layers of glass (or, in large vessels, steel) separated by a vacuum to reduce conduction and convection; the inner surface of a glass vessel is silvered to reduce radiation; and the vessel is stoppered to prevent evaporation. It was devised around 1872 by Sir James Dewar and is also known by its Ürst trade name Thermos flask. See also .
Dewar structure
Dewar structure A proposed structure of *benzene, having a hexagonal ring of six carbon atoms with two opposite atoms joined by a long single bond across the ring and with two double C–C bonds, one on each side of the hexagon. Dewar structures contribute to the resonance hybrid of benzene. dew point The temperature at which the water vapour in the air is saturated. As the temperature falls the dew point is the point at which the vapour begins to condense as droplets of water. dew-point hygrometer See . dextrin An intermediate polysaccharide compound resulting from the hydrolysis of starch to maltose by amylase enzymes. dextrorotatory Denoting a chemical compound that rotates the plane of polarization of plane-polarized light to the right (clockwise as observed by someone facing the oncoming radiation). See  . dextrose See . diabetes See  ; . diacylglycerol See . diagenesis The processes, both chemical and physical, that modify a sediment once it has become buried. A material, such as calcite or silica, may crystallize and form a cement that binds together the particles of a sedimentary rock. Changes in temperature and pressure may convert one type of clay into another. New minerals may be deposited or formed by recrystallization, such as the formation of calcite from aragonite. diagonal relationship A relationship within the periodic table by which certain elements in the second period have a close chemical similarity to their diagonal neighbours in the next group of the third period. This is particularly noticeable with the following pairs. Lithium and magnesium: (1) both form chlorides and bromides that hydrolyse slowly and are soluble in ethanol; (2) both form colourless or slightly coloured crystalline nitrides by direct reaction with nitrogen at high temperatures; (3) both burn in air to give the normal oxide only;
(4) both form carbonates that decompose on heating. Beryllium and aluminium: (1) both form highly refractory oxides with polymorphs; (2) both form crystalline nitrides that are hydrolysed in water; (3) addition of hydroxide ion to solutions of the salts gives an amphoteric hydroxide, which is soluble in excess hydroxide giving beryllate or aluminate ions [Be(OH)4]2– and [Al(OH)4]–; (4) both form covalent halides and covalent alkyl compounds that display bridging structures; (5) both metals dissolve in alkalis. Boron and silicon: (1) both display semiconductor properties; (2) both form hydrides that are unstable in air and chlorides that hydrolyse in moist air; (3) both form acidic oxides with covalent crystal structures, which are readily incorporated along with other oxides into a wide range of glassy materials. The reason for this relationship is a combination of the trends to increase size down a group and to decrease size along a period, and a similar, but reversed, effect in electronegativity, i.e. decrease down a group and increase along a period.
diakinesis The period at the end of the Ürst prophase of *meiosis when the separation of *homologous chromosomes is almost complete and *crossing over has occurred. dialysis A method by which large molecules (such as starch or protein) and small molecules (such as glucose or amino acids) in solution may be separated by selective diffusion through a semipermeable membrane. For example, if a mixed solution of starch and glucose is placed in a closed container made of a semipermeable substance (such as Cellophane), which is then immersed in a beaker of water, the smaller glucose molecules will pass through the membrane into the water while the starch molecules remain behind. The plasma membranes of living organisms are semipermeable, and dialysis takes place naturally in the kidneys for the excretion of nitrogenous waste. An artiÜcial kidney (dialyser) utilizes the principle of dialysis by taking over the functions of diseased kidneys. diamagnetism See . 1,6-diaminohexane (hexamethylenediamine) A solid colourless amine,
diazotization
H2N(CH2)6NH2; m.p. 41°C; b.p. 204°C. It is made by oxidizing cyclohexane to hexanedioic acid, reacting this with ammonia to give the ammonium salt, and dehydrating the salt to give hexanedionitrile (NC(CH2)6CN). This is reduced with hydrogen to the diamine. The compound is used, with hexanedioic acid, for producing *nylon 6,6.
diamond The hardest known mineral (with a hardness of 10 on Mohs' scale). It is an allotropic form of pure *carbon that has crystallized in the cubic system, usually as octahedra or cubes, under great pressure. Diamond crystals may be colourless and transparent or yellow, brown, or black. They are highly prized as gemstones but also have extensive uses in industry, mainly for cutting and grinding tools. Diamonds occur in ancient volcanic pipes of kimberlite; the most important deposits are in South Africa but others are found in Tanzania, the USA, Russia, and Australia. Diamonds also occur in river deposits that have been derived from weathered kimberlite, notably in Brazil, Democratic Republic of Congo, Sierra Leone, and India. Industrial diamonds are increasingly being produced synthetically.
rated from the ends of the bone (see ) by cartilage.
diaspore A mineral form of a mixed aluminium oxide and hydroxide, AlO.OH. See  . diastase See . diastema The gap that separates the biting teeth from the grinding teeth in herbivores. It creates a space in which food can be held in readiness for the grinding action of the teeth. This space is Ülled by large canine teeth in carnivores. diastereoisomers Stereoisomers that are not identical and yet not mirror images. For instance, the d-isomer of tartaric acid and the meso-isomer constitute a pair of diastereoisomers. See  . diastole The phase of a heart beat that occurs between two contractions of the heart, during which the heart muscles relax and the ventricles Üll with blood. Compare . See  . diatomic molecule A molecule formed from two atoms (e.g. H2 or HCl). diatoms See B.
diapause A period of suspended development or growth occurring in many insects and other invertebrates during which metabolism is greatly decreased. It is often triggered by seasonal changes and regulated by an inborn rhythm and enables the animal to survive unfavourable environmental conditions so that its offspring may be produced in more favourable ones. The egg is the most common diapausal stage.
diazepam A *benzodiazepine used medically to treat anxiety, convulsions, insomnia, and alcohol withdrawal. It is widely used, and often known under its tradename Valium.
diaphragm 1. (in optics) An opaque disc with a circular aperture at its centre. Diaphragms of different sizes are used to control the total light Ûux passing through an optical system or to reduce aberration by restricting the light passing through a system to the central portion. An iris diaphragm consists of a number of overlapping crescent-shaped discs arranged so that the central aperture can be continuously varied in diameter. 2. (in anatomy) The muscular membrane that divides the thorax (chest) from the abdomen in mammals. It plays an essential role in breathing (see  ), being depressed during inhalation and raised during exhalation.
diaphysis The shaft of a mammalian limb bone, which in immature animals is sepa-
diazine See . diazo compounds Organic compounds containing two linked nitrogen compounds. The term includes *azo compounds, diazonium compounds, and also such compounds as diazomethane, CH2N2. • Information about IUPAC nomenclature
diazonium salts Unstable salts containing the ion C6H5N2+ (the diazonium ion: see formula). They are formed by *diazotization reactions.
+ N
Diazonium salts. Structure of diazonium ion C6H5N2+.
diazotization The formation of a *diazonium salt by reaction of an aromatic amine
dibasic acid with nitrous acid at low temperature (below 5°C). The nitrous acid is produced in the reaction mixture from sodium nitrite and hydrochloric acid: ArNH2 + NaNO2 + HCl → ArN+N + Cl– + Na+ + OH– + H2O
dibasic acid An *acid that has two acidic hydrogen atoms in its molecules. Sulphuric (H2SO4) and carbonic (H2CO3) acids are common examples. 1,2-dibromoethane A colourless liquid *haloalkane, BrCH2CH2Br; r.d. 2.2; m.p. 9.79°C; b.p. 131.36°C. It is made by addition of bromine to ethene and used as an additive in petrol to remove lead during combustion as the volatile lead bromide. dicarbide See . dicarboxylic acid A *carboxylic acid having two carboxyl groups in its molecules. In systematic chemical nomenclature, dicarboxylic acids are denoted by the sufÜx -dioic; e.g. hexanedioic acid, HOOC(CH2)4COOH. dichasium See  . dichlorine oxide (chlorine monoxide) A strongly oxidizing orange gas, Cl2O, made by oxidation of chlorine using mercury(II) oxide. It is the acid anhydride of chloric(I) acid. dichloroethanoic acid See  . dichloromethane (methylene chloride) A colourless, slightly toxic liquid, CH2Cl2, b.p. 41°C. It has a characteristic odour similar to that of trichloromethane (chloroform), from which it is made by heating with zinc and hydrochloric acid. It is used as a refrigerant and solvent (for paint stripping and degreasing). 2,4-dichlorophenoxyacetic acid See ,-D. dichogamy The condition in which the male and female reproductive organs of a Ûower mature at different times, thereby ensuring that self-fertilization does not occur. Compare . See also ; . dichotomous Describing the type of branching in plants that results when the growing point (apical bud) divides into two equal growing points, which in turn divide in a similar manner after a period of growth,
and so on. Dichotomous branching is common is ferns and mosses.
dichroism The property of some crystals, such as tourmaline, of selectively absorbing light vibrations in one plane while allowing light vibrations at right angles to this plane to pass through. Polaroid is a synthetic dichroic material. See . dichromate(VI) A salt containing the ion Cr2O7–. Solutions containing dichromate(VI) ions are strongly oxidizing. Dicotyledoneae In traditional classiÜcations, one of the two classes of Ûowering plants (see A), distinguished by having two seed leaves (*cotyledons) within the seed. The dicotyledons usually have leaf veins in the form of a net, a ring of vascular bundles in the stem, and Ûower parts in fours or Üves or multiples of these. They include many food plants (e.g. potatoes, peas, beans), ornamentals (e.g. roses, ivies, honeysuckles), and hardwood trees (e.g. oaks, limes, beeches). Dicots are no longer considered a taxonomically valid group. See . Compare M. Dictyoptera An order of insects (sometimes classiÜed as *Orthoptera) comprising the cockroaches (suborder Blattaria) and the mantids (suborder Mantodea), occurring mainly in tropical regions. Cockroaches are oval and Ûattened in shape; some have a single well-developed pair of wings, folded back over the abdomen at rest, while in others the wings may be reduced or absent. They are usually found in forest litter, feeding on dead organic matter, but some species, e.g. the American cockroach (Periplaneta americana), are major household pests, scavenging on starchy foods, fruits, etc. In most species the females produce capsules (oothecae) containing 16–40 eggs. These are either deposited or carried by the female during incubation. dielectric A nonconductor of electric charge in which an applied electric Üeld causes a *displacement of charge but not a Ûow of charge. Electrons within the atoms of a dielectric are, on average, displaced by an applied Üeld with respect to the nucleus, giving rise to a dipole that has an electric moment in the direction of the Üeld. The resulting stress within the dielectric is known as the electric polarization (P) and is deÜned by P = D – Eε0, where D is the dis-
placement, E is the electric Üeld strength, and ε0 is the electric constant. The dielectric constant is now called the relative *permittivity. The dielectric strength is the maximum potential gradient that can be applied to a material without causing it to break down. It is usually expressed in volts per millimetre. See also .
dielectric constant See . dielectric heating The heating of a dielectric material, such as a plastic, by applying a radio-frequency electric Üeld to it. The most common method is to treat the material as the dielectric between the plates of a capacitor. The heat produced is proportional to V2fAφ/t, where V is the applied potential difference, f its frequency, A is the area of the dielectric, t its thickness, and φ is the loss factor of the material (related to its *permittivity). Diels–Alder reaction A type of chemical reaction in which a compound containing two double bonds separated by a single bond (i.e. a conjugated *diene) adds to a suitable compound containing one double bond (known as the dienophile) to give a ring compound. In the dienophile, the double bond must have a carbonyl group on each side. It is named after the German chemists Otto Diels (1876–1954) and Kurt Alder (1902–58), who discovered it in 1928. diene An *alkene that has two double bonds in its molecule. If the two bonds are separated by one single bond, as in buta-1,3diene CH2:CHCH:CH2, the compound is a conjugated diene. dienophile See D–A . Diesel engine See - . diet The food requirements of an organism. The foods that constitute the human diet should contain vitamins, mineral salts (see  ), and dietary *Übre as well as water, carbohydrates and fats (which provide energy), and proteins (required for growth and maintenance). A balanced diet contains of the correct proportions of these *nutrients, which will vary depending on the age, sex, body size, and the level of activity of the individual. An inadequate supply of different food types in the diet can lead to *malnutrition. dietary Übre See .
diffraction grating diethyl ether See . differential calculus See . differential equation An equation in which a derivative of y with respect to x appears as well as the variables x and y. The order of a differential equation is the order of its highest derivative. The degree of the equation is the highest power present of the highest-order derivative. There are many types of differential equation, each having its own method of solution. The simplest type has separable variables, enabling each side of the equation to be integrated separately. differential scanning calorimetry (DSC) See  . differential thermal analysis (DTA) See  . differentiation 1. (in mathematics) The process of Ünding the derivative of a function in differential *calculus. If y = f(x), the derivative of y, written dy/dx or f′(x), is equal to the limit as ∆x → 0 of [f(x + ∆x) – f(x)]/∆x. In general, if y = xn, then dy/dx = nxn–1. On a graph of y = f(x), the derivative dy/dx is the gradient of the tangent to the curve at the point x. 2. (in biology) The changes from simple to more complex forms undergone by developing tissues and organs so that they become specialized for particular functions. Differentiation occurs during embryonic development, *regeneration, and (in plants) meristematic activity (see ). See also  . diffraction The spreading or bending of waves as they pass through an aperture or round the edge of a barrier. The diffracted waves subsequently interfere with each other (see ) producing regions of reinforcement and weakening. First noticed as occurring with light by Francesco Grimaldi (1618–63), the phenomenon gave considerable support to the wave theory of light. Diffraction also occurs with streams of particles such as electrons because of the quantum-mechanical wave nature of such particles. See also F ; F ;  . diffraction grating A device for producing spectra by diffraction and interference. The usual grating consists of a glass or speculum-metal sheet with a very large number of equidistant parallel lines ruled on it (usually of the order of 1000 per mm). Dif-
fracted light after transmission through the glass or reÛection by the speculum produces maxima of illumination (spectral lines) according to the equation mλ = d(sini + sinθ), where d is the distance between grating lines, λ is the wavelength of the light, i is the angle of incidence, θ the direction of the diffracted maximum, and m is the 'order' of the spectral line. ReÛection gratings are also used to produce spectra in the ultraviolet region of the electromagnetic spectrum.
diffusion 1. The process by which different substances mix as a result of the random motions of their component atoms, molecules, and ions. In gases, all the components are perfectly miscible with each other and mixing ultimately becomes nearly uniform, though slightly affected by gravity. The diffusion of a solute through a solvent to produce a solution of uniform concentration is slower, but otherwise very similar to the process of gaseous diffusion. Diffusion of small molecules and ions across a *cell membrane is known as passive transport. In solids, diffusion occurs very slowly at normal temperatures. See also F' ; G' . 2. The scattering of a beam of light by reÛection at a rough surface or by transmission through a translucent (rather than transparent) medium, such as frosted glass. 3. The passage of elementary particles through matter when there is a high probability of scattering and a low probability of capture. diffusion cloud chamber See  . diffusion gradient See  .
most animals; or intracellularly (e.g. by engulÜng phagocytic cells), as occurs in protozoans and cnidarians.
digestive system The system of organs that are involved in the process of *digestion. The digestive system of mammals is divided into the gastrointestinal tract (see  ) and accessory structures, such as teeth, tongue, liver, pancreas, and gall bladder. digit 1. (in mathematics) A symbol used to represent a single number. For example, the number 479 consists of three digits. 2. (in anatomy) A Ünger or toe. In the basic limb structure of terrestrial vertebrates there are Üve digits (see  ). This number is retained in man and other primates, but in some other species the number of digits is reduced. Frogs, for example, have four Üngers and Üve toes, and in ungulate (hooved) mammals, the digits are reduced and their tips are enclosed in horn, forming hooves. digital audio tape See DAT. digital camera see . digital certiÜcate See . digital computer See . digital display A method of indicating a reading of a measuring instrument, clock, etc., in which the appropriate numbers are generated on a Üxed display unit by the varying parameter being measured rather than Üxed numbers on a scale being indicated by a moving pointer or hand. See ; LCD; - .
diffusion pump (condensation pump) A *vacuum pump in which oil or mercury vapour is diffused through a jet, which entrains the gas molecules from the container in which the pressure is to be reduced. The diffused vapour and entrained gas molecules are condensed on the cooled walls of the pump. Pressures down to 10–7 Pa can be reached by sophisticated forms of the diffusion pump.
digital recording A method of recording or transmitting sound in which the sound itself is not transmitted or recorded. Instead the pressure in the sound wave is sampled at least 30 000 times per second and the successive values represented by numbers, which are then transmitted or recorded. Afterwards they are restored to analogue form in the receiver or player. This method is used for very high Üdelity recordings as no distortion or interference occurs during transmission or in the recording process.
digestion The breakdown by a living organism of ingested food material into chemically simpler forms that can be readily absorbed and assimilated by the body. This process requires the action of digestive enzymes and may take place extracellularly (i.e. in the *alimentary canal), as is the case in
digitigrade Describing the gait of most fast-running animals, such as dogs and cats, in which only the toes are on the ground and the rest of the foot is raised off the ground. Compare ; . digitron An electronic gas-discharge tube
dihydrate
that provides a *digital display in calculators, counters, etc. It usually has 10 cold cathodes shaped into the form of the digits 0–9. The cathode selected receives a voltage pulse causing a glow discharge to illuminate the digit. It has now largely been superseded by *light-emitting diodes and liquid-crystal displays (see LCD).
dihedral (dihedron) An angle formed by the intersection of two planes (e.g. two faces of a polyhedron). The dihedral angle is the angle formed by taking a point on the line of intersection and drawing two lines from this point, one in each plane, perpendicular to the line of intersection. dihybrid cross A genetic cross between parents that differ in two characteristics, controlled by genes at different loci. Mendel
performed a dihybrid cross using pea plants and the characteristics of seed colour and texture: the parental plants had either smooth yellow seeds (SSYY) – the dominant characteristics – or wrinkled green seeds (ssyy) – the recessive characteristics. All the offspring had smooth yellow seeds, being heterozygous (SsYy) for the two alleles. Crossing between these offspring produced an F2 generation of plants with smooth yellow, smooth green, wrinkled yellow, and wrinkled green seeds in the ratio 9:3:3:1 (see illustration). Mendel used these results as the basis for his Law of Independent Assortment (see M' ). Compare  .
dihydrate A crystalline hydrate containing two moles of water per mole of compound.
pure-breeding plants with smooth yellow seeds
pure-breeding plants with wrinkled green seeds
P generation
SSYY
gametes
SY SY
F1 generation
hybrid plants with smooth yellow seeds
SsYy SY Sy sY sy
F1 gametes fertilization
male gametes
SY Sy sY sy
female gametes SY
smooth yellow smooth green 9
Dihybrid cross.
wrinkled yellow wrinkled green 3
dihydric alcohol dihydric alcohol See . dihydrogen The normal form of molecular hydrogen, H2, used to distinguish it from hydrogen atoms.
(CH3CH(CH3)NH2) in methanol. Barbiturates give a reddish-violet colour.
diluent A substance added to dilute a solution or mixture (e.g. a *Üller).
1,2-dihydroxybenzene (catechol) A colourless crystalline phenol, C6H4(OH)2; r.d. 1.15; m.p. 105°C; b.p. 245°C. It is used as a photographic developer.
dilute Describing a solution that has a relatively low concentration of solute.
2,3-dihydroxybutanedioic acid See  .
dilution law See O'  .
dikaryon A cell of a fungal hypha or mycelium containing two haploid nuclei of different strains. The nuclei associate in pairs but do not fuse, therefore the cell is not truly diploid. Dikaryosis occurs in the Basidiomycota and Ascomycota. dilatancy See N . dilation (dilatation) 1. An increase in volume. See also . 2. See  . dilatometer A device for measuring the cubic *expansivities of liquids. It consists of a bulb of known volume joined to a graduated capillary tube, which is closed at the top to prevent evaporation. A known mass of liquid is introduced into the device, which is submerged in a bath maintained at different temperatures t1 and t2. The two volumes corresponding to these temperatures, V1 and V2, are read off the calibrated stem. The value of the cubic expansivity (γ) is then given by γ = (V2 – V1)/V1(t2 – t1). dilead(II) lead(IV) oxide A red amorphous powder, Pb3O4; r.d. 9.1; decomposes at 500°C to lead(II) oxide. It is prepared by heating lead(II) oxide to 400°C and has the unusual property of being black when hot and red-orange when cold. The compound is nonstoichiometric, generally containing less oxygen than implied by the formula. It is largely covalent and has Pb(IV)O6 octahedral groups linked together by Pb(II) atoms, each joined to three oxygen atoms. It is used in glass making but its use in the paint industry has largely been discontinued because of the toxicity of lead. Dilead(II) lead(IV) oxide is commonly called red lead or, more accurately, red lead oxide. Dillie–Koppanyi test A presumptive test for barbituates. The Dillie–Koppanyi test reagent has two solutions: a 1% solution of cobalt acetate in methanol, followed by a 5% solution of isopropylamine
dilution The volume of solvent in which a given amount of solute is dissolved.
dimensional analysis A method of checking an equation or a solution to a problem by analysing the dimensions in which it is expressed. It is also useful for establishing the form, but not the numerical coefÜcients, of an empirical relationship. If the two sides of an equation do not have the same dimensions, the equation is wrong. If they do have the same dimensions, the equation may still be wrong, but the error is likely to be in the arithmetic rather than the method of solution. dimensionless units See SI . dimensions The product or quotient of the basic physical quantities, raised to the appropriate powers, in a derived physical quantity. The basic physical quantities of a mechanical system are usually taken to be mass (M), length (L), and time (T). Using these dimensions, the derived physical quantity velocity will have the dimensions L/T and acceleration will have the dimensions L/T2. As force is the product of a mass and an acceleration (see N'   ), force has the dimensions MLT–2. In electrical work in *SI units, current, I, can be regarded as dimensionally independent and the dimensions of other electrical units can be found from standard relationships. Charge, for example, is measured as the product of current and time. It therefore has the dimension IT. Potential difference is given by the relationship P = VI, where P is power. As power is force × distance ÷ time (MLT–2 × L × T–1 = ML2T-3), voltage V is given by V = ML2T–3I–1. dimer An association of two identical molecules linked together. The molecules may react to form a larger molecule, as in the formation of dinitrogen tetroxide (N2O4) from nitrogen dioxide (NO2), or the formation of an *aluminium chloride dimer (Al2Cl6) in the vapour. Alternatively, they may be held by hydrogen bonds. For example, carboxylic
acids form dimers in organic solvents, in which hydrogen bonds exist between the O of the C=O group and the H of the –O–H group.
dimethylbenzenes (xylenes) Three compounds with the formula (CH3)2C6H4, each having two methyl groups substituted on the benzene ring. 1,2-dimethylbenzene is ortho-xylene, etc. A mixture of the isomers (b.p. 135–145°C) is obtained from petroleum and is used as a clearing agent in preparing specimens for optical microscopy. dimethylformamide (DMF) A colourless liquid compound, (CH3)2NCHO; r.d. 0.944; m.p. –61°C; b.p. 153°C. The systematic name is N,N-dimethylmethanamide. It can be made from methanoic acid (formic acid) and dimethylamine, and is widely used as a solvent. dimethylglyoxime (DMG) A colourless solid, (CH3CNOH)2, m.p. 234°C. It sublimes at 215°C and slowly polymerizes if left to stand. It is used in chemical tests for nickel, with which it forms a dark-red complex. dimethyl sulphoxide (DMSO) A colourless solid, (CH3)2SO; m.p. 18°C; b.p. 189°C. It is used as a solvent and as a reagent in organic synthesis. 1,3-dimethylxanthine See . 3,7-dimethylxanthine See . dimictic lake A lake that is stratiÜed by a *thermocline that is not permanent but is turned over twice during one year. The thermocline is disrupted due to seasonal changes in the climate. A meromictic lake is one in which there is a permanent stratiÜcation. dimorphism 1. (in biology) The existence of two distinctly different types of individual within a species. An obvious example is sexual dimorphism in certain animals, in which the two sexes differ in colouring, size, etc. Dimorphism also occurs in some lower plants, such as mosses and ferns, that show an *alternation of generations. 2. (in chemistry) See . dinitrogen The normal form of molecular nitrogen, N2, used to distinguish it from nitrogen atoms. dinitrogen oxide (nitrous oxide) A colourless gas, N2O, d. 1.97 g dm–3; m.p. –90.8°C; b.p. –88.5°C. It is soluble in water,
dinosaur ethanol, and sulphuric acid. It may be prepared by the controlled heating of ammonium nitrate (chloride free) to 250°C and passing the gas produced through solutions of iron(II) sulphate to remove impurities of nitrogen monoxide. It is relatively unreactive, being inert to halogens, alkali metals, and ozone at normal temperatures. It is decomposed on heating above 520°C to nitrogen and oxygen and will support the combustion of many compounds. Dinitrogen oxide is used as an anaesthetic gas ('laughing gas') and as an aerosol propellant.
dinitrogen tetroxide A colourless to pale yellow liquid or a brown gas, N2O4; r.d. 1.45 (liquid); m.p. –11.2°C; b.p. 21.2°C. It dissolves in water with reaction to give a mixture of nitric acid and nitrous acid. It may be readily prepared in the laboratory by the reaction of copper with concentrated nitric acid; mixed nitrogen oxides containing dinitrogen oxide may also be produced by heating metal nitrates. The solid compound is wholly N2O4 and the liquid is about 99% N2O4 at the boiling point; N2O4 is diamagnetic. In the gas phase it dissociates to give nitrogen dioxide N2O4 ˆ 2NO2 Because of the unpaired electron this is paramagnetic and brown. Liquid N2O4 has been widely studied as a nonaqueous solvent (self-ionizes to NO+ and NO3–). Dinitrogen tetroxide, along with other nitrogen oxides, is a product of combustion engines and is thought to be involved in the depletion of stratospheric ozone. Dinomastigota (DinoÛagellata) A phylum of mostly single-celled protists. They are abundant in the marine plankton; many are *photoautotrophs, containing brown xanthophyll pigments in addition to chlorophyll. DinoÛagellates characteristically have two undulipodia (Ûagella) for locomotion and most have a rigid cell wall of cellulose encrusted with silica. Some species (e.g. Noctiluca miliaris) are bioluminescent. dinosaur An extinct terrestrial reptile belonging to a group that constituted the dominant land animals of the Jurassic and Cretaceous periods, 190–65 million years ago. There were two orders. The Ornithischia were typically quadrupedal herbivores, many with heavily armoured bodies, and included Stegosaurus, Triceratops, and Iguanodon. They were all characterized by birdlike pelvic girdles. The Saurischia included many
dinucleotide bipedal carnivorous forms, such as Tyrannosaurus (the largest known carnivore), and some quadrupedal herbivorous forms, such as Apatosaurus (Brontosaurus) and Diplodocus. They all had lizard-like pelvic girdles. Many of the herbivorous dinosaurs were amphibious or semiaquatic.
• A Dino Directory produced by the Natural History Museum, London; contains a wealth of information and images
dinucleotide A compound consisting of two *nucleotides. diode An electronic device with two electrodes. In the obsolescent thermionic diode a heated cathode emits electrons, which Ûow across the intervening vacuum to the anode when a positive potential is applied to it. The device permits Ûow of current in one direction only as a negative potential applied to the anode repels the electrons. This property of diodes was made use of in the Ürst thermionic radios, in which the diode was used to demodulate the transmitted signal (see ). In the semiconductor diode, a p–n junction performs a similar function. The forward current increases with increasing potential difference whereas the reverse current is very small indeed. See ; . dioecious Describing plant species that have male and female Ûowers on separate plants. Examples of dioecious plants are willows. Compare . diol (dihydric alcohol) An *alcohol containing two hydroxyl groups per molecule. dioptre A unit for expressing the power of a lens or mirror equal to the reciprocal of its focal length in metres. Thus a lens with a focal length of 0.5 metre has a power of 1/0.5 = 2 dioptres. The power of a converging lens is usually taken to be positive and that of a diverging lens negative. Because the power of a lens is a measure of its ability to cause a beam to converge, the dioptre is now sometimes called the radian per metre. dioxan A colourless toxic liquid, C4H8O2; r.d. 1.03; m.p. 11°C; b.p. 101.5°C. The molecule has a six-membered ring containing four CH2 groups and two oxygen atoms at opposite corners. It can be made from ethane-1,2-diol and is used as a solvent. dioxin (2,4,7,8-tetrachlorodibenzo-pdioxin) A toxic solid, formed in the manu-
facture of the herbicide *2,4,5-T; it was present as an impurity in Agent Orange, used as a defoliant during the Vietnam War. It is the most toxic member of a group of compounds (called dioxins) that occur widely as environmental pollutants, being produced during combustion processes and as byproducts in various industrial manufacturing processes. Dioxins decompose very slowly and may be concentrated in the food chain; in animals they are stored in fat. Exposure to high levels of dioxins can cause skin disÜgurement (chloracne) and may result in fetal defects. Because of their toxicity, many countries have imposed strict controls to reduce industrial emissions of dioxins.
dioxonitric(III) acid See  . dioxygen The normal form of molecular oxygen, O2, used to distinguish it from oxygen atoms or from ozone (O3). dioxygenyl compounds Compounds containing the positive ion O2+, as in dioxygenyl hexaÛuoroplatinate O2PtF6 – an orange solid that sublimes in vacuum at 100°C. Other ionic compounds of the type O2+[MF6]– can be prepared, where M is P, As, or Sb. dip See . dipeptide A compound consisting of two amino acid units joined at the amino (–NH2) end of one and the carboxyl (–COOH) end of the other. This peptide bond (see ) is formed by a condensation reaction that involves the removal of one molecule of water. diphenylmethanone (benzophenone) A colourless solid, C6H5COC6H5, m.p. 49°C. It has a characteristic smell and is used in making perfumes. It is made from benzene and benzoyl chloride using the *Friedel– Crafts reaction with aluminium chloride as catalyst. diphosgene A colourless liquid, ClCO.O.CCl3, originally used in 1916 by Germany in World War I as a chemical warfare agent. It is now used as a reagent in organic synthesis. See also  . diphosphane (diphosphine) A yellow liquid, P2H4, which is spontaneously Ûammable in air. It is obtained by hydrolysis of calcium phosphide. Many of the references to the spontaneous Ûammability of phosphine (PH3) are in fact due to traces of P2H4 as impurities.
diphosphine See . diphosphonates See . diphyodont Describing a type of dentition that is characterized by two successive sets of teeth: the *deciduous (milk) teeth, which are followed by the *permanent (adult) teeth. Mammals have a diphyodont dentition. Compare ; . diploblastic Describing an animal with a body wall composed only of two layers, *ectoderm and *endoderm, sometimes with a noncellular *mesoglea between them. Cnidarians are diploblastic. Compare . diploid Describing a nucleus, cell, or organism with twice the *haploid number of chromosomes characteristic of the species. The diploid number is designated as 2n. Two sets of chromosomes are present, one set being derived from the female parent and the other from the male. In animals, all the cells except the reproductive cells are diploid. Diplopoda A class of terrestrial *arthropods comprising the millipedes and belonging to the subphylym *Myriapoda. Diplopods are characterized by 20 to over 60 body segments each bearing two pairs of legs. They are slow moving and feed on decaying leaves. diplotene The period in the Ürst prophase of *meiosis when paired *homologous chromosomes begin to move apart. They remain attached at a number of points (see ). Dipnoi A subclass or order of bony Üshes that contains the lungÜshes, which have lungs and breathe air. They are found in Africa, Australia, and South America, where they live in freshwater lakes and marshes that tend to become stagnant or even dry up in summer. They survive in these conditions by burrowing into the mud, leaving a small hole for breathing air, and entering a state of *aestivation, in which they can remain for six months or more. The Dipnoi date from the Devonian era (416–360 million years ago) and share many features with the modern *Amphibia. dipole 1. A pair of separated opposite electric charges. The dipole moment (symbol µ) is the product of the positive charge and the distance between the charges. Dipole mo-
Dirac equation ments are often stated in *debyes; the SI unit is the coulomb metre. In a diatomic molecule, such as HCl, the dipole moment is a measure of the polar nature of the bond (see  ); i.e. the extent to which the average electron charge is displaced towards one atom (in the case of HCl, the electrons are attracted towards the more electronegative chlorine atom). In a polyatomic molecule, the dipole moment is the vector sum of the dipole moments of the individual bonds. In a symmetrical molecule, such as tetrachloromethane (CCl4), there is no overall dipole moment, although the individual C–Cl bonds are polar. 2. An aerial commonly used for frequencies below 30 megahertz, although some are in use above this frequency. It consists of a rod, fed or tapped at its centre. It may be half a wavelength or a full wavelength long.
dipole radiation See  . Diptera An order of insects comprising the true, or two-winged, Ûies. Flies possess only one pair of wings – the forewings; the hindwings are modiÜed to form small clublike halteres that function as balancing organs. Typically Ûuid feeders, Ûies have mouthparts adapted for piercing and sucking or for lapping; the diet includes nectar, sap, decaying organic matter, and blood. Some species prey on insects; others are parasitic. Dipteran larvae (maggots) are typically wormlike with an inconspicuous head. They undergo metamorphosis via a pupal stage to the adult form. Many Ûies or their larvae are serious pests, either by feeding on crops (e.g. fruit Ûies) or as vectors of disease organisms (e.g. the house Ûy (Musca domestica) and certain mosquitoes). Dirac, Paul Adrien Maurice (1902–84) British physicist, who shared the 1933 Nobel Prize for physics with Erwin *Schrödinger for developing Schrödinger's non-relativistic wave equations to take account of relativity. This modiÜed equation predicted the existence and properties of the *positron. Dirac also invented, independently of Enrico Fermi, the form of *quantum statistics known as Fermi–Dirac statistics. Dirac constant See P . Dirac equation A version of the nonrelativistic *Schrödinger equation taking special relativity theory into account. The Dirac equation is needed to discuss the quantum
mechanics of electrons in heavy atoms and, more generally, to discuss Üne-structure features of atomic spectra. The equation was put forward by Paul Dirac in 1928. It can be solved exactly in the case of the hydrogen atom but can only be solved using approximation techniques for more complicated atoms.
direct current (d.c.) An electric current in which the net Ûow of charge is in one direction only. Compare  . direct-current motor See  . direct dye See . direct motion 1. The apparent motion of a planet from west to east as seen from the earth against the background of the stars. 2. The anticlockwise rotation of a planet, as seen from its north pole. Compare  . directrix 1. A plane curve deÜning the base of a *cone. 2. A straight line from which the distance to any point on a *conic is in a constant ratio to the distance from that point to the focus. disaccharide A sugar consisting of two linked *monosaccharide molecules. For example, sucrose comprises one glucose molecule and one fructose molecule bonded together. discharge 1. The conversion of the chemical energy stored in a *secondary cell into electrical energy. 2. The release of electric charge from a capacitor in an external circuit. 3. The passage of charge carriers through a gas at low pressure in a discharge tube. A potential difference applied between cathode and anode creates an electric Üeld that accelerates any free electrons and ions to their appropriate electrodes. Collisions between electrons and gas molecules create more ions. Collisions also produce excited ions and molecules (see ), which decay with emission of light in certain parts of the tube. discontinuous function See  . discontinuous variation (qualitative variation) Clearly deÜned differences in a characteristic that can be observed in a population. Characteristics that are determined by different *alleles at a single locus show
discontinuous variation, e.g. garden peas are either wrinkled or smooth. Compare  .
disease A condition in which the normal function of some part of the body (cells, tissues, or organs) is disturbed. A variety of microorganisms and environmental agents are capable of causing disease. The functional disturbances are often accompanied by structural changes in tissue. disilane See . disinfectant Any substance that kills or inhibits the growth of disease-producing microorganisms and is in general toxic to human tissues. Disinfectants include cresol, bleaching powder, and phenol. They are used to cleanse surgical apparatus, sickrooms, and household drains and if sufÜciently diluted can be used as *antiseptics. disintegration Any process in which an atomic nucleus breaks up spontaneously into two or more fragments in a radioactive decay process or breaks up as a result of a collision with a high-energy particle or nuclear fragment. disintegration constant See . dislocation See   (Feature). disodium hydrogenphosphate(V) (disodium orthophosphate) A colourless crystalline solid, Na2HPO4, soluble in water and insoluble in ethanol. It is known as the dihydrate (r.d. 2.066), heptahydrate (r.d. 1.68), and dodecahydrate (r.d. 1.52). It may be prepared by titrating phosphoric acid with sodium hydroxide to an alkaline end point (phenolphthalein) and is used in treating boiler feed water and in the textile industry. disodium orthophosphate See  (V). disodium tetraborate-10-water See . d-isomer See  . D-isomer
See  .
disordered solid A material that neither has the structure of a perfect *crystal lattice nor of a crystal lattice with isolated *crystal defects. In a random alloy, one type of disordered solid, the order of the different types of atom occurs at random. Another type of disordered solid is formed by introducing a high concentration of defects, with the defects distributed randomly throughout the
solid. In an *amorphous solid, such as glass, there is a random network of atoms with no lattice. The theory of disordered solids is more complicated than the theory of crystals, requiring such concepts as *localization and *spin glasses.
dispersal The dissemination of offspring of plants or sessile animals. Dispersal provides organisms that are not mobile with a better chance of survival by reducing *competition among offspring and parents. It also promotes the colonization of new habitats. Flowering plants produce fruits or seeds that are dispersed by such agents as wind, water, or animals. Specialized structures have evolved in many species to aid dispersal (see ). disperse dye See . disperse phase See . dispersion The splitting up of a ray of light of mixed wavelengths by refraction into its components. Dispersion occurs because the *deviation for each wavelength is different on account of the different speeds at which waves of different wavelengths pass through the refracting medium. If a ray of white light strikes one face of a prism and passes out of another face, the white light will be split into its components and the full visible spectrum will be formed. The dispersive power of a prism (or other medium) for white light is deÜned by (nb – nr)/(ny – 1), where nb, nr, and ny are the *refractive indexes for blue, red, and yellow light respectively. The term is sometimes applied to the separation of wavelengths produced by a *diffraction grating. dispersion forces See   W' . dispersive power See . displacement 1. Symbol s. A speciÜed distance in a speciÜed direction. It is the vector equivalent of the scalar distance. 2. See  . displacement activity An activity shown by an animal that appears to be irrelevant to its situation. Displacement activities are frequently observed when there is conÛict between opposing tendencies. For example, birds in aggressive situations, in which there are simultaneous tendencies to attack and to
dissipative structure Ûee, may preen their feathers as a displacement activity.
displacement current A term of the form ∂D/∂t, where D is the *electric displacement, which is added to the electric current density J to modify *Ampère's law in the fourth of *Maxwell's equations. The necessity for the displacement current term was postulated by Maxwell when he put forward his equations to introduce a degree of symmetry between electricity and magnetism. displacement reaction See  . display behaviour Stereotyped movement or posture that serves to inÛuence the behaviour of another animal. Many displays in *courtship and *aggression are conspicuous and characteristic of the species; special markings or parts of the body may be prominently exhibited (for example, the male peacock spreads its tail in courtship). Other displays are cryptic and make it harder for a predator to recognize the displaying animal as potential prey. For example, geometer moth caterpillars, which look like twigs, hold themselves on plant stems with one end sticking into the air. disproportionation A type of chemical reaction in which the same compound is simultaneously reduced and oxidized. For example, copper(I) chloride disproportionates thus: 2CuCl → Cu + CuCl2 The reaction involves oxidation of one molecule CuI → CuII + e and reduction of the other CuI + e → Cu The reaction of halogens with hydroxide ions is another example of a disproportionation reaction, for example Cl2(g) + 2OH–(aq) ˆ Cl–(aq) + ClO–(aq) + H2O(l) The reverse process is *comproportionation. dissipative structure A state of matter that occurs when a system is driven away from thermal *equilibrium by external constraints that have exceeded certain critical values. A dissipative structure, which is associated with *broken symmetry, is an example of *complexity and *self-organization. An example of dissipative structure is a *Bénard cell. The order in a dissipative structure that
dissipative system is not in thermal equilibrium occurs as a response to such parameters as heat.
dissipative system A system that involves irreversible processes (see ). All real systems are dissipative (in contrast to such idealized systems as the frictionless pendulum, which is invariant under time reversal). In a dissipative system the system is moving towards a state of equilibrium, which can be regarded as moving toward a point *attractor in *phase space; this is equivalent to moving towards the minimum of the *free energy, F. dissociation The breakdown of a molecule, ion, etc., into smaller molecules, ions, etc. An example of dissociation is the reversible reaction of hydrogen iodide at high temperatures 2HI(g) ˆ H2(g) + I2(g) The *equilibrium constant of a reversible dissociation is called the dissociation constant. The term 'dissociation' is also applied to ionization reactions of *acids and *bases in water; for example HCN + H2O ˆ H3O+ + CN– which is often regarded as a straightforward dissociation into ions HCN ˆ H+ + CN– The equilibrium constant of such a dissociation is called the acid dissociation constant or acidity constant, given by Ka = [H+][A–]/[HA] for an acid HA (the concentration of water [H2O] can be taken as constant). Ka is a measure of the strength of the acid. Similarly, for a nitrogenous base B, the equilibrium B + H2O ˆ BH+ + OH– is also a dissociation; with the base dissociation constant, or basicity constant, given by Kb = [BH+][OH–]/[B] For a hydroxide MOH, Kb = [M+][OH–]/[MOH] dissociation pressure When a solid compound dissociates to give one or more gaseous products, the dissociation pressure is the pressure of gas in equilibrium with the solid at a given temperature. For example, when calcium carbonate is maintained at a constant high temperature in a closed container, the dissociation pressure at that temperature is the pressure of carbon dioxide from the equilibrium
CaCO3(s) ˆ CaO(s) + CO2(g)
distal Describing the part of an organ that is farthest from the organ's point of attachment to the rest of the body. For example, hands and feet are at the distal ends of arms and legs, respectively. Compare . distal convoluted tubule (second convoluted tubule) The part of a *nephron that leads from the thick ascending limb of the *loop of Henle and drains into a *collecting duct. The main function of the distal tubule is to absorb sodium chloride and other inorganic salts while retaining water. distillation The process of boiling a liquid and condensing and collecting the vapour. The liquid collected is the distillate. It is used to purify liquids and to separate liquid mixtures (see  ;  ). See also  ;  . distilled water Water puriÜed by distillation so as to free it from dissolved salts and other compounds. Distilled water in equilibrium with the carbon dioxide in the air has a conductivity of about 0.8 × 10–6 siemens cm–1. Repeated distillation in a vacuum can bring the conductivity down to 0.043 × 10–6 siemens cm–1 at 18°C (sometimes called conductivity water). The limiting conductivity is due to self-ionization: H2O ˆ H+ + OH–. distortion The extent to which a system fails to reproduce the characteristics of its input in its output. It is most commonly applied to electronic ampliÜers and to optical systems. See . distribution function A function of some variable, such as velocity, that expresses the probability that a particle, such as a molecule of gas, will have a particular value of that variable. distributive law The mathematical law stating that one operation is independent of being carried out before or after another operation. For example, multiplication is distributive with respect to addition and subtraction, i.e. x(y + z) = xy + xz. Compare  ;  . disulphide bridge (sulphur bridge) A covalent bond (S–S) formed between the thiol groups (–SH) of two cysteine residues, usually in the polypeptide chains of proteins. Easily hydrolysed and prone to rearrange-
ment, these bonds contribute to the tertiary structure of *proteins.
disulphur dichloride (sulphur monochloride) An orange–red liquid, S2Cl2, which is readily hydrolysed by water and is soluble in benzene and ether; r.d. 1.678; m.p. –80°C; b.p. 136°C. It may be prepared by passing chlorine over molten sulphur; in the presence of iodine or metal chlorides sulphur dichloride, SCl2, is also formed. In the vapour phase S2Cl2 molecules have Cl–S–S–Cl chains. The compound is used as a solvent for sulphur and can form higher chlorosulphanes of the type Cl–(S)n–Cl (n < 100), which are of great value in *vulcanization processes. disulphuric(VI) acid (pyrosulphuric acid) A colourless hygroscopic crystalline solid, H2S2O7; r.d. 1.9; m.p. 35°C. It is commonly encountered mixed with sulphuric acid as it is formed by dissolving sulphur trioxide in concentrated sulphuric acid. The resulting fuming liquid, called oleum or Nordhausen sulphuric acid, is produced during the *contact process and is also widely used in the *sulphonation of organic compounds. See also  . dithionate A salt of dithionic acid, containing the ion S2O62–, usually formed by the oxidation of a sulphite using manganese(IV) oxide. The ion has neither pronounced oxidizing nor reducing properties. dithionic acid An acid, H2S2O6, known in the form of its salts (dithionates). dithionite See . dithionous acid See  . diuretic A drug or other agent that increases the rate of urine formation and hence the rate at which water and certain salts are lost from the body. Many diuretic drugs work by decreasing the reabsorption of sodium and chloride ions from the Ültrate in the kidney tubules, so that less water is reabsorbed. They are used to treat Ûuid retention (oedema) arising from disorders of the heart, kidneys, or other organs, and are used in helping to reduce high blood pressure (hypertension). There are several groups of diuretic drugs, with different modes of action. The most powerful are loop diuretics, such as frusemide, which act primarily by blocking Na+/K+/Cl– carriers in cells of the *loop of Henle. Another group consists of the thiazides, such as metolazone, which inhibit
division Na+/Cl– transport in the *distal convoluted tubule. Spironolactone exerts its diuretic effect by blocking the binding of the hormone *aldosterone to its receptors. The osmotic diuretics, such as mannitol, act by increasing the osmolarity of the Ültrate, and hence increasing urine volume.
diurnal Daily; denoting an event that happens once every 24 hours. diurnal rhythm See  . divalent (bivalent) Having a valency of two. divergence (div) The *scalar product of the *gradient operator ∇ with a vector. For a vector u that has components u1, u2, and u3 in the x, y, and z directions, and is a function of x, y, and z, the divergence is given by: divu = ∇.u = ∂ u1/∂x + ∂u2/∂y + ∂u3/∂z. The divergence of a vector at a given point represents the Ûux of the vector per unit volume in the neighbourhood of that point. See also ; L . divergence theorem A theorem that gives the relation between the total Ûux of a vector F out of a surface S, which surrounds the volume V, to the vector inside the volume. The divergence theorem states that ∫vdivFdV = ∫sF·dS. The divergence theorem is also known as Gauss' theorem and Ostrogradsky's theorem (named after the Russian mathematician Michel Ostrogradsky (1801–61), who stated it in 1831). *Gauss' law for electric Üelds is a particular case of the divergence theorem. divergent evolution See  . divergent series See  . diverging lens or mirror A lens or mirror that can refract or reÛect a parallel beam of light into a diverging beam. A diverging lens is predominantly concave; a diverging mirror is convex. Compare    . diverticulum A saclike or tubular outgrowth from a tubular or hollow internal organ. Diverticula may occur as normal structures (e.g. the *caecum and *appendix in the alimentary canal) or abnormally, from a weakened area of the organ. division A category used traditionally in the *classiÜcation of plants that consists of
one or several similar classes. Division names end in -phyta; an example is the Spermatophyta (seed-bearing plants).
major constituent of the *chromosomes within the cell nucleus and plays a central role in the determination of hereditary characteristics by controlling *protein synthesis in cells (see also  ). It is also found in chloroplasts and mitochondria (see  ;  DNA). DNA is a nucleic acid composed of two chains of *nucleotides in which the sugar is deoxyribose and the bases are *adenine, *cytosine, *guanine, and *thymine (compare RNA). The two chains are wound round each other and linked together by hydrogen bonds between speciÜc complementary bases (see  ) to form a spiral ladder-shaped molecule (double helix). See illustration. When the cell divides, its DNA also replicates in such a way that each of the two
dizygotic twins See  .
D-lines Two close lines in the yellow region of the visible spectrum of sodium, having wavelengths 589.0 and 589.6 nm. As they are prominent and easily recognized they are used as a standard in spectroscopy. dl-isomer See  ;  . DMF See . DMG See . DMSO See  . DNA (deoxyribonucleic acid) The genetic material of most living organisms, which is a
– CH 2 O O O P – H O O O
hydrogen bond T
CH 2 O
– O
O – O
Detail of molecular structure of sugar–phosphate backbone. Each deoxyribose unit is attached to a phosphate group and a base, forming a nucleotide A
hydrogen bond thymine adenine (T) (A) H CH3 O H N N N H
HO N H
N G
H N
sugar– phosphate backbone
Double helical structure of DNA
deoxyribose
cytosine (C)
H N H guanine (G)
The four bases of DNA, showing the hydrogen bonding between base pairs
DNA. Its molecular structure.
DNA proÜling
daughter molecules is identical to the parent molecule (see DNA ). See also  DNA.
sures that the foreign DNA (e.g. the complementary DNA used in *gene cloning) is bound to the plasmid into which it is incorporated.
• An animated primer on basic aspects of DNA, genes, and heredity; produced for the Cold Spring Harbor Laboratory.
DNA methylation The addition of methyl groups to constituent bases of DNA. In both prokaryotes and eukaryotes certain bases of the DNA generally occur in a methylated form. In bacteria this methylation protects the cell's DNA from attack by its own restriction enzymes, which cleave foreign unmethylated DNA and thereby help to eliminate viral DNA from the bacterial chromosome. Methylation is also important in helping *DNA repair enzymes to distinguish the parent strand from the progeny strand when repairing mismatched bases in newly replicated DNA, and it also plays a role in controlling the transcription of DNA.
DNAase See DN. DNA blotting See S . DNA chip See DNA . DNA cloning See  . DNA-dependent RNA polymerase See . DNA Üngerprinting See DNA . DNA hybridization A method of determining the similarity of DNA from different sources. Single strands of DNA from two sources, e.g. different bacterial species, are put together and the extent to which double hybrid strands are formed is estimated. The greater the tendency to form these hybrid molecules, the greater the extent of complementary base sequences, i.e. gene similarity. The method is one way of determining the genetic relationships of species. DNA library (gene library; gene bank) A collection of cloned DNA fragments representing the entire genetic material of an organism. This facilitates screening and isolation of any particular gene. DNA libraries are created by fractionating the genomic DNA into fragments using *restriction enzymes and/or physical methods. These fragments are cloned (see  ) and the host cells containing the recombinant fragments are centrifuged and frozen; alternatively, the phage *vectors are maintained in culture. Individual genes in the library are identiÜed using speciÜc *gene probes with the *Southern blotting technique or, via their protein products, using *Western blotting. DNA libraries are thus repositories of raw material for use in genetic engineering. A large genome, such as that of humans, is most conveniently cloned using vectors that can accommodate large fragments of DNA, such as yeast *artiÜcial chromosomes, maintained in cell culture. DNA ligase An enzyme that is able to join together two portions of DNA and therefore plays an important role in *DNA repair. DNA ligase is also used in recombinant DNA technology (see  ) as it en-
DNA microarray (DNA chip) A *microarray containing numerous small DNA molecules. DNA microarrays consist of thousands of short synthetic single-stranded DNA molecules, each comprising 20–25 nucleotides and all with unique sequences designed to complement and bind to speciÜc target nucleotide sequences. They can be used to quantify gene expression by determining the total output of messenger RNAs (mRNAs) (i.e. the *transcriptome) of a cell or tissue. This involves adding Ûuorescent labels to the mRNAs, then incubating these with the microarray so that they bind to complementary oligonucleotides on it. A computerized scanner measures the intensity of Ûuorescence and thereby the amount of bound mRNA. DNA microarrays can also detect mutations in particular genes, for example the BRCA genes involved in hereditary forms of breast cancer. An individual's DNA is denatured, and its binding to a microarray is compared with that of normal (control) DNA on the same microarray. Any disparities between the two binding patterns will pinpoint sequences from the individual with possible abnormalities, enabling closer examination. DNA polymerase See . DNA probe See  . DNA proÜling (genetic Üngerprinting) A technique in which an individual's DNA is analysed to reveal the pattern of repetition of certain short nucleotide sequences, called short tandem repeats: (STRs), throughout the genome. This pattern is claimed to be
unique to the individual concerned, and the technique is therefore used for identiÜcation purposes in forensic science and paternity disputes, and in veterinary science. SufÜcient DNA can be obtained from very small samples of body tissue, such as blood, semen, or hair.
DNA repair A variety of mechanisms that help to ensure that the genetic sequence, as expressed in the DNA, is maintained and that errors that occur during *DNA replication, by mutation, are not allowed to accumulate. An error in the genetic sequence could cause cell death by interfering with the replication process. The mechanisms work because DNA is made up of two strands, each of which contains a copy of the genetic sequence. A damaged section of a strand, or a mismatched base, can be recognized and removed by enzymes and replaced by the correct form by DNA *polymerases. The phosphodiester backbone is then sealed by *DNA ligase. DNA replication The process whereby DNA makes exact copies of itself, which is controlled by the enzyme DNA *polymerase. Replication occurs at rates of between 50 nucleotides per second (in mammals) and 500 nucleotides per second (in bacteria). The hydrogen bonds between the complementary bases on the two strands of the parent DNA molecule break and the strands unwind, each strand acting as a template for the synthesis of a new one complementary to itself. DNA polymerases move down the two single strands linking free nucleotides to their complementary bases (see  ) on the templates. The process continues until all the nucleotides on the templates have joined with appropriate free nucleotides and two identical molecules of DNA have been formed. This process is known as semiconservative replication as each new molecule contains half of the original parent DNA molecule. Sometimes mutations occur that may cause the exact sequence of the parent DNA not to be replicated. However, *DNA repair mechanisms reduce this possibility. DNase (DNAase; deoxyribonuclease) An enzyme that catalyses the cleavage of DNA. DNase I is a digestive enzyme, secreted by the pancreas, that degrades DNA into shorter nucleotide fragments. Many other *endonucleases and *exonucleases cleave DNA, including the *restriction enzymes and
enzymes involved in DNA repair and replication.
DNA sequencing (gene sequencing) The process of elucidating the nucleotide sequence of a DNA fragment. The method now used almost universally is the Sanger method (named after Frederick *Sanger), also called the dideoxy method, which involves synthesizing a new DNA strand using as template single-stranded DNA from the gene being sequenced. Synthesis of the new strand can be stopped at any of the four bases by adding the corresponding dideoxy (dd) derivative of the deoxyribonucleoside phosphates; for example, by adding ddATP the synthesis terminates at an adenosine; by adding ddGTP it terminates at a guanosine, etc. The fragments, which comprise Ûuorescently labelled nucleotides, are subjected to electrophoresis and scanned by a Ûuorescence detector. A big advantage of the Sanger method is that it can easily be adapted to sequencing RNA, by making single-stranded DNA from the RNA template using the enzyme *reverse transcriptase. This enables, for example, sequencing of ribosomal RNA for use in *molecular systematics. After separation of the fragments, the products of all four reactions are detected by Ûuorescence spectroscopy and analysed by computer, which gives a printout of the base sequence. DNA sequencing using fully automated sequencers is now employed on a major scale, for example in determining the nucleotide sequence of entire genomes (see  ). DNS See   . Döbereiner's triads A set of triads of chemically similar elements noted by Johann Döbereiner (1780–1849) in 1817. Even with the inaccurate atomic mass data of the day it was observed that when each triad was arranged in order of increasing atomic mass, then the mass of the central member was approximately the average of the values for the other two. The chemical and physical properties were similarly related. The triads are now recognized as consecutive members of the groups of the periodic table. Examples are: lithium, sodium, and potassium; calcium, strontium, and barium; and chlorine, bromine, and iodine.
• Döbereiner's original paper
Document Object Model (DOM) A stan-
dard interface for representing *XML and *HTML documents. The data is parsed into a tree of objects, which a programmer can navigate and manipulate. It is especially relevant to web browsers, and in particular to the implementation of dynamic HTML. The World Wide Web Consortium released the Ürst version of the DOM (level 1) in 1998, although a previous intermediate version (level 0) had already been incorporated in HTML 4; levels 2 and 3 have subsequently been released. Adoption of the DOM by web browsers – which had previously implemented idiosyncratic and incompatible models – was initially slow and is still not uniform.
• The WSC's DOM page
dodecanoic acid (lauric acid) A white crystalline *fatty acid, CH3(CH2)10COOH; r.d. 0.868; m.p. 44°C; b.p. 131°C. Glycerides of the acid are present in natural fats and oils (e.g. coconut and palm-kernel oil). dodecene A straight-chain alkene, CH3(CH2)9CH:CH2, obtained from petroleum and used in making *dodecylbenzene. dodecylbenzene A hydrocarbon, CH3(CH2)11C6H5, manufactured by a Friedel–Crafts reaction between dodecene (CH3(CH2)9CH:CH2) and benzene. It can be sulphonated, and the sodium salt of the sulphonic acid is the basis of common *detergents. dolomite A carbonate mineral consisting of a mixed calcium–magnesium carbonate, CaCO3.MgCO3, crystallizing in the rhombohedral system. It is usually white or colourless. The term is also used to denote a rock with a high ratio of magnesium to calcium carbonate. See . DOM See D O M. Domagk, Gerhard (1895–1964) German biochemist who went to work for IG Farbenindustrie to investigate new drugs. In 1935 he discovered the antibacterial properties of a dye, Prontosil, which became the Ürst sulpha drug (see ). He was offered the 1939 Nobel Prize for physiology or medicine but was forced by Hitler to refuse; he Ünally received the award in 1947. domain 1. (in taxonomy) The highest taxonomic category, consisting of one or more *kingdoms. Living organisms are divided into three domains: *Archaea (archaebacte-
ria), Eubacteria (see ), and Eukarya (see ). 2. (in physics) See .
domain name system (DNS) In computing, a system that provides mappings between the human-oriented names of users or services in a network, and the machineoriented network addresses of the named entity. It is used primarily on *TCP/IP networks, primarily the Internet, to map such human-oriented names as www.oup.co.uk to the equivalent IP address; however, other networks have similar facilities. Names are usually hierarchical, and in general terms the boundaries of a *domain will coincide with some form of natural boundary within the network environment, such as a country, a community of users within a country, or the users on a site. The above example consists of the four domains 'uk' (the top-level domain), 'co', 'oup', and 'www'. While the arrangement of IP addresses is also usually hierarchical, there is no assumption in the mapping between the two that the hierarchies are in any way equivalent.
• Introduction to the Internet domain name system • Domain names – implementation and specification
dominance hierarchy See . dominant 1. (in genetics) Describing the *allele that is expressed in the *phenotype when two different alleles of a gene are present in the cells of an organism. For example, the height of garden peas is controlled by two alleles, 'tall' (T) and 'dwarf' (t). When both are present (Tt), i.e. when the cells are *heterozygous, the plant is tall since T is dominant and t is *recessive. See also ;  . 2. (in ecology) Describing the most conspicuously abundant and characteristic species in a *community. The term is usually used of a plant species in plant ecology; for example, pine trees in a pine forest. 3. (in animal behaviour) Describing an animal that is allowed priority in access to food, mates, etc., by others of its species because of its success in previous aggressive encounters. Less dominant animals frequently show *appeasement behaviour towards a more dominant individual, so overt *aggression is minimized. In a stable group there may be a linear dominance hierarchy or peck order (so called because it was Ürst observed in do-
donor mestic fowl), with each animal being subservient to those above it in the hierarchy and taking precedence over those below it.
donor 1. (in chemistry) An atom, ion, or molecule that provides a pair of electrons in forming a coordinate bond. 2. (in electronics) A substance added as an impurity to a *semiconductor because it can donate electrons to the conduction bands, causing ntype conduction by electrons. Compare . 3. (in medicine) An individual whose tissues or organs are transferred to another (the recipient). Donors may provide blood for transfusion or a kidney or heart for transplantation. 4. (in genetics) A cell that contributes genetic material for insertion into another cell, for example to produce a transgenic cell by genetic engineering. donor levels Energy levels of a donor atom in a *semiconductor, such as arsenic in silicon. These energy levels are very near the bottom of the conduction band, thus causing n-type conduction. See also  . dopa (dihydroxyphenylalanine) A derivative of the amino acid tyrosine. It is found in particularly high levels in the adrenal glands and is a precursor in the synthesis of *dopamine, *noradrenaline, and *adrenaline. The laevorotatory form, L-dopa, is administered in the treatment of Parkinson's disease, in which brain levels of dopamine are reduced. dopamine A *catecholamine that is a precursor in the synthesis of *noradrenaline and *adrenaline. It also functions as a neurotransmitter, especially in the brain. doping See . Doppler broadening Broadening of a spectral line caused by the *Doppler effect. Shifts in frequency occur according to whether the emitting atom or molecule is moving towards or away from the observer. The effect depends on temperature and is used for temperature measurement in astronomy. Doppler cooling See  . Doppler effect The apparent change in the observed frequency of a wave as a result of relative motion between the source and the observer. For example, the sound made by a low-Ûying aircraft as it approaches appears to fall in pitch as it passes and Ûies away. In fact, the frequency of the aircraft
engine remains constant but as it is approaching more sound waves per second impinge on the ear and as it recedes fewer sound waves per second impinge on the ear. The apparent frequency, F, is given by F = f(c – uo)/(c – us), where f is the true frequency, c is the speed of sound, and uo and us are the speeds of the observer and the source, respectively. Although the example of sound is most commonly experienced, the effect was suggested by Christian Johann Doppler (1803–53), an Austrian physicist, as an attempt to explain the coloration of stars. In fact the Doppler effect cannot be observed visually in relation to the stars, although the effect does occur with electromagnetic radiation and the *redshift of light from receding stars can be observed spectroscopically. The Doppler effect is also used in radar to distinguish between stationary and moving targets and to provide information regarding the speed of moving targets by measuring the frequency shift between the emitted and reÛected radiation. For electromagnetic radiation, the speed of light, c, features in the calculation and as there is no Üxed medium to provide a frame of reference, relativity has to be taken into account, so that F = f√[(1 – v/c)/(1 + v/c)], where v is the speed at which source and observer are moving apart. If v2/c2 is small compared to 1, i.e. if the speed of separation is small compared to the speed of light, this equation simpliÜes to F = f(1 – v/c).
d-orbital See . dormancy An inactive period in the life of an animal or plant during which growth slows or completely ceases. Physiological changes associated with dormancy help the organism survive adverse environmental conditions. Annual plants survive the winter as dormant seeds while many perennial plants survive as dormant tubers, rhizomes, or bulbs. *Hibernation and *aestivation in animals help them survive extremes of cold and heat, respectively. dorsal Describing the surface of a plant or animal that is farthest from the ground or other support, i.e. the upper surface. In vertebrates, the dorsal surface is that down which the backbone runs. Thus in upright (bipedal) mammals, such as man and kanga-
double circulation
roos, it is the backward-directed (*posterior) surface. Compare .
dorsal root The part of a spinal nerve that enters the *spinal cord on the dorsal side and contains only sensory Übres. The cell bodies of these Übres form the dorsal root ganglion (see ), a swelling in the root that lies just outside the cord. Compare  . dose A measure of the extent to which matter has been exposed to *ionizing radiation. The absorbed dose is the energy per unit mass absorbed by matter as a result of such exposure. The SI unit is the gray, although it is often measured in rads (1 rad = 0.01 gray; see  ). The maximum permissible dose is the recommended upper limit of absorbed dose that a person or organ
should receive in a speciÜed period according to the International Commission on Radiological Protection. See also   .
dosimeter Any device used to measure absorbed *dose of ionizing radiation. Methods used include the *ionization chamber, photographic Ülm, or the rate at which certain chemical reactions occur in the presence of ionizing radiation. dot product See  . double bond See  . double circulation The type of circulatory system that occurs in mammals, in which the blood passes through the heart twice before completing a full circuit of the body (see illustration). Blood is pumped
pulmonary vein
pulmonary artery
left atrium left ventricle
right ventricle heart
liver hepatic portal vein
others parts of body = oxygenated blood = deoxygenated blood = direction of blood
Double circulation.
double decomposition
from the heart to the lungs and returns to the heart before being distributed to the other organs and tissues of the body. The heart is divided into two separate compartments to prevent oxygenated blood returning from the lungs from mixing with deoxygenated blood from the other parts of the body. See also  ;  . Compare  .
double decomposition See . double fertilization A process, unique to Ûowering plants, in which two male nuclei, which have travelled down the pollen tube, separately fuse with different female nuclei in the *embryo sac. The Ürst male nucleus fuses with the egg cell to form the zygote; the second male nucleus fuses with the two polar nuclei to form a triploid nucleus that develops into the endosperm. double helix See DNA. double recessive An organism with two *recessive alleles for a particular characteristic. double refraction The property, possessed by certain crystals (notably calcite), of forming two refracted rays from a single incident ray. The ordinary ray obeys the normal laws of refraction. The other refracted ray, called the extraordinary ray, follows different laws. The light in the ordinary ray is polarized at right angles to the light in the extraordinary ray. Along an *optic axis the ordinary and extraordinary rays travel with the same speed. Some crystals, such as calcite, quartz, and tourmaline, have only one optic axis; they are uniaxial crystals. Others, such as mica and selenite, have two optic axes; they are biaxial crystals. The phenomenon is also known as birefringence and the double-refracting crystal as a birefringent crystal. See also . double salt A crystalline salt in which there are two different anions and/or cations. An example is the mineral dolomite, CaCO3.MgCO3, which contains a regular arrangement of Ca2+ and Mg2+ ions in its crystal lattice. *Alums are double sulphates. Double salts only exist in the solid; when dissolved they act as a mixture of the two separate salts. Double oxides are similar. doublet 1. A pair of optical lenses of different shapes and made of different materials used together so that the chromatic aberra-
tion produced by one is largely cancelled by the reverse aberration of the other. 2. A pair of associated lines in certain spectra, e.g. the two lines that make up the sodium D-lines.
down feathers (plumules) Small soft feathers that cover and insulate the whole body of a bird. In nestlings they are the only feathers; in adults they lie between and beneath the *contour feathers. Down feathers have a Ûuffy appearance as their *barbs are not joined together to form a smooth vane. Downs process A process for extracting sodium by the electrolysis of molten sodium chloride. The Downs cell has a central graphite anode surrounded by a cylindrical steel cathode. Chlorine released is led away through a hood over the anode. Molten sodium is formed at the cathode and collected through another hood around the top of the cathode cylinder (it is less dense than the sodium chloride). The two hoods and electrodes are separated by a coaxial cylindrical steel gauze. A small amount of calcium chloride is added to the sodium chloride to lower its melting point. The sodium chloride is melted electrically and kept molten by the current through the cell. More sodium chloride is added as the electrolysis proceeds. Down's syndrome A congenital form of mental retardation due to a chromosome defect in which there are three copies of chromosome no. 21 instead of the usual two (see ). The affected individual has a short broad face and slanted eyes (as in the Mongolian races), short Üngers, and weak muscles. Down's syndrome can be detected before birth by *amniocentesis. It is named after the British physician John Down (1828–96), who Ürst studied the incidence of the disorder. Dragendorff test A *presumptive test for alkaloids. The Dragendorff reagent has two solutions. One is bismuth nitrate in acetic acid. This is followed by a sodium nitrate solution. Alkaloids are indicated by a reddishbrown deposit. dragonÛies See O. drain See . dreikanter (German: three edges) A faceted stone or pebble formed in desert regions by erosion by wind-blown sand. The stones are too heavy to be blown along the desert Ûoor and are merely battered back and forth. Most have three curved facets.
drift-tube accelerator See  . drone A fertile male in a colony of social bees, especially the honeybee (Apis mellifera). The drones die after mating with the queen bee as the male reproductive organs explode within the female. Drosophila A genus of fruit Ûies often used in genetic research because the larvae possess giant chromosomes in their salivary glands. These chromosomes have resulted from repeated duplication without separation of the chromatids; they have conspicuous transverse bands, which can be studied microscopically to reveal gene activity. Fruit Ûies have a short life cycle and produce a large number of offspring, which also makes them a good model animal for genetic research. drug Any chemical substance that alters the physiological state of a living organism. Drugs are widely used in medicine for the prevention, diagnosis, and treatment of diseases; they include *analgesics, *antibiotics, anaesthetics, *antihistamines, and *anticoagulants. Some drugs are taken solely for the pleasurable effects they induce; these include *narcotics; stimulants, such as cocaine and *amphetamine; *hallucinogens, such as *LSD; and some tranquillizers. Many of these drugs are habit-forming and their use is illegal. drupe (pyrenocarp) A Ûeshy fruit that develops from either one or several fused carpels and contains one or many seeds. The seeds are enclosed by the hard protective endocarp (see ) of the fruit. Thus the stone of a peach is the endocarp containing the seed. Plums, cherries, coconuts, and almonds are other examples of one-seeded drupes; holly and elder fruits are examples of many-seeded drupes. See also . dry cell A primary or secondary cell in which the electrolytes are restrained from Ûowing in some way. Many torch, radio, and calculator batteries are *Leclanché cells in which the electrolyte is an ammonium chloride paste and the container is the negative zinc electrode (with an outer plastic wrapping). Various modiÜcations of the Leclanché cell are used in dry cells. In the zinc chloride cell, the electrolyte is a paste of zinc chloride rather than ammonium chloride. The electrical characteristics are similar to those of the Leclanché cell but the cell
works better at low temperatures and has more efÜcient depolarization characteristics. A number of alkaline secondary cells can be designed for use as dry cells. In these, the electrolyte is a liquid (sodium or potassium hydroxide) held in a porous material or in a gel. Alkaline dry cells typically have zinc–manganese dioxide, silver oxide–zinc, nickel–cadmium, or nickel–iron electrode systems (see – ). For specialized purposes, dry cells and batteries have been produced with solid electrolytes. These may contain a solid crystalline salt, such as silver iodide, an ion-exchange membrane, or an organic wax with a small amount of dissolved ionic material. Such cells deliver low currents. They are used in miniature cells for use in electronic equipment.
dry ice Solid carbon dioxide used as a refrigerant. It is convenient because it sublimes at –78°C (195 K) at standard pressure rather than melting. drying oil A natural oil, such as linseed oil, that hardens on exposure to the air. Drying oils contain unsaturated fatty acids, such as linoleic and linolenic acids, which polymerize on oxidation. They are used in paints, varnishes, etc. dry mass The mass of a biological sample after the water content has been removed, usually by placing the sample in an oven. The dry mass is used as a measure of the *biomass of a sample. Dryopithecus A genus of extinct apes, fossils of which have been found in Europe and Asia and dated to the mid-Miocene (about 9–13 million years ago). Fossils of Dryopithecus and of the similar genus Proconsul are often referred to as dryopithecines. Dryopithecines are believed to have split into several lines, some of which survived to give rise to the chimpanzees, gorillas, early hominids, and orang-utans. DSC Differential scanning calorimetry. See  . D-series See  .
DTA Differential thermal analysis. See  . DTD Document type deÜnition. A Üle associated with an *SGML or *XML document, giving a formalized description of the tags used and the structure of the document, as well as special entities that may be present.
dubnium DTDs are optional for XML documents but compulsory for SGML documents. To validate a document is to check its structure against its DTD; a document that passes this test without errors is said to be 'valid' or to conform to its DTD.
dubnium Symbol Db. A radioactive *transactinide element; a.n. 105. It was Ürst reported in 1967 by a group at Dubna near Moscow and was conÜrmed in 1970 at Dubna and at Berkeley, California. It can be made by bombarding californium-249 nuclei with nitrogen-15 nuclei. Only a few atoms have ever been made.
duct A tube or passage in an organism that is involved in the secretion or excretion of substances (see ). ductility The ability of certain metals, such as copper, to retain their strength when their shape is changed, especially the ability of such metals to be drawn into a thin wire without cracking or breaking. ductless gland See  . ductus arteriosus A channel that connects the pulmonary artery with the aorta in the mammalian fetus and therefore allows blood to bypass the inactive lungs of the fetus. It normally closes soon after birth. Dulong and Petit's law For a solid element the product of the relative atomic mass and the speciÜc heat capacity is a constant equal to about 25 J mol–1 K–1. Formulated in these terms in 1819 by the French scientists Pierre Dulong (1785–1838) and Alexis Petit (1791–1820), the law in modern terms states: the molar heat capacity of a solid element is approximately equal to 3R, where R is the *gas constant. The law is only approximate but applies with fair accuracy at normal temperatures to elements with a simple crystal structure.
• A translation of the original 1819 paper in Annals of Philosophy
Dumas, Jean Baptiste André (1800–84) French chemist, who became an apothecary in Geneva, where in 1818 he investigated the use of iodine to treat goitre. He then took up chemistry and moved to Paris. In 1826 he devised a method of measuring *vapour density. He went on to discover various organic
compounds, including anthracene (1832), urethane (1833), and methanol (1834), which led him in 1840 to propose the theory of types (functional groups).
Dumas' method 1. A method of Ünding the amount of nitrogen in an organic compound. The sample is weighed, mixed with copper(II) oxide, and heated in a tube. Any nitrogen present in the compound is converted into oxides of nitrogen, which are led over hot copper to reduce them to nitrogen gas. This is collected and the volume measured, from which the mass of nitrogen in a known mass of sample can be found. 2. A method of Ünding the relative molecular masses of volatile liquids by weighing. A thin-glass bulb with a long narrow neck is used. This is weighed full of air at known temperature, then a small amount of sample is introduced and the bulb heated (in a bath) so that the liquid is vaporized and the air is driven out. The tip of the neck is sealed and the bulb cooled and weighed at known (room) temperature. The volume of the bulb is found by Ülling it with water and weighing again. If the density of air is known, the mass of vapour in a known volume can be calculated. The techniques are named after J. B. A. Dumas. dune A mound or ridge of unconsolidated sand formed by the action of wind. Dunes are characteristic of desert regions and some coastlines. Coastal dunes are usually anchored by vegetation, whereas desert dunes generally move gradually in the direction of the prevailing wind. There are various types, often named after their shapes. Crescentshaped dunes are called barchans, and longitudinal dunes are seif dunes; others include sinuous anklé dunes, star dunes, whaleback dunes and tail dunes, which form in the lee of an obstacle. A large seif dune may be up to 100 m high and 10 km long. Similar structures to dunes may also form underwater by the action of currents. duodenum The Ürst section of the *small intestine of vertebrates. It is the site where food from the stomach is subjected to the action of bile (from the bile duct) and pancreatic enzymes (from the pancreatic duct) as well as the enzymes secreted by digestive glands in the duodenum itself, which are required in the breakdown of proteins, carbohydrates, and fats. By neutralizing the acidic secretions of the stomach, the duodenum
provides an alkaline environment necessary for the action of the intestinal enzymes. See also  .
duplet A pair of electrons in a covalent chemical bond. duplex Describing a biological molecule comprising two cross-linked polymeric chains oriented lengthways side by side. The term is applied particularly to the doublestranded structure of *DNA. duplication (in genetics) The doubling or repetition of part of a chromosome, which generally originates during the *crossing over phase of meiosis. Occasionally this type of *chromosome mutation may have beneÜcial effects on a population. For example, a beneÜcial duplication resulted in the evolution of four types of haemoglobin in man and apes from a single form. One of these types of haemoglobin (gamma or fetal haemoglobin) has a greater afÜnity for oxygen and maximizes fetal uptake of oxygen from the mother's blood. Duralumin Trade name for a class of strong lightweight aluminium alloys containing copper, magnesium, manganese, and sometimes silicon. Duralumin alloys combine strength with lightness and are extensively used in aircraft, racing cars, etc. dura mater The outermost and toughest of the three membranes (*meninges) that surround the central nervous system in vertebrates. It lies adjacent to the skull and its purpose is to protect the delicate inner meninges (the *arachnoid membrane and the *pia mater). duramen See . dust core See . Dutch metal An alloy of copper and zinc, which can be produced in very thin sheets and used as imitation gold leaf. It spontaneously inÛames in chlorine. DVD Digital versatile disk: a disk format similar to a compact disk (see CD-ROM) but containing much more data. It was introduced in 1996. DVD disks are the same 120 mm diameter as CDs with potential capacities of up to 4.7 gigabytes for a singlesided single-layer disk. The technology involved in DVD storage is similar to that in compact disks, but more precise. The extra capacity is achieved in a number of ways. The tracks on a DVD are closer and the pits
are smaller, allowing more pits per unit area. The key to this was the use of a shorter wavelength laser (typically 635 or 650 nm in the red region for DVDs as opposed to 780 nm in the infrared for CDs). Moreover, a DVD can have two layers on the same side of the disk. The top layer is translucent and the bottom layer opaque. Data can be read from either layer by refocusing the laser. In addition DVDs may be double-sided. DVD formats also have a more efÜcient error-correction system. The potential capacity of a doublesided double-layer DVD is up to 17 gigabytes. DVDs have been increasingly used in computing as a higher-capacity version of compact disks. As with compact disks, there are various types. DVD-ROM (DVD readonly memory) is similar to CD-ROM. DVD-R (DVD-recordable) is similar to CD-R. There are also different rewritable formats: DVDRAM, DVD+RW, and DVD-RW.
• The DVD Forum home page
dwarf galaxy A galaxy made up of only a few billion stars rather than the 200 to 400 billion that form the Milky Way System. Dwarf galaxies are classiÜed like ordinary galaxies. In the *Local Group, several dwarf galaxies are satellites of the group's three largest galaxies. dwarf planet A *small solar system body that is in direct orbit around the sun, not around another planet, and has sufÜcient mass to have contracted to a spherical or nearly spherical shape but has not cleared its orbital zone of *planetesimals. Five objects have been classiÜed as dwarf planets since 2006, including *Pluto (formerly the ninth planet of the solar system and now recognized as a *Kuiper belt object [KBO]), *Eris (the largest dwarf planet so far known), the asteroid (1) Ceres, and the KBOs (136472) Makemake and (136108) Haumea.
• International Astronomical Union 2006 General Assembly: Result of the IAU resolution votes. This press release carries the official definition of a planet & dwarf planet • NASA's explanation of the IAU's definitions of 'planet' and 'dwarf planet'
dwarf star A star, such as the sun, that lies on the main sequence in a *Hertzsprung– Russell diagram and is of average or normal size in relation to its mass. See also  ;  ;  ;  .
dye laser
dye laser A type of laser in which the active material is a dye dissolved in a suitable solvent (e.g. Rhodanine G in methanol). The dye is excited by an external source. The solvent broadens the states into bands and consequently laser action can be obtained over a range of wavelengths. This allows one to select a speciÜc wavelength (using a grating) and to change the wavelength of the laser. Such a device is called a tuneable laser. Dye lasers are also used in producing very short pulses of radiation. The technique is to use a dye that stops absorbing radiation when a high proportion of its molecules become excited. The cavity then becomes resonant and a pulse of radiation is produced. This technique can give pulses of about 10 nanoseconds duration and is used in *femtochemistry. dyes Substances used to impart colour to textiles, leather, paper, etc. Compounds used for dyeing (dyestuffs) are generally organic compounds containing conjugated double bonds. The group producing the colour is the *chromophore; other noncoloured groups that inÛuence or intensify the colour are called *auxochromes. Dyes can be classiÜed according to the chemical structure of the dye molecule. For example, azo dyes contain the –N=N– group (see  ). In practice, they are classiÜed according to the way in which the dye is applied or is held on the substrate. Acid dyes are compounds in which the chromophore is part of a negative ion (usually an organic sulphonate RSO2O–). They can be used for protein Übres (e.g. wool and silk) and for polyamide and acrylic Übres. Originally, they were applied from an acidic bath. Metallized dyes are forms of acid dyes in which the negative ion contains a chelated metal atom. Basic dyes have chromophores that are part of a positive ion (usually an amine salt or ionized imino group). They are used for acrylic Übres and also for wool and silk, although they have only moderate fastness with these materials. Direct dyes are dyes that have a high afÜnity for cotton, rayon, and other cellulose Übres. They are applied directly from a neutral bath containing sodium chloride or sodium sulphate. Like acid dyes, they are usually sulphonic acid salts but are distinguished by their greater substantivity (afÜnity for the substrate), hence the alternative name substantive dyes. Vat dyes are insoluble substances used for
cotton dyeing. They usually contain keto groups, C=O, which are reduced to C–OH groups, rendering the dye soluble (the leuco form of the dye). The dye is applied in this form, then oxidized by air or oxidizing agents to precipitate the pigment in the Übres. Indigo and anthroquinone dyes are examples of vat dyes. Sulphur dyes are dyes applied by this technique using sodium sulphide solution to reduce and dissolve the dye. Sulphur dyes are used for cellulose Übres. Disperse dyes are insoluble dyes applied in the form of a Üne dispersion in water. They are used for cellulose acetate and other synthetic Übres. Reactive dyes are compounds that contain groups capable of reacting with the substrate to form covalent bonds. They have high substantivity and are used particularly for cellulose Übres.
dyke A sheetlike vertical *intrusion of igneous rock cutting across the strata of older rocks. Dykes vary in thickness from a few centimetres to several metres. Several dykes may be grouped, radially or in parallel, as a dyke swarm. Depending on composition and how long it took to cool, a dyke's grain structure may be coarse, medium, or Üne. dynamical meteorology The branch of meteorology concerned with motions in the atmosphere. It is based on hydrodynamics and thermodynamics. dynamic equilibrium See . dynamics The branch of mechanics concerned with the motion of bodies under the action of forces. Time intervals, distances, and masses are regarded as fundamental and bodies are assumed to possess *inertia. Bodies in motion have an attribute called *momentum (see N'   ), which can only be changed by the application of a force. Compare ; . dynamite Any of a class of high explosives based on nitroglycerin. The original form, invented in 1867 by Alfred Nobel, consisted of nitroglycerin absorbed in kieselguhr. Modern dynamites, which are used for blasting, contain sodium or ammonium nitrate sensitized with nitroglycerin and use other absorbers (e.g. wood pulp). dynamo An electric *generator, especially one designed to provide *direct current. Alternating-current generators can be called
dystrophic
dynamos but are more often called alternators.
dynamo action The generation of electrical current and magnetic Üeld by the motion of an electrically conducting Ûuid. It is generally believed that the magnetic Üelds of the earth and the sun are produced by dynamo action in the molten iron–nickel core of the earth and in the plasma of the solar interior. dynamometer 1. An instrument used to measure a force, often a spring balance. 2. A device used to measure the output power of an engine or motor. 3. (current dynamometer) A variety of *current balance, for measuring electric current. dyne The unit of force in the *c.g.s. system; the force required to give a mass of one gram an acceleration of 1 cm s–2. 1 dyne = 10–5 newton. dysprosium Symbol Dy. A soft silvery metallic element belonging to the *lan-
thanoids; a.n. 66; r.a.m. 162.50; r.d. 8.551 (20°C); m.p. 1412°C; b.p. 2562°C. It occurs in apatite, gadolinite, and xenotime, from which it is extracted by an ion-exchange process. There are seven natural isotopes and twelve artiÜcial isotopes have been identiÜed. It Ünds limited use in some alloys as a neutron absorber, particularly in nuclear technology. It was discovered by Paul Lecoq de Boisbaudran (1838–1912) in 1886.
dystectic mixture A mixture of substances that has a constant maximum melting point. dystrophic Describing a body of water, such as a lake, that contains large amounts of undecomposed organic matter derived from terrestrial plants. Dystrophic lakes are poor in dissolved nutrients and therefore unproductive; they are common in peat areas and may develop into peat bogs.
E e The irrational number deÜned as the limit as n tends to inÜnity of (1 + 1/n)n. It has the value 2.718 28…. It is used as the base of natural *logarithms and occurs in the *exponential function, ex. EAC Emergency action code. See  . ear The sense organ in vertebrates that is specialized for the detection of sound and the maintenance of balance. It can be divided into the *outer ear and *middle ear, which collect and transmit sound waves, and the *inner ear, which contains the organs of balance and (except in Üsh) hearing (see illustration). eardrum See . early universe The study of *cosmology at the time very soon after the *big bang. Theories of the early universe have led to a mutually beneÜcial interaction between cosmology and the theory of *elementary particles, particularly *grand uniÜed theories. Because there were very high tempera-
tures in the early universe many of the *broken symmetries in *gauge theories were unbroken symmetries. As the universe cooled after the big bang there is thought to have been a sequence of transitions to broken symmetry states. Combining cosmology with grand uniÜed theories helps to explain why the observed universe appears to consist of matter with no antimatter. This means that one has a nonzero *baryon number for the universe. This solution relies on the fact that there were nonequilibrium conditions in the early universe due to its rapid expansion after the big bang. An important idea in the theory of the early universe is that of inflation – the idea that the nature of the *vacuum state gave rise, after the big bang, to an exponential expansion of the universe. The hypothesis of the inflationary universe solves several long-standing problems in cosmology, such as the Ûatness and homogeneity of the universe. In particular, it is thought that quantum Ûuctuations in the early universe
utriculus
ear ossicles pinna
perilymph
malleus stapes (hammer) (stirrup) incus (anvil)
ampulla auditory nerve sacculus oval window nerve fibre
tympanum (eardrum) external auditory meatus
outer ear
Ear. Structure of the mammalian ear.
cochlea endolymph round window middle ear
Eustachian tube
were responsible for the emergence of largescale structures in the universe, such as galaxies.
ear ossicles Three small bones – the incus (anvil), malleus (hammer), and stapes (stirrup) – that lie in the mammalian *middle ear, forming a bridge between the tympanum (eardrum) and the *oval window. The function of the ossicles is to transmit (and amplify) vibrations of the tympanum across the middle ear to the oval window, which transfers them to the *inner ear. Muscles of the middle ear constrict the movement of the ossicles. This serves to safeguard the ear from damage caused by excessively loud noise. earth The planet that orbits the sun between the planets Venus and Mars at a mean distance from the sun of 149 600 000 km. It has a mass of about 5.974 × 1024 kg and an equatorial diameter of 12 756.3 km. The earth consists of three layers: the gaseous atmosphere (see ' ), the liquid *hydrosphere, and the solid *lithosphere. The solid part of the earth also consists of three layers: the crust with a mean thickness of about 32 km under the land and 10 km under the seas; the mantle, which extends some 2900 km below the crust; and the core, part of which is believed to be liquid. The crust has a relative density of about 2.7 to 3.0 and consists largely of sedimentary rocks overlaying igneous rocks. The composition of the crust is: oxygen 47%, silicon 28%, aluminium 8%, iron 4.5%, calcium 3.5%, sodium and potassium 2.5% each, and magnesium 2.2%. Hydrogen, carbon, phosphorus, and sulphur are all present to an extent of less than 1%. The mantle reaches a relative density of about 5.5 at its maximum depth and is believed to consist mainly of silicate rocks. The core is believed to have a maximum relative density of 13 and a maximum temperature of 6400 K. See also ;  .
• NASA's earth observatory
earthquake A sudden movement or fracturing within the earth's lithosphere, causing a series of shocks. This may range from a mild tremor to a large-scale earth movement causing extensive damage over a wide area. The point at which the earthquake originates is known as the seismic focus; the point on the earth's surface directly above this is the epicentre (or hypocentre). See 
earth's atmosphere . Earthquakes result from a build-up of stresses within the rocks until they are strained to the point beyond which they will fracture. They occur in narrow continuous belts of activity, which correspond with the junction of lithospheric plates, including the circum-PaciÜc belt, the Alpine– Himalayan belt, and mid-ocean ridges. The scale of the shock of an earthquake is known as the magnitude; the most commonly used scale for comparing the magnitude of earthquakes is the logarithmic *Richter scale (9.5 is the highest recorded magnitude on the scale).
earth's atmosphere The gas that surrounds the earth. The composition of dry air at sea level is: nitrogen 78.08%, oxygen 20.95%, argon 0.93%, carbon dioxide 0.03%, neon 0.0018%, helium 0.0005%, krypton 0.0001%, and xenon 0.00001%. In addition to water vapour, air in some localities contains sulphur compounds, hydrogen peroxide, hydrocarbons, and dust particles. The lowest level of the atmosphere, in which most of the weather occurs, is called the troposphere. Its thickness varies from about 7 km at the poles to 28 km at the equator and in this layer temperature falls with increasing height. The next layer is the stratosphere, which goes up to about 50 km. Here the temperature remains approximately constant. Above this is the ionosphere, which extends to about 1000 km, with the temperature rising and the composition changing substantially. At about 100 km and above most of the oxygen has dissociated into atoms; at above 150 km the percentage of nitrogen has dropped to nil. In the ionosphere the gases are ionized by the absorption of solar radiation. This enables radio transmissions to be made round the curved surface of the earth as the ionized gas acts as a reÛector for certain wavelengths. The ionosphere is divided into three layers. The D-layer (50–90 km) contains a low concentration of free electrons and reÛects lowfrequency radio waves. The E-layer (90–150 km) is also called the Heaviside layer or Heaviside–Kennelly layer as its existence was predicted independently by Oliver Heaviside (1850–1925) and Arthur E. Kennelly (1861–1939). This layer reÛects medium-frequency waves. The F-layer (150–1000 km) is also called the Appleton layer after its discoverer Sir Edward Appleton (1892–1965). It has the highest concentration of free electrons and is the most
earth sciences useful for radio transmission. Wavelengths between 8 mm and 20 m are not reÛected by the ionosphere but escape into space. Therefore television transmissions, which utilize this range, require artiÜcial *satellites for reÛection (or reception, ampliÜcation, and retransmission). From about 400 km, the outermost region of the atmosphere is also called the exosphere. See illustration.
earth sciences A group of sciences concerned with the study of the earth. The chief earth sciences are geology, physical geography, oceanography, meteorology, geophysics, and geochemistry. earthshine Sunlight reÛected from the surface of the earth. An observer in space may see nearby objects dimly illuminated by earthshine, as things on earth may be illuminated by moonlight. Under certain conditions near new moon the dark disc of the moon can be seen faintly illuminated by earthshine – a phenomenon called 'the old moon in the new moon's arms'. earthslide The movement of a layer of dry
soil down a slope. The soil layer is inherently unstable and shears, sometimes brought about by the action of water. But if the soil becomes very wet, and the slope is steep, an earthÛow may occur. Contributory factors are sparse vegetation and sudden rainfall.
earth's magnetic Üeld See . earwigs See D. ebonite See . ebullioscopic constant See    . ebullioscopy The use of *elevation of boiling point to determine relative molecular masses. eccentricity See . Eccles, Sir John Carew (1903–97) Australian physiologist, who was educated in Melbourne and Oxford, and held appointments in Britain, Australia, New Zealand and, Ünally, the USA. While in Australia he carried out his best-known work, on the transmission of nerve impulses across
height (km)
F region
300 250 ionosphere
(Appleton layer) (Heaviside–Kennelly layer)
200 150 E region 100 D region 50
stratosphere troposphere
log10 density (kg m–3)
Earth's atmosphere.
synapses, which he attributed to the neurotransmitter acetylcholine. He shared the 1963 Nobel Prize for physiology or medicine with Sir Alan Hodgkin (1914–98) and Sir Andrew Huxley (1917– ), who worked in the same area of biology.
ecdysis (moulting) 1. The periodic loss of the outer cuticle of arthropods. It starts with the reabsorption of some materials in the inner part of the old cuticle and the formation of a new soft cuticle. The remains of the old cuticle then split; the animal emerges and absorbs water or swallows air and increases in size while the new cuticle is still soft. This cuticle is then hardened with chitin and lime salts. In insects and crustaceans ecdysis is controlled by the hormone *ecdysone. 2. The periodic shedding of the outer layer of the epidermis of reptiles (except crocodiles) to allow growth to occur. ecdysone A steroid hormone, produced by insects and crustaceans, that stimulates moulting (see ) and metamorphosis. It acts on speciÜc gene loci, stimulating the synthesis of proteins involved in these bodily changes. e-cell A computer simulation of a living cell based on information obtained from genomics, proteomics, metabolomics, and other data drawn from real cells. ECG See . echelon A form of *interferometer consisting of a stack of glass plates arranged stepwise with a constant offset. It gives a high resolution and is used in spectroscopy to study hyperÜne line structure. In the transmission echelon the plates are made equal in optical thickness to introduce a constant delay between adjacent parts of the wavefront. The reflecting echelon has the exposed steps metallized and acts like an exaggerated *diffraction grating. Echinodermata A phylum of marine invertebrates that includes the sea urchins, starÜsh, brittlestars, and sea cucumbers. Echinoderms have an exoskeleton (test) of calcareous plates embedded in the skin. In many species (e.g. sea urchins) spines protrude from the test. A system of water-Ülled canals (the water vascular system) provides hydraulic power for thousands of tube feet: saclike protrusions of the body wall used for locomotion, feeding, and respiration. Echinoderms have a long history: fossils of primi-
tive echinoderms are known from rocks over 500 million years old.
• Overview of echinoderm characteristics and phylogeny from the Tree of Life Project
echo The reÛection of a wave by a surface or object so that a weaker version of it is detected shortly after the original. The delay between the two is an indication of the distance of the reÛecting surface. An echo sounder is an apparatus for determining the depth of water under a ship. The ship sends out a sound wave and measures the time taken for the echo to return after reÛection by the sea bottom. Sonar (sound navigation ranging) is a technique for locating underwater objects by a similar method. Echoes also occur with radio waves; reÛection of waves causes an echo in radio transmission and ghosts in television pictures. See also . echolocation 1. See ; . 2. A method used by some animals (such as bats, dolphins, and certain birds) to detect objects in the dark. The animal emits a series of high-pitched sounds that echo back from the object and are detected by the ear or some other sensory receptor. From the direction of the echo and from the time between emission and reception of the sounds the object is located, often very accurately. ECL See - . eclipse The obscuring of light from a celestial body as it passes behind or through the shadow of another body. In a total eclipse, the light is completely cut off; in a partial eclipse, the light is only partly hidden. A lunar eclipse occurs when the sun, earth, and moon are in a straight line and the shadow of the earth falls on the moon. A solar eclipse occurs when the sun, moon, and earth are aligned in such a way that the moon blocks out light from the sun's *photosphere and casts its shadow on the earth. An annular eclipse occurs when the moon is too far away for its shadow to reach the earth: during an eclipse, the rim of the photosphere is visible as a Üery ring (annulus) around the dark silhouette of the moon. See illustrations overleaf.
• NASA's eclipse Website, a fully comprehensive site maintained by Fred Espenak at the Goddard Space Flight Center, near Washington DC
eclipsed conformation
earth's orbit
Solar and lunar eclipses
moon's motion
earth's motion
path of totality
Moon's shadow in solar eclipse
Eclipse. eclipsed conformation See . ecliptic The *great circle on the *celestial sphere that traces the sun's apparent passage through the sky against the background of the *zodiac constellations over the course of a year. In reality, it is the earth that is travels around the sun, and so the ecliptic delineates the plane of the earth's orbit. See also ; . ECM 1. See  . 2. See   . ECMAScript See JS. E. coli See E . ecological niche The status or role of an organism in its environment. An organism's niche is deÜned by its food supply, predators, temperature tolerances, etc. Two species cannot coexist stably if they occupy identical niches. ecology The study of the interrelationships between organisms and their natural
environment, both living and nonliving. For this purpose, ecologists study organisms in the context of the *populations and *communities in which they can be grouped and the *ecosystems of which they form a part. The study of ecological interactions provides important information on the nature and mechanisms of evolutionary change. Advances made in ecology over the last 30 years have led to increased concern about the effects of human activities on the environment (notably the effects of *pollution), which has resulted in a greater awareness of the importance of *conservation.
ecosystem A biological *community and the physical environment associated with it. Nutrients pass between the different organisms in an ecosystem in deÜnite pathways; for example, nutrients in the soil are taken up by plants, which are then eaten by herbivores, which in turn may be eaten by carnivores (see  ). Organisms are classiÜed on the basis of their position in an ecosystem into various *trophic levels. Nutrients and energy move round ecosystems in loops or cycles (in the case above, for example, nutrients are returned to the soil via animal wastes and decomposition). See  ;  .
• The latest Millennium Ecosystem Assessment Reports commissioned by the United Nations.
ecosystem services The beneÜts that people receive from ecosystems. These beneÜts can be classiÜed into several categories. Provisioning services include the products that humans derive from ecosystems, such as food, Übres, fuel, drinking water, and medicinal products. Regulating services encompass such processes as control of the climate, puriÜcation of air and water, amelioration of erosion, reduction of pests and diseases, and provision of pollination mechanisms. Cultural services comprise the nonmaterial beneÜts that accrue from the landscape, such as recreational pursuits, spiritual renewal, and aesthetic experiences. Underlying all these are the fundamental supporting services of all ecosystems, such as primary production (i.e. photosynthesis by green plants, algae, etc.), nutrient cycling (e.g. carbon, nitrogen, sulphur cycles), water cycling, and soil formation. In recent decades the widespread degradation of many of the world's ecosystems has seen the concept of ecosystem ser-
vices acquire greater prominence, chieÛy as a means of assessing the economic value of particular ecological beneÜts and the cost of their destruction.
ecotype A subgroup of a species that has characteristic genetically determined adaptations to its local environment. In some cases individuals belonging to different ecotypes cannot interbreed, for example where accumulated genetic differences are too great. ecstasy (methylenedioxymethamphetamine; MDMA) A designer drug based on methamphetamine (see ). Originally intended as an appetite suppressant, it produces a feeling of euphoria and is widely used as a club drug. It is a class A drug in the UK. ectoderm The external layer of cells of the *gastrula, which will develop into the epidermis and the nervous system in the adult. See also  . ectoparasite A parasite that lives on the outside of its host's body. See . ectoplasm See . Ectoprocta See B. ectotherm (poikilotherm) An animal that maintains its body temperature by absorbing heat from the surrounding environment. All animals except mammals and birds are ectotherms; they are often described as being cold-blooded and are unable to regulate their body temperature metabolically. See . Compare . edaphic factor A factor relating to the physical or chemical composition of the soil found in a particular area. For example, very alkaline soil may be an edaphic factor limiting the variety of plants growing in a region. Eddington limit A limit for the maximum value of the brightness of a star of a given mass. This limit exists because the radiation pressure caused by the nuclear fusion reactions powering the star has to counter, but not exceed, the gravitational force that would cause gravitational collapse of the star. The existence of this limit was Ürst pointed out by the English astrophysicist Sir Arthur Stanley Eddington (1882–1944). eddy current A current induced in a conductor situated in a changing magnetic Üeld or moving in a Üxed one. Any imagined circuit within the conductor will change its
efÜciency magnetic Ûux linkage, and the consequent induced e.m.f. will drive current around the circuit. In a substantial block of metal the resistance will be small and the current therefore large. Eddy currents occur in the cores of transformers and other electrical machines and represent a loss of useful energy (the eddy-current loss). To reduce this loss to a minimum metal cores are made of insulated sheets of metal, the resistance between these laminations reducing the current. In high-frequency circuits *ferrite cores can be used. Eddy currents in a moving conductor interact with the magnetic Üeld producing them to retard the motion of the conductor. This enables some electrical instruments (moving-coil type) to utilize eddy currents to create damping. Eddy currents are also used in *induction heating.
Edison cell See – . EDTA Ethylenediaminetetraacetic acid, (HOOCCH2)2N(CH2)2N(CH2COOH)2 A compound that acts as a chelating agent, reversibly binding with iron, magnesium, and other metal ions. It is used in certain culture media bound with iron, which it slowly releases into the medium, and also in some forms of quantitative analysis. EEG See . effective temperature See . effective value See -- . effector A cell or organ that produces a physiological response when stimulated by a nerve impulse. Examples include muscles and glands. effector neuron A nerve cell, such as a motor neuron, that transmits impulses from the central nervous system to an *effector in order to bring about a physiological response to changes in the environment. efferent Carrying (nerve impulses, blood, etc.) away from the centre of a body or organ towards peripheral regions. The term is usually applied to types of nerve Übres or blood vessels. Compare . effervescence The formation of gas bubbles in a liquid by chemical reaction. efÜciency A measure of the performance of a machine, engine, etc., being the ratio of the energy or power it delivers to the energy
efÛorescence
or power fed to it. In general, the efÜciency of a machine varies with the conditions under which it operates and there is usually a load at which it operates with the highest efÜciency. The thermal efficiency of a heat engine is the ratio of the work done by the engine to the heat supplied by the fuel. For a reversible heat engine this efÜciency equals (T1 – T2)/T1, where T1 is the thermodynamic temperature at which all the heat is taken up and T2 is the thermodynamic temperature at which it is given out (see C ). For real engines it is always less than this.
efÛorescence The process in which a crystalline hydrate loses water, forming a powdery deposit on the crystals. effusion The Ûow of a gas through a small aperture. The relative rates at which gases effuse, under the same conditions, is approximately inversely proportional to the square roots of their densities. egestion The expulsion from the body of waste food materials that have never left the gut, particularly the expulsion of undigested materials from the gut through the anus (see ). Egestion should not be confused with *excretion, in which the waste materials are produced by metabolic activity in the body's tissues. egg 1. The fertilized ovum (*zygote) in egglaying animals, e.g. birds and insects, after it emerges from the body. The egg is covered by *egg membranes that protect it from environmental damage, such as drying. 2. (egg cell) The mature female reproductive cell in animals and plants. See ; . egg membrane The layer of material that covers an animal egg cell. Primary membranes develop in the ovary and cover the egg surface in addition to the normal plasma membrane. The primary membrane is called the vitelline membrane in insects, molluscs, birds, and amphibians, the chorion in tunicates and Üsh, and the zona pellucida in mammals. Insects have a second thicker membrane, also called the chorion. Secondary membranes are secreted by the oviducts and parts of the genital system while the egg is passing to the outside. They include the jelly coat of frogs' eggs and the albumen and shell of birds' eggs. Ehrlich, Paul (1854–1915) German bacteriologist, who graduated as a physician in 1878. After working in a Berlin hospital for nine years he taught at Berlin University (un-
paid because he was a Jew). In 1890 he went to work with Robert Koch (1843–1910) to study tuberculosis, cholera, and other diseases. In 1910 he discovered Salvarsan, an arsenical drug effective against syphilis. He was awarded the 1908 Nobel Prize for physiology or medicine for his earlier work on serum therapy.
eigenfunction An allowed *wave function of a system in quantum mechanics. The associated energies are eigenvalues. Einstein, Albert (1879–1955) Germanborn US physicist, who took Swiss nationality in 1901. A year later he went to work in the Bern patent ofÜce. In 1905 he published Üve enormously inÛuential papers, one on *Brownian movement, one on the *photoelectric effect, one on the special theory of *relativity, and one on energy and inertia (which included the famous expression E = mc2). In 1915 he published the general theory of relativity, concerned mainly with gravitation. In 1921 he was awarded the Nobel Prize for physics. In 1933, as a Jew, Einstein decided to remain in the USA (where he was lecturing), as Hitler had come to power. For the remainder of his life he sought a uniÜed Üeld theory. In 1939 he informed President Roosevelt that an atom bomb was feasible and that Germany might be able to make one.
• A comprehensive website about his life and work run by The American Institute of Physics Center for History of Physics
Einstein coefÜcients CoefÜcients used in the *quantum theory of radiation, related to the probability of a transition occurring between the ground state and an excited state (or vice versa) in the processes of *induced emission and *spontaneous emission. For an atom exposed to *electromagnetic radiation, the rate of absorption Ra is given by Ra = Bρ, where ρ is the density of electromagnetic radiation and B is the Einstein B coefficient associated with absorption. The rate of induced emission is also given by Bρ, with the coefÜcient B of induced emission being equal to the coefÜcient of absorption. The rate of spontaneous emission is given by A, where A is the Einstein A coefficient of spontaneous emission. The A and B coefÜcients are related by A = 8πhν3B/c3, where h is the *Planck constant, ν is the frequency of electromagnetic radiation, and c is the speed of light. The coefÜcients were put forward by
Einstein equation 1. The mass–energy relationship announced by Einstein in 1905 in the form E = mc2, where E is a quantity of energy, m its mass, and c is the speed of light. It presents the concept that energy has mass. See also . 2. The relationship Emax = hf – W, where Emax is the maximum kinetic energy of the electrons emitted in the photoemissive effect, h is the Planck constant, f the frequency of the incident radiation, and W the *work function of the emitter. This is also written Emax = hf – φe, where e is the electronic charge and φ a potential difference, also called the work function. (Sometimes W and φ are distinguished as work function energy and work function potential.) The equation can also be applied to photoemission from gases, when it has the form E = hf – I, where I is the ionization potential of the gas. einsteinium Symbol Es. A radioactive metallic transuranic element belonging to the *actinoids; a.n. 99; mass number of the most stable isotope 254 (half-life 270 days). Eleven isotopes are known. The element was Ürst identiÜed by Albert Ghiorso and associates in debris from the Ürst hydrogen bomb explosion in 1952. Microgram quantities of the element did not become available until 1961.
Einstein shift See . ejaculation The propulsion of semen out of the erect penis due to powerful rhythmic contractions of the urethra. An ejaculation coincides with the peak of sexual excitement (orgasm) and is accompanied by various physiological effects in the body, such as increased respiration rate and heart rate. ejecta Rocks and other material thrown up when a crater is formed. Some craters and their associated ejecta are volanic in origin, but most (especially on the moon, Mercury, and other planets and their satellites) are caused by the impact of meteorites. Often these are surrounded by an ejecta blanket, which is thickest near the crater's rim. ekpyrotic universe A model of the universe that postulates that the big bang oc-
curred because of the collision of two parallel branes of a type predicted to occur in string theory (see  ). The term 'ekpyrotic' means 'coming out of Üre'. It has been developed into a cyclic model of the universe, which avoids the problems of entropy associated with the *oscillatory universe. There are some differences between the predictions of the ekpyrotic theory and those of the more established inÛationary universe (see  ).
Elasmobranchii See C. elastance The reciprocal of *capacitance. It is measured in farad–1 (sometimes called a 'daraf'). elastic cartilage See . elastic collision A collision in which the total kinetic energy of the colliding bodies after collision is equal to their total kinetic energy before collision. Elastic collisions occur only if there is no conversion of kinetic energy into other forms, as in the collision of atoms. In the case of macroscopic bodies this will not be the case as some of the energy will become heat. In a collision between polyatomic molecules, some kinetic energy may be converted into vibrational and rotational energy of the molecules, but otherwise molecular collisions appear to be elastic. elastic Übres See . elasticity The property of certain materials that enables them to return to their original dimensions after an applied *stress has been removed. In general, if a stress is applied to a wire, the *strain will increase in proportion (see OA on the illustration) until a certain point called the limit of proportionality is reached. This is in accordance with *Hooke's law. Thereafter there is at Ürst a slight increase in strain with increased load until a point L is reached. This is the elastic
C load
Albert Einstein in 1916–17 in his analysis of the quantum theory of radiation.
elastic limit D breaking L B A stress yield point breaking point proportional limit
Elasticity.
limit; up to this point the deformation of the specimen is elastic, i.e. when the stress is removed the specimen returns to its original length. Beyond the point L there is permanent deformation when the stress is removed, i.e. the material has ceased to be elastic and has become plastic. In the plastic stages individual materials vary somewhat; in general, however, at a point B there is a sudden increase in strain with further increases of stress – this is the yield point. Beyond the point C, the breaking stress, the wire will snap (which occurs at point D).
elastic modulus The ratio of the *stress applied to a body to the *strain produced. The Young modulus of elasticity, named after Thomas Young, refers to longitudinal stress and strain. The bulk modulus is the ratio of the pressure on a body to its fractional decrease in volume. The shear (or rigidity) modulus is the tangential force per unit area divided by the angular deformation in radians. elastin A Übrous protein that is the major constituent of the yellow elastic fibres of *connective tissue. It is rich in glycine, alanine, proline, and other nonpolar amino acids that are cross-linked, making the protein relatively insoluble. Elastic Übres can stretch to several times their length and then return to their original size. Elastin is particularly abundant in elastic *cartilage, bloodvessel walls, ligaments, and the heart. elastomer A natural or synthetic rubber or rubberoid material, which has the ability to undergo deformation under the inÛuence of a force and regain its original shape once the force has been removed. electret A permanently electriÜed substance or body that has opposite charges at its extremities. Electrets resemble permanent magnets in many ways. An electret can be made by cooling certain waxes in a strong electric Üeld. electrical energy A form of energy related to the position of an electric charge in an electric Üeld. For a body with charge Q and an electric potential V, its electrical energy is QV. If V is a potential difference, the same expression gives the energy transformed when the charge moves through the p.d. electric arc A luminous discharge between two electrodes. The discharge raises the electrodes to incandescence, the result-
ing thermal ionization largely providing the carriers to maintain the high current between the electrodes.
electric-arc furnace A furnace used in melting metals to make alloys, especially in steel manufacture, in which the heat source is an electric arc. In the direct-arc furnace, such as the Héroult furnace, an arc is formed between the metal and an electrode. In the indirect-arc furnace, such as the Stassano furnace, the arc is formed between two electrodes and the heat is radiated onto the metal. electric bell A device in which an electromagnetically operated hammer strikes a bell (see illustration). Pressing the bell-push closes a circuit, causing current to Ûow from a battery or mains step-down transformer through an electromagnet. The electromagnet attracts a piece of soft iron attached to the hammer, which strikes the bell and at the same time breaks the circuit. The hammer springs back into its original position again, closing the circuit and causing the magnet to attract the soft iron. This process continues until the bell-push is released. bell hammer
light iron trembler
soft iron core coil spring switch cell
Electric bell. electric charge See . electric constant See . electric current See . electric displacement (electric Ûux den-
sity) Symbol D. The charge per unit area that would be displaced across a layer of conductor placed across an *electric Üeld. This describes also the charge density on an extended surface that could be causing the Üeld. electric Üeld A region in which an electric charge experiences a force usually because of a distribution of other charges. The electric field strength or electric intensity (E) at any point in an electric Üeld is deÜned as the force per unit charge experienced by a small charge placed at that point. This is equivalent to a potential gradient along the Üeld and is measured in volts per metre. The strength of the Üeld can alternatively be described by its *electric displacement D. The ratio D/E for measurements in a vacuum is the electric constant ε0. In a substance the observed potential gradient is reduced by electron movement so that D/E appears to increase: the new ratio (ε) is called the *permittivity of the substance. An electric Üeld can be created by an isolated electric charge, in which case the Üeld strength at a distance r from a point charge Q is given by E = Q/4πr2ε, where ε is the permittivity of the intervening medium (see C' ). An electric Üeld can also be created by a changing magnetic Üeld. electric Ûux Symbol Ψ. In an *electric Üeld, the product of the electric Ûux density and the relevant area. See  . electric Ûux density See  . electricity Any effect resulting from the existence of stationary or moving electric charges. electric lighting Illumination provided by electric currents. The devices used are the arc lamp, the light bulb (incandescent Ülament lamp), and the Ûuorescent tube. In the arc lamp, which is no longer used as a general means of illumination, an electric current Ûows through a gap between two carbon electrodes, between which a high potential difference is maintained. The current is carried by electrons and ions in the vapour produced by the electrodes and a mechanism is required to bring the electrodes closer together as they are vaporized. The device produces a strong white light but has many practical disadvantages. However, arcs enclosed in an inert gas (usually xenon) are
electric motor increasingly used for such purposes as cinema projectors. The common light bulb is a glass bulb containing a tungsten Ülament and usually an inert gas. The passage of an electric current through the Ülament heats it to a white heat. Inert gas is used in the bulb to minimize blackening of the glass by evaporation of tungsten. In the Ûuorescent tube a glass tube containing mercury vapour (or some other gas) at a low pressure has its inner surface coated with a Ûuorescent substance. A discharge is created within the tube between two electrodes. Electrons emitted by the cathode collide with gas atoms or molecules and raise them to an excited state (see ). When they fall back to the *ground state they emit photons of ultraviolet radiation, which is converted to visible light by the coating of phosphor on the inner walls of the tube. In some lamps, such as the *sodium-vapour and *mercury-vapour lamps used in street lighting, no Ûuorescent substance is used, the light being emitted directly by the excited atoms of sodium or mercury. Vapour lights are more efÜcient than Ülament lights as less of the energy is converted into heat.
electric motor A machine for converting electrical energy into mechanical energy. They are quiet, clean, and have a high efÜciency (75–95%). They work on the principle that a current passing through a coil within a magnetic Üeld will experience forces that can be used to rotate the coil. In the induction motor, alternating current is fed to a stationary coil (the stator), which both creates the magnetic Üeld and induces a current in the rotating coil (rotor), which it surrounds. The advantage of this kind of motor is that current does not have to be fed through a commutator to a moving part. In the synchronous motor, alternating current fed to the stator produces a magnetic Üeld that rotates and locks with the Üeld of the rotor, in this case an independent magnet, causing the rotor to rotate at the same speed as the stator Üeld rotates. The rotor is either a permanent magnet or an electromagnet fed by a direct current through slip rings. In the universal motor, current is fed to the stator and, through a commutator, to the rotor. In the series-wound motor the two are in series; in the shunt-wound motor they are in parallel. These motors can be used with either a.c. or d.c. but some small motors use a permanent magnet as the stator and require d.c. for the
electric organ rotor (via the commutator). See also  .
electric organ An organ occurring on the body or tail of certain Üsh, such as the electric ray (Torpedo) and electric eel (Electrophorus electricus). It gives an electric shock when touched and is used either to stun prey or predators or, in some species, to maintain a weak electric Üeld in the surrounding water that is used in navigation. The organ is composed of modiÜed muscle cells (electroplate cells), nervous stimulation of which greatly increases the potential difference across the cell. The electroplates are in series so a high overall voltage can be achieved. electric polarization See . electric potential Symbol V. The energy required to bring unit electric charge from inÜnity to the point in an electric Üeld at which the potential is being speciÜed. The unit of electric potential is the volt. The potential difference (p.d.) between two points in an electric Üeld or circuit is the difference in the values of the electric potentials at the two points, i.e. it is the work done in moving unit charge from one point to the other. electric power The rate of expending energy or doing work in an electrical system. For a direct-current circuit, it is given by the product of the current passing through a system and the potential difference across it. In alternating-current circuits, the power is given by VIcosφ, where V and I are the RMS values and φ is the *phase angle. Cosφ is called the power factor of the circuit. electric spark The transient passage of an electric current through a gas between two points of high opposite potential, with the emission of light and sound. *Lightning consists of a spark between a cloud and earth or between two oppositely charged parts of the same cloud. electric susceptibility See . electrocardiogram (ECG) A tracing or graph of the electrical activity of the heart. Recordings are made from electrodes fastened over the heart and usually on both arms and a leg. Changes in the normal pattern of an ECG may indicate heart irregularities or disease. electrochemical cell See .
electrochemical equivalent Symbol z. The mass of a given element liberated from a solution of its ions in electrolysis by one coulomb of charge. See F'  (of electrolysis). electrochemical series See  . electrochemistry The study of chemical properties and reactions involving ions in solution, including electrolysis and electric cells. electrochromatography See . electrode 1. A conductor that emits or collects electrons in a cell, thermionic valve, semiconductor device, etc. The anode is the positive electrode and the cathode is the negative electrode. 2. See  . electrodeposition The process of depositing one metal on another by electrolysis, as in *electroforming and *electroplating. electrode potential The potential difference produced between the electrode and the solution in a *half cell. It is not possible to measure this directly since any measurement involves completing the circuit with the electrolyte, thereby introducing another half cell. Standard electrode potentials EŠ are deÜned by measuring the potential relative to a standard *hydrogen half cell using 1.0 molar solution at 25°C. The convention is to designate the cell so that the oxidized form is written Ürst. For example, Pt(s)|H2(g)H+(aq)|Zn2+(aq)|Zn(s) The e.m.f. of this cell is –0.76 volt (i.e. the zinc electrode is negative). Thus the standard electrode potential of the Zn2+|Zn half cell is –0.76 V. Electrode potentials are also called reduction potentials. See also  . electrodialysis A method of obtaining pure water from water containing a salt, as in *desalination. The water to be puriÜed is fed into a cell containing two electrodes. Between the electrodes is placed an array of *semipermeable membranes alternately semipermeable to positive ions and negative ions. The ions tend to segregate between alternate pairs of membranes, leaving pure water in the other gaps between membranes. In this way, the feed water is separated into two streams: one of pure water and the other of more concentrated solution.
electrodynamics The study of electric charges in motion, the forces created by electric and magnetic Üelds, and the relationship between them. Compare . electroencephalogram (EEG) A tracing or graph of the electrical activity of the brain. Electrodes taped to the scalp record electrical waves from different parts of the brain. The pattern of an EEG reÛects an individual's level of consciousness and can be used to detect such disorders as epilepsy, tumours, or brain damage. See also  . electroendosmosis See . electroforming A method of forming intricate metal articles or parts by *electrodeposition of the metal on a removable conductive mould. electroluminescence See . electrolysis The production of a chemical reaction by passing an electric current through an electrolyte. In electrolysis, positive ions migrate to the cathode and negative ions to the anode. The reactions occurring depend on electron transfer at the electrodes and are therefore redox reactions. At the anode, negative ions in solution may lose electrons to form neutral species. Alternatively, atoms of the electrode can lose electrons and go into solution as positive ions. In either case the reaction is an oxidation. At the cathode, positive ions in solution can gain electrons to form neutral species. Thus cathode reactions are reductions.
• Faraday's 1834 paper on electrical decomposition from Philosophical Transactions of the Royal Society
electrolyte A liquid that conducts electricity as a result of the presence of positive or negative ions. Electrolytes are molten ionic compounds or solutions containing ions, i.e. solutions of ionic salts or of compounds that ionize in solution. Liquid metals, in which the conduction is by free electrons, are not usually regarded as electrolytes. Solid conductors of ions, as in the sodium–sulphur cell, are also known as electrolytes. electrolytic capacitor See . electrolytic cell A cell in which electrolysis occurs; i.e. one in which current is passed
electromagnetic induction through the electrolyte from an external source.
electrolytic corrosion Corrosion that occurs through an electrochemical reaction. See . electrolytic gas (detonating gas) The highly explosive gas formed by the electrolysis of water. It consists of two parts hydrogen and one part oxygen by volume. electrolytic rectiÜer A *rectiÜer consisting of two dissimilar electrodes immersed in an electrolyte. By suitable choice of electrodes and electrolyte the cell can be made to pass current easily in one direction but hardly at all in the other. Examples include a lead–aluminium cell with ammonium phosphate(V) electrolyte and a tantalum–lead cell with sulphuric acid as the electrolyte. electrolytic reÜning The puriÜcation of metals by electrolysis. It is commonly applied to copper. A large piece of impure copper is used as the anode with a thin strip of pure copper as the cathode. Copper(II) sulphate solution is the electrolyte. Copper dissolves at the anode: Cu → Cu2+ + 2e, and is deposited at the cathode. The net result is transfer of pure copper from anode to cathode. Gold and silver in the impure copper form a so-called anode sludge at the bottom of the cell, which is recovered. electrolytic separation A method of separating isotopes by exploiting the different rates at which they are released in electrolysis. It was formerly used for separating deuterium and hydrogen. On electrolysis of water, hydrogen is formed at the cathode more readily than deuterium, thus the water becomes enriched with deuterium oxide. electromagnet A magnet consisting of a soft ferromagnetic core with a coil of insulated wire wound round it. When a current Ûows through the wire the core becomes magnetized; when the current ceases to Ûow the core loses its magnetization. Electromagnets are used in switches, solenoids, electric bells, metal-lifting cranes, and many other applications. electromagnetic induction The production of an electromotive force in a conductor when there is a change of magnetic Ûux linkage with the conductor or when there is relative motion of the conductor across a magnetic Üeld. The magnitude of the e.m.f. is proportional (and in modern
electromagnetic interaction systems of units equal) to the rate of change of the Ûux linkage or the rate of cutting Ûux dΦ/dt; the sense of the induced e.m.f. is such that any induced current opposes the change causing the induction, i.e. E = –dΦ/dt. See F' ; L' ; N' ; .
electromagnetic interaction See  . electromagnetic pump A pump used for moving liquid metals, such as the liquidsodium coolant in a fast nuclear reactor. The liquid is passed through a Ûattened pipe over two electrodes between which a direct current Ûows. A magnetic Üeld at right angles to the current causes a force to be created directly on the liquid, along the axis of the tube. The pump has no moving parts and is therefore safe and trouble free. electromagnetic radiation Energy resulting from the acceleration of electric charge and the associated electric Üelds and magnetic Üelds. The energy can be regarded as waves propagated through space (requiring no supporting medium) involving oscillating electric and magnetic Üelds at right angles to each other and to the direction of propagation. In a vacuum the waves travel with a constant speed (the speed of light) of 2.9979 × 108 metres per second; if material is present they are slower. Alternatively, the energy can be regarded as a stream of *photons travelling at the speed of light, each photon having an energy hc/λ, where h is the Planck constant, c is the speed of light, and λ is the wavelength of the associated wave. A fusion of these apparently conÛicting concepts is possible using the methods of *quantum mechanics or *wave mechanics. The characteristics of the radiation depend on its wavelength. See  . electromagnetic spectrum The range of wavelengths over which *electromagnetic radiation extends. The longest waves (105–10–3 metres) are radio waves, the next longest (10–3–10–6 m) are infrared waves, then comes the narrow band (4–7 × 10–7 m) of visible light, followed by ultraviolet waves (10–7–10–9 m), X-rays (10–9–10–11 m), and gamma rays (10–11–10–14 m). electromagnetic units (e.m.u.) A system of electrical units formerly used in the *c.g.s. system. The e.m.u. of electric current is the abampere (all e.m.u. have the preÜx
ab- attached to the names of practical units). The abampere is the current that, Ûowing in an arc of a circle (1 centimetre in diameter), exerts a force of 1 dyne on unit magnetic pole at the centre of the circle. In e.m.u. the magnetic constant is of unit magnitude. The system has now been replaced by *SI units for most purposes. Compare  ; G ; H–L .
electromagnetic wave See  ; . electrometallurgy The uses of electrical processes in the separation of metals from their ores, the reÜning of metals, or the forming or plating of metals. electrometer A measuring instrument for determining a voltage difference without drawing an appreciable current from the source. Originally electrostatic instruments based on the electroscope, they are now usually based on operational ampliÜers, solidstate devices with high input impedances. Electrometers are also used to measure low currents (nanoamperes), by passing the current through a high resistance. electromotive force (e.m.f.) The greatest potential difference that can be generated by a particular source of electric current. In practice this may be observable only when the source is not supplying current, because of its *internal resistance. electromotive series (electrochemical series) A series of chemical elements arranged in order of their *electrode potentials. The hydrogen electrode (H+ + e → ½H2) is taken as having zero electrode potential. Elements that have a greater tendency than hydrogen to lose electrons to their solution are taken as electropositive; those that gain electrons from their solution are below hydrogen in the series and are called electronegative. The series shows the order in which metals replace one another from their salts; electropositive metals will replace hydrogen from acids. The chief metals and hydrogen, placed in order in the series, are: potassium, calcium, sodium, magnesium, aluminium, zinc, cadmium, iron, nickel, tin, lead, hydrogen, copper, mercury, silver, platinum, gold. This type of series is sometimes referred to as an activity series. electron An *elementary particle, classed as a *lepton, with a rest mass (symbol me) of 9.109 3897(54) × 10–31 kg and a negative
charge of 1.602 177 33(49) × 10–19 coulomb. Electrons are present in all atoms in groupings called shells around the nucleus; when they are detached from the atom they are called free electrons. The antiparticle of the electron is the positron.
electron afÜnity Symbol A. The energy change occurring when an atom or molecule gains an electron to form a negative ion. For an atom or molecule X, it is the energy released for the electron-attachment reaction X(g) + e → X–(g) Often this is measured in electronvolts. Alternatively, the molar enthalpy change, ∆H, can be used. electron biprism An arrangement of Üelds that splits a beam of electrons or other charged particles in an analogous way to an optical biprism. electron capture 1. The formation of a negative ion by an atom or molecule when it acquires an extra free electron. 2. A radioactive transformation in which a nucleus acquires an electron from an inner orbit of the atom, thereby transforming, initially, into a nucleus with the same mass number but an atomic number one less than that of the original nucleus (capture of the electron transforms a proton into a neutron). This type of capture is accompanied by emission of an X-ray photon or Auger electron as the vacancy in the inner orbit is Ülled by an outer electron. electron counting rules Rules for counting the numbers of electrons that give rise to stable structures in atoms, molecules, and solids. For example, the result that the number of electrons in a full electronic shell in an atom is n2, where n is the principal quantum number associated with the shell, is an example of an electron counting rule. Various rules exist for predicting stable structures in chemical compounds, especially in certain types of cluster compound. electron cyclotron maser (ECM) A type of *maser in which the operation is based on the stimulated emission of radiation caused by the motion of high-energy electrons moving as in a *cyclotron. A full understanding of this type of maser requires the special theory of relativity. There are many practical applications of the device, including plasma heating in fusion research and telecommunications. This mechanism of maser emission may account for short intense
electron diffraction pulses in solar microwave radiation and for certain ionospheric radio emissions.
electron-deÜcient compound A compound in which there are fewer electrons forming the chemical bonds than required in normal electron-pair bonds. Such compounds use *multicentre bonds. See . electron diffraction *Diffraction of a beam of electrons by atoms or molecules. The fact that electrons can be diffracted in a similar way to light and X-rays shows that particles can act as waves (see  B ). An electron (mass m, charge e) accelerated through a potential difference V acquires a kinetic energy mv2/2 = eV, where v is the velocity of the electron. The (nonrelativistic) momentum (p) of the electron is √(2eVm). The de Broglie wavelength (λ) of an electron is given by h/p, where h is the Planck constant, thus λ = h/√(2eVm). For an accelerating voltage of 3600 V, the wavelength of the electron beam is 0.02 nanometre, some 3 × 104 times shorter than visible radiation. Electrons then, like X-rays, show diffraction effects with molecules and crystals in which the interatomic spacing is comparable to the wavelength of the beam. They have the advantage that their wavelength can be set by adjusting the voltage. Unlike X-rays they have very low penetrating power. The Ürst observation of electron diffraction was by George Thomson (1892–1975) in 1927, in an experiment in which he passed a beam of electrons in a vacuum through a very thin gold foil onto a photographic plate. Concentric circles were produced by diffraction of electrons by the lattice. The same year Clinton J. Davisson (1881–1958) and Lester Germer (1896–1971) performed a classic experiment in which they obtained diffraction patterns by glancing an electron beam off the surface of a nickel crystal. Both experiments were important veriÜcations of de Broglie's theory and the new quantum theory. Electron diffraction, because of the low penetration, cannot easily be used to investigate crystal structure. It is, however, employed to measure bond lengths and angles of molecules in gases. Moreover, it is extensively used in the study of solid surfaces and absorption. The main techniques are lowenergy electron diffraction (LEED) in which the electron beam is reÛected onto a Ûuorescent screen, and high-energy electron dif-
electronegative
fraction (HEED) used either with reÛection or transmission in investigating thin Ülms.
thermionic valves, resistors, capacitors, and inductors. See Chronology.
electronegative Describing elements that tend to gain electrons and form negative ions. The halogens are typical electronegative elements. For example, in hydrogen chloride, the chlorine atom is more electronegative than the hydrogen and the molecule is polar, with negative charge on the chlorine atom. There are various ways of assigning values for the electronegativity of an element. Mulliken electronegativities are calculated from E = (I + A)/2, where I is ionization potential and A is electron afÜnity. More commonly, Pauling electronegativities are used. These are based on bond dissociation energies using a scale in which Ûuorine, the most electronegative element, has a value 4. Some other values on this scale are B 2, C 2.5, N 3.0, O 3.5, Si 1.8, P 2.1, S 2.5, Cl 3.0, Br 2.8.
electron lens A device used to focus an electron beam. It is analogous to an optical lens but instead of using a refracting material, such as glass, it uses a coil or coils to produce a magnetic Üeld or an arrangement of electrodes between which an electric Üeld is created. Electron lenses are used in *electron microscopes and *cathode-ray tubes.
electron Ûow The transfer of electrons along a series of carrier molecules in the *electron transport chain. electron gun A device used in *cathoderay tubes (including television tubes), electron microscopes, etc., to produce a steady narrow beam of electrons. It usually consists of a heated cathode, control grid, and two or more annular anodes inserted in an evacuated tube. The electrons emitted by the cathode are attracted to the Ünal anode, through which they pass. The intensity of the beam is regulated by the control grid and potential differences between the anodes create electric Üelds that focus the diverging electrons into a narrow beam.
electron microscope A form of microscope that uses a beam of electrons instead of a beam of light (as in the optical microscope) to form a large image of a very small object. In optical microscopes the resolution is limited by the wavelength of the light. High-energy electrons, however, can be associated with a considerably shorter wavelength than light; for example, electrons accelerated to an energy of 105 electronvolts have a wavelength of 0.004 nanometre (see  B ) enabling a resolution of 0.2–0.5 nm to be achieved. The transmission electron microscope (see illustration) has an electron beam, sharply focused by *electron lenses, passing through a very thin metallized specimen (less than 50 nanometres thick) onto a Ûuorescent screen, where a visual image is formed. This image can be photographed. The scanning elec-
source of electrons
condenser lens
electronic mail (e-mail) Messages, documents, etc., sent between users of computer systems, the computer systems being used to transport and hold the e-mail. The service itself is also referred to as electronic mail. The sender and recipient(s) need not be at their computers at the same time to communicate, and the computer systems may be situated worldwide. The sender creates an email by means of a mail-sending computer program, and a mail transport system then takes responsibility for delivering the e-mail to the indicated address(es).
electronics The study and design of control, communication, and computing devices that rely on the movement of electrons in circuits containing semiconductors,
final image
objective lens
intermediate image projector lens
Electron microscope. Principle of the transmission electron microscope.
ELECTRONICS 1887
Radio waves are discovered by Heinrich Hertz.
Oliver Lodge invents the 'coherer' for detecting radio waves. Marconi develops radio telegraphy.
J. J. Thomson discovers the electron.
US engineer Reginald Fessenden (1866–1932) develops radio telephony.
Danish engineer Valdemar Poulsen (1869–1942) invents the arc transmitter for radio telegraphy.
British engineer Ambrose Fleming (1849–1945) invents the diode thermionic valve.
US engineer Lee De Forest (1873–1961) invents the triode thermionic valve. US electrical engineer Greenleaf Pickard (1877–1956) patents the crystal detector for radios. Fessenden introduces amplitude modulation in radio broadcasting.
German physicist Karl Braun (1850–1918) invents cathode-ray tube scanning.
Fessenden develops the heterodyne radio receiver.
US electrical engineer Edwin Armstrong (1890–1954) develops the superheterodyne radio receiver.
US physicist Albert Hull (1880–1966) invents the magnetron microwavegenerating valve.
Russian-born US engineer Vladimir Zworykin (1889–1982) invents the iconoscope television camera-tube.
Scottish inventor John Logie Baird (1888–1946) and Vladimir Zworykin independently develop television.
Swedish-born US electronics engineer Ernst Alexanderson (1878–1975) invents an all-electronic television system.
US electrical engineer Edwin Armstrong (1890–1954) develops frequency modulation radio broadcasting.
US physicists John Bardeen, Walter Brattain (1902–87), and William Shockley (1910–89) invent the point-contact transistor.
US engineers develop the Videcon television camera tube.
Chinese-born US computer engineer An Wang (1920–90) invents the magnetic core computer memory.
US physicist Charles Townes (1915– ) and Soviet physicists Nikolai Basov and Aleksandr Prokhorov (1916–2002) independently develop the maser.
US electronics engineers Jack Kilby and Robert Noyce (1927–90) develop integrated circuits.
US physicist Theodore Maiman (1927–2007) invents the ruby laser.
US electronics engineer Steven Hofstein develops the field-effect transistor.
US electronics engineer Marcian Edward Hoff (1937– first microprocessor (Intel 4004).
US engineers transmit television signals along optical fibres.
) designs the
electron–nuclear double resonance
tron microscope can be used with thicker specimens and forms a perspective image, although the resolution and magniÜcation are lower. In this type of instrument a beam of primary electrons scans the specimen and those that are reÛected, together with any secondary electrons emitted, are collected. This current is used to modulate a separate electron beam in a TV monitor, which scans the screen at the same frequency, consequently building up a picture of the specimen. The resolution is limited to about 10–20 nm. See also - ; - .
• A tutorial featuring virtual scanning electron microscopy, created by the Optical Microscopy Division of the National High Magnetic Field Laboratory
electron–nuclear double resonance See ENDOR. electron optics The study of the use of *electron lenses in the *electron microscope, *cathode-ray tubes, and other similar devices. The focusing of beams of positive or negative ions also relies on these methods. electron paramagnetic resonance (EPR) A spectroscopic method of locating electrons within the molecules of a paramagnetic substance (see ) in order to provide information regarding its bonds and structure. The spin of an unpaired electron is associated with a *magnetic moment that is able to align itself in one of two ways with an applied external magnetic Üeld. These two alignments correspond to different *energy levels, with a statistical probability, at normal temperatures, that there will be slightly more in the lower state than in the higher. By applying microwave radiation to the sample a transition to the higher state can be achieved. The precise energy difference between the two states of an electron depends on the surrounding electrons in the atom or molecule. In this way the position of unpaired electrons can be investigated. The technique is used particularly in studying free radicals and paramagnetic substances such as inorganic complexes. It is also called electronspin resonance (ESR). See also   . electron probe microanalysis (EPM) A method of analysing a very small quantity of a substance (as little as 10–13 gram). The
method consists of directing a very Ünely focused beam of electrons on to the sample to produce the characteristic X-ray spectrum of the elements present. It can be used quantitatively for elements with atomic numbers in excess of 11.
electron spectroscopy Any of a number of techniques in which information is obtained by analysing the energy spectrum of electrons. *Photoelectron spectroscopy is a typical example. electron-spin resonance See   . electron-transfer reaction A chemical reaction that involves the transfer, addition, or removal of electrons. Electron-transfer reactions often involve complexes of transition metals. In such complexes one general mechanism for electron transfer is the inner-sphere mechanism, in which two complexes form an intermediate, with ligand bridges enabling electrons to be transferred from one complex to another complex. The other main mechanism is the outer-sphere mechanism, in which two complexes retain all their ligands, with electrons passing from one complex to the other. The rates of electron-transfer reactions vary enormously. These rates can be explained in terms of the way in which molecules of the solvent solvating the reactants rearrange so as to solvate the products in the case of the outer-sphere mechanism. In the case of the inner-sphere (ligand-bridged) reactions the rate of the reaction depends on the intermediate and the way in which the electron is transferred. electron transport chain (electron transport system) A sequence of biochemical reduction–oxidation reactions that effects the transfer of electrons through a series of carriers. An electron transport chain, also known as the respiratory chain, forms the Ünal stage of *aerobic respiration. It results in the transfer of electrons or hydrogen atoms derived from the *Krebs cycle to molecular oxygen, with the formation of water. At the same time it conserves energy from food or light in the form of *ATP. The chain comprises a series of *carrier molecules that undergo reversible reduction–oxidation reactions, accepting electrons and then donating them to the next carrier in the chain – a process known as electron flow. In the mitochondria, NADH and FADH2, generated by the Krebs cycle, transfer their electrons to a
chain comprising Ûavin mononucleotide (FMN), *ubiquinone, and a series of *cytochromes. This process is coupled to the formation of ATP at three sites along the chain (see  ). The ATP is then carried across the mitochondrial membrane in exchange for ADP. An electron transport chain also occurs in *photosynthesis.
electronvolt Symbol eV. A unit of energy equal to the work done on an electron in moving it through a potential difference of one volt. It is used as a measure of particle energies although it is not an *SI unit. 1 eV = 1.602 × 10–19 joule. electroorganic reaction An organic reaction produced in an electrolytic cell. Electroorganic reactions are used to synthesize compounds that are difÜcult to produce by conventional techniques. An example of an electroorganic reaction is *Kolbe's method of synthesizing alkanes. electroosmosis The movement of a polar liquid through a membrane under the inÛuence of an applied electric Üeld. The linear velocity of Ûow divided by the Üeld strength is the electroosmotic mobility. Electroosmosis was formerly called electroendosmosis. electroosmotic mobility See . electropherogram See  . electrophile An ion or molecule that is electron deÜcient and can accept electrons. Electrophiles are often reducing agents and Lewis *acids. They are either positive ions (e.g. NO2+) or molecules that have a positive charge on a particular atom (e.g. SO3, which has an electron-deÜcient sulphur atom). In organic reactions they tend to attack negatively charged parts of a molecule. Compare . electrophilic addition An *addition reaction in which the Ürst step is attack by an electrophile (e.g. a positive ion) on an electron-rich part of the molecule. An example is addition to the double bonds in alkenes. electrophilic substitution A *substitution reaction in which the Ürst step is attack by an electrophile. Electrophilic substitution is a feature of reactions of benzene (and its compounds) in which a positive ion ap-
electrophorus proaches the delocalized pi electrons on the benzene ring.
electrophoresis (cataphoresis) A technique for the analysis and separation of colloids, based on the movement of charged colloidal particles in an electric Üeld. There are various experimental methods. In the simplest, the sample is placed in a U-tube and a buffer solution added to each arm, so that there are sharp boundaries between buffer and sample. An electrode is placed in each arm, a voltage applied, and the motion of the boundaries under the inÛuence of the Üeld is observed. The rate of migration of the particles depends on the Üeld, the charge on the particles, and on other factors, such as the size and shape of the particles. Electrophoresis can also be carried out using an adsorbent, such as a strip of Ülter paper, soaked in a buffer with two electrodes making contact. The sample is placed between the electrodes and a voltage applied. Different components of the mixture migrate at different rates, so the sample separates into zones. The components can be identiÜed by the rate at which they move. In gel electrophoresis the medium is a gel, typically made of polyacrylamide, agarose, or starch. In modern automated DNA sequencers, electrophoresis is carried out in capillary tubes, less than 0.5mm in diameter and about 48cm long, containing the gel. Electrophoresis, which has also been called electrochromatography, is used extensively in studying mixtures of proteins, nucleic acids, carbohydrates, enzymes, etc. In clinical medicine it is used for determining the protein content of body Ûuids. electrophoretic deposition A technique for coating a material making it an electrode in a bath containing a colloidal suspension of charged particles. Under suitable conditions, the particles are attracted to, and deposited on, the electrode. Electrophoretic deposition is used extensively in industry; for example, in applying paint to metal components. electrophoretic effect The effect in which the mobility of ions in solution moving under the inÛuence of an applied electric Üeld is affected by the Ûow of ions of opposite charge in the opposite direction. electrophorus An early form of *electrostatic generator. It consists of a Ûat dielectric plate and a metal plate with an insulated handle. The dielectric plate is charged by
electroplating friction and the metal plate is placed on it and momentarily earthed, which leaves the metal plate with an induced charge of opposite polarity to that of the dielectric plate. The process can be repeated until all of the original charge has leaked away.
electroplating A method of plating one metal with another by *electrodeposition. The articles to be plated are made the cathode of an electrolytic cell and a rod or bar of the plating metal is made the anode. Electroplating is used for covering metal with a decorative, more expensive, or corrosion-resistant layer of another metal. electropositive Describing elements that tend to lose electrons and form positive ions. The alkali metals are typical electropositive elements. electroscope A device for detecting electric charge and for identifying its polarity. In the gold-leaf electroscope two rectangular gold leaves are attached to the end of a conducting rod held in an insulated frame. When a charge is applied to a plate attached to the other end of the conducting rod, the leaves move apart owing to the mutual repulsion of the like charges they have received. electrospray ionization (ESI) A technique for producing ions for mass spectrometry, used especially for obtaining ions from large molecules. The sample is dissolved in a volatile solvent, which may also contain volatile acids or bases so that the sample exists in an ionic form. The solution is forced through a charged metal capillary tube and forms an aerosol. Evaporation of the solvent results in single ions of the sample, which are analysed by the mass spectrometer. electrostatic Üeld The *electric Üeld that surrounds a stationary charged body. electrostatic generator A device used to build up electric charge to an extreme potential usually for experimental purposes. The *electrophorus and the *Wimshurst machine were early examples; a more usual device now is the *Van de Graaff generator. electrostatic precipitation A method of removing solid and liquid particles from suspension in a gas. The gas is exposed to an electric Üeld so that the particles are attracted to and deposited on a suitably placed electrode. Electrostatic precipitation is widely used to remove dust and other pollu-
tants from waste gases and from air. See also C .
electrostatics The study of electric charges at rest, the forces between them (see C' ), and the electric Üelds associated with them. Compare . electrostatic units (e.s.u.) A system of electrical units in the *c.g.s. system. The e.s.u. of electric charge is the statcoulomb (all e.s.u. have the preÜx stat- attached to the names of practical units). The statcoulomb is the quantity of electric charge that will repel an equal quantity 1 centimetre distant with a force of 1 dyne. In e.s.u. the electric constant is of unit magnitude. The system has now been replaced for most purposes by *SI units. Compare  ; G ; H–L . electrostriction A change in the dimensions of a body as a result of reorientation of its molecules when it is placed in an electric Üeld. If the Üeld is not homogeneous the body will tend to move; if its relative permittivity is higher than that of its surroundings it will tend to move into a region of higher Üeld strength. Compare . electrovalent bond See  . electroweak theory A *gauge theory (sometimes called quantum flavourdynamics, or QFD) that gives a uniÜed description of the electromagnetic and weak interactions (see  ). A successful electroweak theory was proposed in 1967 by Steven Weinberg and Abdus Salam, known as the Weinberg–Salam model or WS model. Because early developments of these ideas were put forward by Sheldon Glashow, it is sometimes known as the Glashow–Weinberg–Salam model or GWS model. In this electroweak theory the gauge group is non-Abelian and the gauge symmetry is a *broken symmetry. The electroweak interaction is mediated by photons and by intermediate vector bosons, called the *W boson and the *Z boson. The observation of these particles in 1983–84, with their predicted energies, was a major success of the theory. The theory successfully accounts for existing data for electroweak processes and also predicts the existence of a heavy particle with spin 0, the *Higgs boson. electrum 1. An alloy of gold and silver containing 55–88% of gold. 2. A *German sil-
ver alloy containing 52% copper, 26% nickel, and 22% zinc.
element A substance that cannot be decomposed into simpler substances. In an element, all the atoms have the same number of protons or electrons, although the number of neutrons may vary. There are 92 naturally occurring elements. See also  ;  ;  . elementary particles The fundamental constituents of all the matter in the universe. By the beginning of the 20th century, the electron and the proton had been discovered, but it was not until 1932 that the existence of the neutron was deÜnitely established. Since 1932, it had been known that atomic nuclei consist of both protons and neutrons (except hydrogen, whose nucleus consists of a lone proton). Between 1900 and 1930, *quantum mechanics was also making progress in the understanding of physics on the atomic scale. Non-relativistic quantum theory was completed in an astonishingly brief period (1923–26), but it was the relativistic version that made the greatest impact on our understanding of elementary particles. Dirac's discovery in 1928 of the equation that bears his name led to the discovery of the positive electron or *positron. The mass of the positron is equal to that of the negative electron while its charge is equal in magnitude but opposite in sign. Pairs of particles related to each other in this way are said to be antiparticles of each other. Positrons have only a transitory existence; that is, they do not form part of ordinary matter. Positrons and electrons are produced simultaneously in high-energy collisions of charged particles or gamma rays with matter in a process called *pair production. The union of *relativity and quantum mechanics therefore led to speculation as early as 1932 that there might also be antiprotons and antineutrons, bearing a similar relationship to their respective ordinary particles as the positron does to the electron. However, it was not until 1955 that particle beams were made sufÜciently energetic to enable these antimatter particles to be observed. It is now understood that all known particles have antimatter equivalents, which are predicted by relativistic quantum equations. By the mid-1930s the list of known and theoretically postulated particles was still small but steadily growing. At this time the
elementary particles Japanese physicist Hideki Yukawa (1907–81) was studying the possible *fundamental interactions that could hold the nucleus together. Since the nucleus is a closely packed collection of positively charged protons and neutral neutrons, clearly it could not be held together by an electromagnetic force; there had to be a different and very large force capable of holding proton charges together at such close proximity. This force would necessarily be restricted to the short range of nuclear dimensions, because evidence of its existence only arose after the discovery of the constituents of the atomic nucleus. Guided by the properties required of this new force, Yukawa proposed the existence of a particle called the *meson, which was responsible for transmitting nuclear forces. He suggested that protons and neutrons in the nucleus could interact by emitting and absorbing mesons. For this reason this new type of force was called an *exchange force. Yukawa was even able to predict the mass of his meson (meaning 'middle weight'), which turned out to be intermediate between the proton and the electron. Only a year after Yukawa had made this suggestion, a particle of intermediate mass was discovered in *cosmic radiation. This particle was named the µ-meson or muon. The µ– has a charge equal to the electron, and its antiparticle µ+ has a positive charge of equal magnitude. However, physicists soon discovered that muons do not interact with nuclear particles sufÜciently strongly to be Yukawa's meson. It was not until 1947 that a family of mesons with the appropriate properties was discovered. These were the πmesons or pions, which occur in three types: positive, negative, and neutral. Pions, which interact strongly with nuclei, have in fact turned out to be the particles predicted by Yukawa in the 1930s. The nuclear force between protons and neutrons was given the name 'strong interaction' (see  ) and until the 1960s it was thought to be an exchange force as proposed by Yukawa. A theory of the weak interaction was also in its infancy in the 1930s. The weak interaction is responsible for *beta decay, in which a radioactive nucleus is transformed into a slightly lighter nucleus with the emission of an electron. However, beta decays posed a problem because they appeared not to conserve energy and momentum. In 1931 *Pauli proposed the existence of a neutral particle that might be able to carry off the missing
elementary particles
energy and momentum in a beta decay and escape undetected. Three years later, *Fermi included Pauli's particle in a comprehensive theory of beta decay, which seemed to explain many experimentally observed results. Fermi called this new particle the *neutrino, the existence of which was Ünally established in the 1950s. A plethora of experiments involving the neutrino revealed some remarkable properties for this new particle. The neutrino was found to have an intimate connection with the electron and muon, and indeed never appeared without the simultaneous appearance of one or other of these particles. A conservation law was postulated to explain this observation. Numbers were assigned to the electron, muon, and neutrino, so that during interactions these numbers were conserved; i.e. their algebraic sums before and after these interactions were equal. Since these particles were among the lightest known at the time, these assigned numbers became known as lepton numbers (lepton: 'light ones'). In order to make the assignments of lepton number agree with experiment, it is necessary to postulate the existence of two types of neutrino. Each of these types is associated with either the electron or muon; there are thus muon neutrinos and electron neutrinos. In 1978 the tau particle or tauon was discovered and was added to the list of particles with assigned lepton numbers. The conservation of lepton number in the various interactions involving the tau requires the existence of an equivalent tau neutrino. The six particles with assigned lepton numbers are now known as *leptons. Neutrinos have zero charge and were originally thought to have zero rest mass, but there has been increasing indirect experimental evidence to the contrary. In 1985 a Soviet team reported a measurement, for the Ürst time, of a non-zero neutrino mass. The
mass measured was extremely small (10 000 times less than the mass of the electron), but subsequent attempts independently to reproduce these results did not succeed. More recently (1998–99), Japanese and US groups have put forward theories and corroborating experimental evidence to suggest, indirectly, that neutrinos do have mass. In these experiments neutrinos are found to apparently 'disappear'. Since it is unlikely that momentum and energy are actually vanishing from the universe, a more plausible explanation is that the types of neutrinos detected are changing into types that cannot be detected. Present theoretical considerations imply that the masses of neutrinos involved cannot be equal to one another, and therefore they cannot all be zero. This speculative work has not yet yielded estimates of the neutrino masses, which is indicated by the use of asterisks in the table. In the 1960s, the development of highenergy accelerators and more sophisticated detection systems led to the discovery of many new and exotic particles. They were all unstable and existed for only small fractions of a second; nevertheless they set into motion a search for a theoretical description that could account for them all. The large number of these apparently fundamental particles suggested strongly that they do not, in fact, represent the most fundamental level of the structure of matter. Physicists found themselves in a position similar to Mendeleev when the *periodic table was being developed. Mendeleev realized that there had to be a level of structure below the elements themselves, which explained the chemical properties and the interrelations between elements. Murray Gell-Mann and his collaborators proposed the particle-physics equivalent of the periodic table in 1961. In this structure, leptons were indeed regarded as fundamental particles, but the short-lived particles dis-
Charge (electron charges)
Rest mass (MeV/c 2)
electron electron neutrino muon muon neutrino tauon tau neutrino
e– νe µ– νµ τ–
0.511 * 105.7 * 1784
ντ
Elementary particles. Table of leptons.
covered in the 1960s were not. These particles were found to undergo strong interactions, which did not seem to affect the leptons. Gell-Mann called these strongly interacting particles the *hadrons and proposed that they occurred in two different types: baryons and mesons. These two different types corresponded to the two different ways of constructing hadrons from constituent particles, which Gell-Mann called quarks. These quarks came in three Ûavours, up (u), down (d), and strange (s). These three quarks were thought to be the fundamental constituents of hadrons, i.e. matter that undergoes strong interactions: baryons are composed of three quarks (u, d, or s) or three antiquarks (ū, đ, or ); mesons are composed of (u, d, or s) quark–antiquark pairs. No other combinations seemed to be necessary to describe the full variation of the observed hadrons. This scheme even led to the prediction of other particles that were not known to exist in 1961. For example, in 1961 Gell-Mann not only predicted the Ω– (omega-minus) particle, but more importantly told experimentalists exactly how to produce it. The Ω– particle was Ünally discovered in 1964. Gell-Mann called his scheme 'the eightfold way', after the similarly named Buddhist principle. The scheme requires that quarks have properties not previously allowed for fundamental particles. For example, quarks have fractional electric charges, i.e. charges of 1/3 and 2/3 of the electron charge. Quarks also have a strong afÜnity for each other through a new kind of charge known as colour charge. Thus colour charge is responsible for strong interactions, and the force is known as the colour force. This is a revision of Yukawa's proposal in 1930. Yukawa's strong force was mediated by π-mesons. The strong force is now thought to be mediated by exchange of particles carrying colour charge called gluons. The theory governing these colour charge combinations is modelled on *quantum electrodynamics and is known as *quantum chromodynamics. In November 1974 the discovery of the ψ (psi) particle initiated what later came to be known as 'the November revolution'. At the time, any known hadron could be described as some combination of u, d, or s quarks. These hadrons were very short-lived with lifetimes of about 10–23 s. The ψ particle, however, had a lifetime of 10–20 s; i.e. a thousand times longer. This suggested a com-
pletely different species of particle. It is now universally accepted that the ψ represents a meson-bound state of a new fourth quark, the charm (c) quark and its antiquark. In 1977 the list of quarks once again increased with the discovery of a new even heavier meson, called the Y (upsilon) meson. This meson was found to have an even longer lifetime than the ψ, and was quickly identiÜed as the carrier of a Üfth quark, bottom (b). Thus, by the end of 1977, Üve Ûavours of quark (u, d, s, c, b) were known to exist together with six Ûavours of lepton (e, µ, τ, νe, τµ, ντ). Assuming that quarks and leptons are the fundamental constituents of matter, many of the strong and weak interactions of hadrons and the weak interactions of leptons can be explained. However, anticipating a symmetry in nature's building blocks, it was expected that a sixth quark would eventually reveal itself. This quark, labelled top (t), would be the 2/3 electronic charge partner to the b quark (see table). Quark symbol
u d c s t b
up down charm strange top bottom
Elementary particles. Table of quarks (mass is not shown because quarks are never observed alone).
In 1998 the top quark was found at CERN in Geneva and the symmetry of six quarks with six leptons was Ünally veriÜed. In 1978 the standard model was proposed as the deÜnitive theory of the fundamental constituents of matter. In the current view, all matter consists of three kinds of particles: leptons, quarks, and mediators (see table overleaf). The mediators are the particles by which the four fundamental interactions are mediated. In the standard model, each of these interactions has a particle mediator. For the electromagnetic reaction it is the *photon. For weak interactions the force is mediated by three particles called W+, W–, and Z° *bosons; for the strong force it is the gluon. Current theories of quantum gravity propose the *graviton as the mediator for the gravitational interaction, but this work is highly
elements of an orbit
Mediator (exchange particle)
Rest mass (GeV/c2)
strong electromagnetic weak gravitational
gluon photon W+, W–, Z° graviton
0 0 81,81,93 0
0 0 +1,–1,0 0
Elementary particles. Table of mediators. speculative and the graviton has never been detected.
elements of an orbit Six parameters used to deÜne the path of a celestial body. The shape of the orbit is deÜned by its eccentricity (see ) and semimajor axis. The orientation of the orbit is speciÜed by the *inclination of the orbital plane to the reference plane (usually the *ecliptic) and by the longitude of the ascending *node (the angular distance from the vernal equinox to the ascending node). The position of the body in its orbit is deÜned by its eccentric *anomaly and the position as a function of time is calculated from the periapsis passage (see ). elevation of boiling point An increase in the boiling point of a liquid when a solid is dissolved in it. The elevation is proportional to the number of particles dissolved (molecules or ions) and is given by ∆t = kBC, where C is the molal concentration of solute. The constant kB is the ebullioscopic constant of the solvent and if this is known, the molecular weight of the solute can be calculated from the measured value of ∆t. The elevation is measured by a Beckmann thermometer. See also  . elimination reaction A reaction in which one molecule decomposes into two, one much smaller than the other. Elinvar Trade name for a nickel– chromium steel containing about 36% nickel, 12% chromium, and smaller proportions of tungsten and manganese. Its elasticity does not vary with temperature. ELISA (enzyme-linked immunosorbent assay) A sensitive technique (see ) for accurately determining the amount of protein or other antigen in a given sample by means of an enzyme-catalysed colour change. Antibody speciÜc to the test
protein is adsorbed onto a solid substrate, such as a PVC sheet, and a measured amount of the sample is added; all molecules of the test protein in the sample are bound by the antibody. A second antibody speciÜc for a second site on the test protein is added; this is conjugated with an enzyme, which catalyses a colour change in the fourth reagent, added Ünally to the sheet. The colour change can be measured photometrically and compared against a standard curve to give the concentration of protein in the sample. ELISA is widely used for diagnostic and other purposes.
Ellingham diagram A diagram used to show the conditions under which a metal oxide can be reduced to a metal. The standard Gibbs free energy of formation of the oxide is considered, for example, M + ½O2 → MO This value, ∆GŠ, is plotted against temperature. In general, the result is a straight line. In some cases, there is an abrupt change in the line's slope at a point because of a phase change. The value of ∆GŠ for the reducing agent is also plotted. For example, if the reducing agent is carbon, forming carbon dioxide, it is ∆GŠ for the reaction C + O2 → CO2 Reduction can occur in the range of temperatures in which the carbon curve is lower than the metal curve. The diagram was devised by the physical chemist H. J. T. Ellingham. ellipse A *conic formed by the intersection of a plane with a right circular cone, so that the plane is inclined to the axis of the cone at an angle in excess of half the apex angle of the cone. The ellipse has two vertices, which are joined by a line called the major axis. The centre of the ellipse falls on this line, midway between the vertices. The minor axis is the line perpendicular to the major axis that passes through the centre and joins two points on the ellipse. The foci of an el-
283 latus rectum
minor axis major axis
centre focus
An ellipse. lipse are two points on the major axis so placed that for any point on the ellipse the sum of the distances from that point to each focus is constant. (See illustration.) The area of an ellipse is πab, where a and b are half the major and minor axes, respectively. For an ellipse centred at the origin, the equation in Cartesian coordinates is x2/a2 + y2/b2 = 1. The foci are at (ea, 0) and (–ea, 0), where e is the eccentricity. Each of the two chords of the ellipse passing through a focus and parallel to the minor axis is called a latus rectum and has a length equal to 2b2/a.
ellipsoid A solid body formed when an *ellipse is rotated about an axis. If it is rotated about its major axis it is a prolate ellipsoid; if it is rotated about its minor axis it is an oblate ellipsoid. For an ellipsoid centred at the origin the equation in Cartesian coordinates is: x2/a2 + y2/b2 + z2/c2 = 1. elliptical galaxy See .
solved in the eluent is the eluate. Elution is the process used to wash components of a mixture through a *chromatography column.
elutriation The process of suspending Ünely divided particles in an upward Ûowing stream of air or water to wash and separate them into sized fractions. elytra The thickened horny forewings of the *Coleoptera (beetles), which cover and protect the membranous hindwings when the insect is at rest. e-mail See  . emanation The former name for the gas radon, of which there are three isotopes: Rn–222 (radium emanation), Rn–220 (thoron emanation), and Rn–219 (actinium emanation). emasculation The removal of the anthers of a Ûower in order to prevent self-pollination or the undesirable pollination of neighbouring plants. EMBASE (Excerpta Medica Database) A bibliographic database containing citations, abstracts, indexing terms, and codes covering over 5000 biological and medical journals from more than 70 countries, especially Europe. It contains over 11 million records, with some half a million new ones added each year, and has a particular emphasis on drug-related Üelds.
elliptical polarization See   .
Embden–Meyerhof pathway See .
El Niño A surge of warm ocean water (the Peru current) that occurs every 5 to 8 years off the eastern coast of South America. See Feature overleaf.
embryo 1. An animal in the earliest stages of its development, from the time when the fertilized ovum starts to divide (see ), while it is contained within the egg or reproductive organs of the mother, until hatching or birth. A human embryo (see illustration p. 286) is called a *fetus after the Ürst eight weeks of pregnancy. 2. The structure in plants that develops from the zygote prior to germination. In seed plants the zygote is situated in the *embryo sac of the ovule. It divides by mitosis to form the embryonic cell and a structure called the suspensor, which embeds the embryo in the surrounding nutritive tissue. The embryonic cell divides continuously and eventually gives rise to the *radicle (young root), *plumule (young shoot), and one or two *cotyledons (seed leaves). Changes also take place in the surrounding tissues of the ovule,
Elton, Charles Sutherland (1900–91) British zoologist and ecologist, who founded the Bureau of Animal Population at Oxford in 1932 and the same year became editor of the new Journal of Animal Ecology. The Ürst zoologist to study animals in relation to their environment, he explored the nature of food chains and studied population Ûuctuations. eluate See ; . eluent See ; . elution The process of removing an adsorbed material (adsorbate) from an adsorbent by washing it in a liquid (eluent). The solution consisting of the adsorbate dis-
EL NIÑ O
A phenomenon reoccurring every few years in the equatorial part of the Pacific ocean, characterized by movement of a mass of warm water eastward towards the west coast of South America. This change in ocean conditions has long been recognized in Peru, where sailors noticed that an unusual counter-current appeared in certain years around the area of the port of Paita. They named this current El Niño – 'the Christ Child' – because it usually appeared immediately after Christmas. It was also known that the appearance of this current coincided with different weather conditions, particularly increased rainfall and sometimes flooding. The phenomenon has dramatic effects on the climate and ecology of this part of South America. In particular, it interrupts the Humbolf current, which is a cold ocean current carrying plankton from Arctic regions. El Niño occurs every 4–7 years and the effects last for about 8 months. In the 1960s interest developed in the phenomenon and it is now recognized that El Niño has effects on climate much wider than those observed on the west coast of South America. In extreme cases, as in 1986–87 and 1997–98, it can cause tropical cyclones over the whole Pacific area, drought in southeastern Asia and Australia, and increased rainfall and flooding in parts of North America.
The mechanism of El Niño There is no definite agreement about what induces an El Niño event, but the physical mechanism of how it occurs is fairly well understood. The large-scale movement of water in the world's oceans is influenced by, and in turn influences, the prevailing wind patterns. In tropical regions there are persistent trade winds flowing from east to west. In the Pacific these push large amounts of water westward towards the coasts of Indonesia. This causes a significant difference in sea level between opposite sides of the Pacific. For instance, the sea level in the Philippines in the west is around 60 cm higher than that on the coast of Panama in the eastern Pacific. The water in the west is also much warmer. This mass of warm ocean in the western Pacific gives Indonesia its high rainfall under normal climatic conditions.
normal trade winds
warm normal ocean currents
waters AUSTRALIA
In normal years the trade winds blow from east to west across the Pacific. This causes a difference in sea level and a mass of warm water builds up in the western Pacific, creating a warm area of ocean off the east coast of Indonesia. This area has some of the highest rainfall in the world.
Every few years there is a significant change in the prevailing wind pattern, involving a fall in the intensity of the trade winds. In extreme cases, a reversal of direction of the winds may occur. As a result, the warm water that is piled up in the west flows back eastwards across the Pacific towards the west coast of South America. The event lasts until the normal conditions and wind patterns are reestablished.
The Southern Oscillation and La Niña El Niño events are connected with another periodic phenomenon occurring not in the ocean but in the atmosphere. The meteorologist Sir Gilbert Walker noted in 1923 that when pressure was high in the Pacific it tends to be low in the Indian ocean, and vice versa. The extent of this is now measured by taking the difference between the surface atmospheric pressure at Darwin in Australia and at Tahiti in the south Pacific. A high pressure at one site is usually accompanied by a low pressure at the other and every few years the pattern reverses. There is a large mass of air slowly oscillating (with a period of a few years) across tropical regions. Walker called this the Southern Oscillation. The Southern Oscillation is part of a large general cycle of coupled air and water flow known as the El Niño–Southern Oscillation (ENSO). El Niño is the warm phase of this cycle. In some years, as part of the ENSO cycle, a cold region develops in the eastern tropical Pacific. This is known as La Niña ('the little girl'). The Southern oscillation is not the only atmospheric oscillation that occurs. For example there is a North Atlantic Oscillation (NAO) measured by the pressure difference between Iceland and the Azores, which is thought to have a major influence on climatic conditions and on the ecosystems of this part of the globe. There is however no Atlantic analogue of El Niño. The causes of El Niño Although the flows of air and water are understood, there is no concensus about why an El Niño forms. Some workers have suggested that the frequency and intensities of El Niño events may be increasing because of global warming. However, there is no direct evidence for this .
El Niño current
weakened trade winds
increased surface temperatures
In certain years there is a reduction in the intensity of the trade winds across the Pacific. This allows the mass of warm water in the west to flow across to the east, creating a warm area of ocean off the west coast of South America. This brings rain to Peru but causes drought in southeast Asia and Australia.
286 chorionic villi
uterus wall
fallopian tube
blood vessel yolk sac umbilical cord
amnion embryo
chorion amniotic fluid
cervix vagina
Embryo. A developing human embryo. which becomes the *seed enclosing the embryo plant.
embryology The study of the development of animals from the fertilized egg to the new adult organism. It is sometimes limited to the period between fertilization of the egg and hatching or birth (see ).
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embryonic stem cell See  . embryophyte A true plant, i.e. one that develops from an embryo and therefore is necessarily multicellular. The term underlines the distinction between plants and algae, which lack embryos. embryo sac A large cell that develops in antipodal cells
embryo sac polar nuclei integuments oosphere egg synergids apparatus
Embryo sac.
the *ovule of Ûowering plants. It is equivalent to the female *gametophyte of lower plants, although it is very much reduced. Typically, it contains eight nuclei formed by division of the original female gamete (see illustration). One forms the *oosphere (egg cell), which is fertilized by a male nucleus and becomes the *embryo. The two polar nuclei fuse with a second male nucleus to form a triploid nucleus that gives rise to the *endosperm. The three remaining nuclei form the antipodal cells.
emerald The green gem variety of *beryl: one of the most highly prized gemstones. The Ünest specimens occur in the Muzo mines, Colombia. Other occurrences include the Ural Mountains, the Transvaal in South Africa, and Kaligunan in India. Emeralds can also be successfully synthesized. emergence A key concept in *complexity theory in which certain features of a complex system occur as a result of collective behaviour of the system. For example, in the kinetic theory of gases the concept of temperature emerges from the average kinetic energy of the very large number of particles in the system. It has been suggested that space and time in physics should ultimately be emergent quantities. emergency action code See  . emery A rock composed of corundum
endo-
(natural aluminium oxide, Al2O3) with magnetite, haematite, or spinel. It occurs on the island of Naxos (Greece) and in Turkey. Emery is used as an abrasive and polishing material and in the manufacture of certain concrete Ûoors.
e.m.f. See  . emission spectrum See . emissivity Symbol ε. The ratio of the power per unit area radiated by a surface to that radiated by a *black body at the same temperature. A black body therefore has an emissivity of 1 and a perfect reÛector has an emissivity of 0. The emissivity of a surface is equal to its *absorptance. emittance See . emitter See . emitter-coupled logic (ECL) A set of integrated *logic circuits. The input part of an ECL consists of an emitter-coupled *transistor pair which is a very good differential ampliÜer. The output is through an *emitter follower. ECL circuits are very rapid logic circuits. emitter follower An amplifying circuit using a bipolar junction *transistor with a *common-collector connection. The output is taken from the emitter. empirical Denoting a result that is obtained by experiment or observation rather than from theory. empirical formula See . emulsiÜcation (in digestion) The breakdown of fat globules in the duodenum into tiny droplets, which provides a larger surface area on which the enzyme pancreatic *lipase can act to digest the fats into fatty acids and glycerol. EmulsiÜcation is assisted by the action of the bile salts (see ). emulsion A *colloid in which small particles of one liquid are dispersed in another liquid. Usually emulsions involve a dispersion of water in an oil or a dispersion of oil in water, and are stabilized by an emulsifier. Commonly emulsiÜers are substances, such as *detergents, that have lyophobic and lyophilic parts in their molecules. Dietary fats are reduced to an emulsion in the duodenum to facilitate their subsequent digestion (see ). en The symbol for ethylene diamine (1,2-
diaminoethane) functioning as a bidentate ligand, used in formulae.
enamel The material that forms a covering over the crown of a *tooth (i.e. the part that projects above the gum). Enamel is smooth, white, and extremely hard, being rich in minerals containing calcium, especially *apatite. It is produced by certain cells (ameloblasts) of the oral epithelium and protects the underlying dentine of the tooth. Enamel may also be found in the placoid *scales of certain Üsh, which demonstrates the common developmental origin of scales and teeth. enamine A type of compound with the general formula R1R2C=C(R3)–NR4R5, where R is a hydrocarbon group or hydrogen. Enamines can be produced by condensation of an aldehyde or ketone with a secondary amine. enantiomers See  . enantiomorphism See  . enantiotropy See . encephalin See . endangered species A plant or animal species deÜned by the IUCN (International Union for the Conservation of Nature and Natural Resources) as being in immediate danger of *extinction because its population numbers have reached a critical level or its habitats have been drastically reduced. If these causal factors continue the species is unlikely to survive. A list of endangered species is published by the IUCN, which also deÜnes other categories of threatened species.
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endemic 1. Describing a plant or animal species that is restricted to one or a few localities in its distribution. Endemic species are usually conÜned to islands and are vulnerable to extinction. 2. Describing a disease or a pest that is always present in an area. For example, malaria is endemic in parts of Africa. endergonic reaction A chemical reaction in which energy is absorbed. Compare  . endo- (in chemistry) PreÜx used to designate a bridged ring molecule with a substituent on the ring that is on the same side
endocannabinoids as the bridge. If the substituent is on the opposite side the compound is designated exo-.
endocannabinoids See . endocarp See .
endocrine gland (ductless gland) Any gland in an animal that manufactures *hormones and secretes them directly into the bloodstream to act at distant sites in the body (known as target organs or cells). Endocrine glands tend to control slow longterm activities in the body, such as growth and sexual development. In mammals they include the *pituitary, *adrenal, *thyroid, and *parathyroid glands, the *ovary and *testis, the *placenta, and part of the pancreas (see   L). The activity of endocrine glands is controlled by negative feedback, i.e. a rise in output of hormone inhibits a further increase in its production, either directly or indirectly via the target organ or cell. See also  . Compare  . endocrinology The study of the structure and functions of the *endocrine glands and of the *hormones they produce. endocytosis The process by which materials enter a cell without passing through the plasma membrane. The membrane folds around material outside the cell, resulting in the formation of a saclike vesicle into which the material is incorporated. This vesicle is then pinched off from the cell surface so that it lies within the cell. Both *phagocytosis and *pinocytosis are forms of endocytosis. Compare . endoderm (entoderm) The internal layer of cells of the *gastrula, which will develop into the alimentary canal (gut) and digestive glands of the adult. See also  . endodermis The innermost layer of the root *cortex of a plant, lying immediately outside the vascular tissue. Various modiÜcations of the endodermal cell walls enable them to regulate the passage of materials both into and out of the vascular system. An endodermis may also be seen in the stems of some plants. endoergic Denoting a nuclear process that absorbs energy. Compare . endogamy The fusion of reproductive cells from closely related parents, i.e. *inbreeding. Compare .
endogenous Describing a substance, stimulus, organ, etc., that originates from within an organism. For example, growth rhythms not directed by environmental stimuli are termed endogenous rhythms. Lateral roots, which always grow from inside the main root rather than from its surface, are said to arise endogenously. Compare . endolymph The Ûuid that Ülls the membranous labyrinth of the vertebrate *inner ear. See ;  . Compare . endometrium The mucous membrane that lines the *uterus of mammals. It is comprised of an upper mucus-secreting layer, which is shed during menstruation, and a basal layer, which proliferates to form the upper layer. See also  . endonuclease An enzyme that catalyses the internal cleavage of nucleic acids. See also  . Compare . endoparasite A parasite that lives inside its host's body. See . endopeptidase A protein-digesting enzyme that cleaves a polypeptide chain at speciÜc sites between amino acids. For example, *chymotrypsin cleaves the chain next to aromatic amino acids, such as phenylalanine; *trypsin cleaves the chain next to basic amino acids, such as lysine or arginine; and *pepsin cleaves the chain next to tyrosine and phenylalanine. Compare . endoplasm See . endoplasmic reticulum (ER) A system of membranes within the cytoplasm of plant and animal *cells. It forms a link between the plasma and nuclear membranes and is the site of protein synthesis. It is also concerned with the transport of proteins and lipids within the cell. Rough ER has *ribosomes attached to its surface; proteins synthesized on the ribosomes are enclosed in vesicles and transported to the *Golgi apparatus. Smooth ER lacks ribosomes; it is the site of important metabolic reactions, including phospholipid and fatty-acid synthesis. ENDOR Electron–nuclear double resonance. A magnetic resonance technique involving exitation of both electron spins and nuclear spins. Two sources of radiation are used. One is a Üxed source at microwave frequency, which partially saturates the elec-
tron spins. The other is a variable radiofrequency source, which excites the atomic nuclear spins. Excitation of the nuclear spins affects the electron spins by hyperÜre coupling, increasing the relaxation time of the excited electron spins and increasing the signal strength. The technique, which is usually done at low temperatures, is used to investigate paramagnetic molecules. See also   .
end organ The structure at the end of a peripheral nerve. Examples of end organs are the muscle *end plate at the end of a motor neuron and the *receptor at the end of a sensory neuron. endorphin Any of a class of three endogenous *opioids – α-, β-, and γ- endorphins – found naturally in brain and other tissues that have pain-relieving effects similar to those of morphine. They are all peptides or polypeptides derived from the precursor pro-opiomelanocortin; for example, βendorphin is a 31-amino-acid peptide. The endorphins mediate their analgesic effects by binding to opioid receptors. See also . endoskeleton A supporting framework that lies entirely within the body of an animal, such as the bony *skeleton of vertebrates or the spicules of a sponge. The function of an endoskeleton is to support the body and in vertebrates it also protects the organs and provides a system of levers on which the muscles can act to produce movement. Compare . endosperm A nutritive tissue, characteristic of Ûowering plants, that surrounds the developing embryo in a seed. It develops from nuclei in the *embryo sac and its cells are triploid. In endospermic seeds it remains and increases in size; in nonendospermic seeds it disappears as the food is absorbed by the embryo, particularly the *cotyledons. Many plants with endospermic seeds, such as cereals and oil crops, are cultivated for the rich food reserves in the endosperm. endospore The resting stage of certain bacteria, formed in response to adverse conditions. The bacterial cell becomes enclosed in a tough resistant protein coat. On return to favourable conditions the spore germinates and reverts to the normal vegetative form of the organism. Endospores can remain viable for long periods, perhaps several thousands of years.
endosymbiont theory A theory, devised principally by US biologist Lynn Margulis (1938– ), that eukaryotic organisms evolved from symbiotic associations between prokaryotic ancestors. Free-living aerobic bacteria and chloroxybacteria (see C) became incorporated inside larger nucleated prokaryotic cells, where they acted as forerunners of the mitochondria and chloroplasts seen in modern eukaryotes. Such events are held to have occurred on several occasions, producing various lineages of both heterotrophic and phototrophic protoctists, from which evolved ancestors of animals, plants, and fungi. There is strong evidence for the theory, particularly the Ünding that mitochondria and chloroplasts have DNA similar in form to that of eubacteria, and that they contain prokaryotic-type ribosomes. endothelium A single layer of thin platelike cells that line the inner surfaces of blood and lymph vessels and the heart. Endothelium is derived from the *mesoderm. Compare ; . endotherm (homoiotherm) An animal that can generate and maintain heat within its body independently of the environmental temperature. Mammals and birds are endotherms; they are often described as being warm-blooded. See . Compare . endothermic Denoting a chemical reaction that takes heat from its surroundings. Compare . endotoxin See . end plate The area of the plasma membrane of a muscle cell that lies immediately beneath a motor nerve ending at a *neuromuscular junction. Release of a *neurotransmitter at the end plate induces contraction of the muscle Übre. end point The point in a titration at which reaction is complete as shown by the *indicator. energy A measure of a system's ability to do work. Like work itself, it is measured in joules. Energy is conveniently classiÜed into two forms: potential energy is the energy stored in a body or system as a consequence of its position, shape, or state (this includes gravitational energy, electrical energy, nuclear energy, and chemical energy); kinetic energy is energy of motion and is usually
energy bands
deÜned as the work that will be done by the body possessing the energy when it is brought to rest. For a body of mass m having a speed v, the kinetic energy is mv2/2 (classical) or (m – m0)c2 (relativistic). The rotational kinetic energy of a body having an angular velocity ω is Iω2/2, where I is its moment of inertia. The *internal energy of a body is the sum of the potential energy and the kinetic energy of its component atoms and molecules.
energy bands Ranges of energies that electrons can have in a solid. In a single atom, electrons exist in discrete *energy levels. In a crystal, in which large numbers of atoms are held closely together in a lattice, electrons are inÛuenced by a number of adjacent nuclei and the sharply deÜned levels of the atoms become bands of allowed energy (see illustration); this approach to energy levels in solids is often known as the band theory. Each band represents a large number of allowed quantum states. Between the bands are forbidden bands. The outermost electrons of the atoms (i.e. the ones responsible for chemical bonding) form the valence band of the solid. This is the band, of those occupied, that has the highest energy. The band structure of solids accounts for their electrical properties. In order to move through the solid, the electrons have to change from one quantum state to another. This can only occur if there are empty quantum states with the same energy. In general, if the valence band is full, electrons cannot change to new quantum states in the same band. For conduction to occur, the electrons have to be in an unÜlled band – the conducE
tion band. Metals are good conductors either because the valence band and the conduction band are only half-Ülled or because the conduction band overlaps with the valence band; in either case vacant states are available. In insulators the conduction band and valence band are separated by a wide forbidden band and electrons do not have enough energy to 'jump' from one to the other. In intrinsic *semiconductors the forbidden gap is narrow and, at normal temperatures, electrons at the top of the valence band can move by thermal agitation into the conduction band (at absolute zero, a semiconductor would act as an insulator). Doped semiconductors have extra bands in the forbidden gap.
energy Ûow (in ecology) The Ûow of energy that occurs along a *food chain. Energy enters the food chain at the level of the *producers (usually plants) in the form of solar energy. The plants convert solar energy into chemical energy in the process of *photosynthesis. Chemical energy is passed from one trophic level to the next through feeding. Since a large proportion of energy is lost at each trophic level, mostly in the form of heat energy due to respiration, a food chain does not normally consist of more than Üve trophic levels: the Üfth trophic level does not contain enough energy to support further levels. Energy is also lost from the food chain in excretory products and the remains of dead organisms; this is converted into heat energy by the action of *decomposers. See also ;   . energy landscape A multidimensional surface in which the energy of a system is
forbidden band
electron distribution valence band valence band insulator
Energy bands.
valence band conductor
plotted against parameters characterizing that system. For example, in molecular spectroscopy the parameters are the distances between atoms in the molecule. The concept of energy landscape is very useful in many topics in physics and chemistry, including nuclear Üssion and protein folding. See also  .
energy level A deÜnite Üxed energy that a system described by *quantum mechanics, such as a molecule, atom, electron, or nucleus, can have. In an atom, for example, the atom has a Üxed energy corresponding to the *orbitals in which its electrons move around the nucleus. The atom can accept a quantum of energy to become an excited atom (see ) if that extra energy will raise an electron to a permitted orbital. Between the ground state, which is the lowest possible energy level for a particular system, and the Ürst excited state there are no permissible energy levels. According to the *quantum theory, only certain energy levels are possible. An atom passes from one energy level to the next without passing through fractions of that energy transition. These levels are usually described by the energies associated with the individual electrons in the atoms, which are always lower than an arbitrary level for a free electron. The energy levels of molecules also involve quantized vibrational and rotational motion. Engel's salt See  . engine Any device for converting some forms of energy into mechanical work. See  ; C ;  ;  . enkephalin (encephalin) Any of a class of endogenous *opioids consisting of Üve amino acids and found principally in the central nervous system. They bind to opioid receptors, chieÛy in the brain, and their release controls levels of pain and other sensations. See also . enols Compounds containing the group –CH=C(OH)– in their molecules. See also – . enrichment The process of increasing the abundance of a speciÜed isotope in a mixture of isotopes. It is usually applied to an increase in the proportion of U–235, or the addition of Pu–239 to natural uranium for use in a nuclear reactor or weapon. Ensembl A joint venture between the Eu-
ropean Molecular Biology Laboratory (EMBL), the European Bioinformatics Institute (EBI), and the Wellcome Trust's Sanger Institute that provides both sequence data from various eukaryotic genomes and opensource (free) software for researchers handling the data.
ensemble A set of systems of particles used in *statistical mechanics to describe a single system. The concept of an ensemble was put forward by the US scientist Josiah Willard Gibbs (1839–1903) in 1902 as a way of calculating the time average of the single system, by averaging over the systems in the ensemble at a Üxed time. An ensemble of systems is constructed from knowledge of the single system and can be represented as a set of points in *phase space with each system of the ensemble represented by a point. Ensembles can be constructed both for isolated systems and for open systems. entanglement See  . enterokinase (enteropeptidase) An enzyme in the small intestine that activates trypsinogen to *trypsin. enteron (coelenteron; gastrovascular cavity) The body cavity of the coelenterates, which has one opening functioning both as mouth and anus. See C. enthalpy Symbol H. A thermodynamic property of a system deÜned by H = U + pV, where H is the enthalpy, U is the internal energy of the system, p its pressure, and V its volume. In a chemical reaction carried out in the atmosphere the pressure remains constant and the enthalpy of reaction, ∆H, is equal to ∆U + p∆V. For an exothermic reaction ∆H is taken to be negative. entoderm See . entomology The study of insects. entomophily Pollination of a Ûower in which the pollen is carried on an insect. Entomophilous Ûowers are usually brightly coloured and scented and often secrete nectar. In some species (e.g. primulas) there are structural differences between the Ûowers to ensure that cross-pollination occurs. Other examples of entomophilous Ûowers are orchids and antirrhinums. Compare ; . Entrez An information retrieval service, or browser, provided by the National Center for
entropy Biotechnology Information (NCBI), a division of the US National Library of Medicine. It gives access to a collection of NCBI databases, both bibliographic and biomolecular, including PubMed, *MEDLINE, and OMIM (Online Mendelian Inheritance in Man).
entropy Symbol S. A measure of the unavailability of a system's energy to do work; in a closed system an increase in entropy is accompanied by a decrease in energy availability. When a system undergoes a reversible change the entropy (S) changes by an amount equal to the energy (Q) transferred to the system by heat divided by the thermodynamic temperature (T) at which this occurs, i.e. ∆S = ∆Q/T. However, all real processes are to a certain extent irreversible changes and in any closed system an irreversible change is always accompanied by an increase in entropy. In a wider sense entropy can be interpreted as a measure of disorder; the higher the entropy the greater the disorder (see B ). As any real change to a closed system tends towards higher entropy, and therefore higher disorder, it follows that the entropy of the universe (if it can be considered a closed system) is increasing and its available energy is decreasing (see     ). This increase in the entropy of the universe is one way of stating the second law of *thermodynamics.
• Translations of papers by Clausius on entropy (1850) and the second law (1865), published in Annalen der Physik und Chemie
environment (in ecology) The physical, chemical, and biological conditions of the region in which an organism lives. See also ; . environmental resistance The sum total of the factors that prevent populations from continually growing and therefore tend to keep populations at constant levels. These factors include predators, disease, and a shortage of any of the various requirements for survival, such as food, water, shelter, and light (which is particularly important for plants). See also  . enyl complex A type of complex in which there is a link between the metal atom or ion and the pi electrons of a double bond. *Zeise's salt was the Ürst known example. enzyme A protein that acts as a *catalyst in
biochemical reactions. Each enzyme is speciÜc to a particular reaction or group of similar reactions. Many require the association of certain nonprotein *cofactors in order to function. The molecule undergoing reaction (the substrate) binds to a speciÜc *active site on the enzyme molecule to form a short-lived intermediate (see –  ): this greatly increases (by a factor of up to 1020) the rate at which the reaction proceeds to form the product. Enzyme activity is inÛuenced by substrate concentration and by temperature and pH, which must lie within a certain range. Other molecules may compete for the active site, causing *inhibition of the enzyme or even irreversible destruction of its catalytic properties. Enzyme production is governed by a cell's genes. Enzyme activity is further controlled by pH changes, alterations in the concentrations of essential cofactors, feedback inhibition by the products of the reaction, and activation by another enzyme, either from a less active form or an inactive precursor (*zymogen). Such changes may themselves be under the control of hormones or the nervous system. See also  . Enzymes are classiÜed into six major groups, according to the type of reaction they catalyse: (1) *oxidoreductases; (2) *transferases; (3) *hydrolases; (4) *lyases; (5) *isomerases; (6) *ligases. The names of most individual enzymes also end in -ase, which is added to the names of the substrates on which they act. Thus *lactase is the enzyme that acts to break down lactose; it is classiÜed as a hydrolase.
enzyme inhibition See . enzyme kinetics The study of the rates of enzyme-catalysed reactions. Rates of reaction are usually measured by using the puriÜed enzyme in vitro with the substrate and then observing the formation of the product or disappearance of the substrate. As the concentration of the substrate is increased the rate of reaction increases proportionally up to a certain point, after which any further increase in substrate concentration no longer increases the reaction rate (see M–M ). At this point, all active sites of the enzyme are saturated with substrate; any further increase in the rate of reaction will occur only if more
enzyme is added. Reaction rates are also affected by the presence of inhibitors (see ), temperature, and pH (see ).
enzyme-linked immunosorbent assay See ELISA. enzyme–substrate complex The intermediate formed when a substrate molecule interacts with the *active site of an enzyme. Following the formation of an enzyme–substrate complex, the substrate molecule undergoes a chemical reaction and is converted into a new product. Various mechanisms for the formation of enzyme–substrate complexes have been suggested, including the *lock-and-key mechanism. Eocene The second geological epoch of the *Palaeogene period. It extended from the end of the Palaeocene epoch, about 55 million years ago, to the beginning of the Oligocene epoch, about 34 million years ago. The term was Ürst proposed by Sir Charles Lyell in 1833. Mammals were dominant in the Eocene: rodents, artiodactyls, carnivores, perissodactyls (including early horses), and whales were among the groups to make their Ürst appearance. eosin One of a series of acidic dyes, used in optical microscopy, that colours cytoplasm pink and cellulose red. It is frequently used as a counterstain with *haematoxylin for colouring tissue smears and sections of animal tissue. ephedrine An alkaloid, C6H5CH(OH)CH(CH3)NHCH3, found in plants of the genus Ephedra, once used as a bronchodilator in the treatment of asthma. It is also used as a stimulant and appetite suppressant. Structurally, it is a phenylethylamine and is similar to amphetamines, although less active. It is, however, widely used in the illegal synthesis of methamphetamine. The molecule has two chiral centres. If the stereochemical conformations are opposite (i.e. 1R,2S or 1S,1R) the name ephedrine is used. If the conformations are the same (1R,2R or 1S,2S) then the compound is called pseudoephedrine. ephemeral 1. (in botany) An *annual plant that completes its life cycle in considerably less than one growing season. A number of generations can therefore occur in one year. Many troublesome weeds, such as groundsel and willowherb, are ephemerals. Certain desert plants are also ephemerals, completing their life cycles in a short period
following rain. 2. (in zoology) A short-lived animal, such as a mayÛy.
ephemeris A tabulation showing the calculated future positions of the sun, moon, and planets, together with other useful information for astronomers and navigators. It is published at regular intervals. ephemeris time (ET) A time system that has a constant uniform rate as opposed to other systems that depend on the earth's rate of rotation, which has inherent irregularities. It is reckoned from an instant in 1900 (Jan 0d 12h) when the sun's mean longitude was 279.696 677 8°. The unit by which ephemeris time is measured is the tropical year, which contains 31 556 925.9747 ephemeris seconds. This fundamental deÜnition of the *second was replaced in 1964 by the caesium second of atomic time. epicalyx A ring of bracts below a Ûower that resembles a calyx. It is seen, for example, in the strawberry Ûower. epicarp See . epicentre The point on the surface of the earth directly above the focus of an earthquake or directly above or below a nuclear explosion. epicotyl The region of a seedling stem above the stalks of the seed leaves (*cotyledons) of an embryo plant. It grows rapidly in seeds showing *hypogeal germination and lifts the stem above the soil surface. Compare . epicycle A small circle whose centre rolls around the circumference of a larger Üxed circle. The curve traced out by a point on the epicycle is called an epicycloid. epidemiology The study of diseases that affect large numbers of people. Traditionally, epidemiologists have been concerned primarily with infectious diseases, such as typhoid and inÛuenza, that arise and spread rapidly among the population as epidemics. However, today the discipline also covers noninfectious disorders, such as diabetes, heart disease, and back pain. Typically the distribution of a disease is charted in order to discover patterns that might yield clues about its mode of transmission or the susceptibility of certain groups of people. This in turn may reveal insights about the causes of the disease and possible preventive measures.
epidermis 1. (in zoology) The outermost layer of cells of the body of an animal. In invertebrates the epidermis is normally only one cell thick and is covered by an impermeable *cuticle. In vertebrates the epidermis is the thinner of the two layers of *skin (compare ). It consists of a basal layer of actively dividing cells (see M ), covered by layers of cells that become impregnated with keratin (see ). The outermost layers of epidermal cells (the *stratum corneum) form a water-resistant protective layer. The epidermis may bear a variety of specialized structures (e.g. *feathers, *hairs). 2. (in botany) The outermost layer of cells covering a plant. It is overlaid by a *cuticle and its functions are principally to protect the plant from injury and to reduce water loss. Some epidermal cells are modiÜed to form guard cells (see ) or hairs of various types (see  ). In woody plants the functions of the shoot epidermis are taken over by the periderm tissues (see  ) and in mature roots the epidermis is sloughed off and replaced by the *hypodermis. epidiascope An optical instrument used by lecturers, etc., for projecting an enlarged image of either a translucent object (such as a slide or transparency) or an opaque object (such as a diagram or printed page) onto a screen. epididymis A long coiled tube in which spermatozoa are stored in vertebrates. In reptiles, birds, and mammals it is attached at one end to the *testis and opens into the sperm duct (*vas deferens) at the other. epigamic Serving to attract a mate. Epigamic characters include the bright plumage of some male birds. epigeal Describing seed germination in which the seed leaves (cotyledons) emerge from the ground and function as true leaves. Examples of epigeal germination are seen in sycamore and sunÛower. Compare . epiglottis A Ûexible Ûap of cartilage in mammals that is attached to the wall of the pharynx near the base of the tongue. During swallowing (see ) it covers the *glottis (the opening to the respiratory tract) and helps to prevent food from entering the trachea (windpipe), although it is not essential for this purpose.
epilimnion The upper layer of water in a lake. Compare . See . epimerism A type of optical isomerism in which a molecule has two chiral centres; two optical isomers (epimers) differ in the arrangement about one of these centres. See also  . epinephrine See . epiphysis The terminal section of a growing bone (especially a long limb bone) in mammals. It is separated from the bone shaft (diaphysis) by cartilage. New bone is produced on the side of the cartilage facing the diaphysis, while new cartilage is produced on the other side of the cartilage disc. When the bone reaches adult length the epiphysis merges with the diaphysis. epiphyte A plant that grows upon another plant but is neither parasitic on it nor rooted in the ground. Epiphytes include many mosses and lichens and some tropical orchids. episome A genetic element that can exist and replicate either independently of its host cell's chromosomes or as an integrated part of the chromosomes. Examples include certain bacterial *plasmids. epistasis A gene interaction in which one gene suppresses the effect of another gene that is situated at a different *locus on the chromosome. For example, in guinea pigs the gene that controls the production of melanin is epistatic to the gene that regulates the deposition of melanin. A dominant allele (C) is responsible for the production of melanin, while the amount of melanin deposited is controlled by a second gene, which determines whether the coat colour is black or brown. If an animal is homozygous recessive (cc) for melanin production, the coat colour will be white regardless of the alleles that produce black or brown coloration. epitaxy (epitaxial growth) Growth of a layer of one substance on a single crystal of another, such that the crystal structure in the layer is the same as that in the substrate. It is used in making semiconductor devices. epithelium A tissue in vertebrates consisting of closely packed cells in a sheet with little intercellular material. It covers the outer surfaces of the body and the walls of the internal cavities (coeloms). It also forms glands
equation of state
and parts of sense organs. Its functions are protective, absorptive, secretory, and sensory. The types of cell vary, giving rise to squamous, cuboidal, columnar, and ciliated epithelia. Stratified epithelium (e.g. in the skin) is made up of several layers of cells. Epithelium is derived from *ectoderm and *endoderm. Compare ; .
epithermal neutron A neutron with an energy in excess of that associated with a thermal neutron (see ) but less than that of a *fast neutron, i.e. a neutron having an energy in the range 0.1 to 100 eV. EPM See   . epoch 1. (in astronomy) An arbitrary date used as a Üxed reference point, especially for the celestial coordinates of a star or deep-sky object or the orbital elements of a planet. Because the coordinates *right ascension and *declination change over time, the position of an astronomical object given in a catalogue must always be referred to a particular epoch, e.g. the date and time that an observation was made, the start of a year. 2. The starting date of a calendar or chronological era. 3. A unit of geological time in which a series of rocks is formed. epoetin (EPO) A drug used medically to treat anaemia, and also used illegally by athletes participating in endurance events. It is a genetically engineered form of the hormone erythropoietin, which regulates the production of red blood cells. epoxides Compounds that contain oxygen atoms in their molecules as part of a threemembered ring (see formula). Epoxides are thus cyclic ethers. CH3
CH3 O
Epoxides. The functional group in epoxides. epoxyethane (ethylene oxide) A colourless Ûammable gas, C2H4O; m.p. –111°C; b.p. 13.5°C. It is a cyclic ether (see ) that is made by the catalytic oxidation of ethene. It can be hydrolysed to ethane-1,2-diol and also polymerizes to: … –O–C2H4–O–C2H4–…, which is used for lowering the viscosity of water (e.g. in Üre Üghting).
epoxy resins Synthetic resins produced by copolymerizing epoxide compounds with phenols. They contain –O– linkages and epoxide groups and are usually viscous liquids. They can be hardened by addition of agents, such as polyamines, that form crosslinkages. Alternatively, catalysts may be used to induce further polymerization of the resin. Epoxy resins are used in electrical equipment and in the chemical industry (because of resistance to chemical attack). They are also used as adhesives. EPR See   . epsomite A mineral form of *magnesium sulphate heptahydrate, MgSO4. 7H2O. Epsom salt See  . Epstein–Barr virus See . equation of motion (kinematic equation) Any of four equations that apply to bodies moving linearly with uniform acceleration (a). The equations, which relate distance covered (s) to the time taken (t), are: v = u + at s = (u + v)t/2 s = ut + at2/2 v2 = u2 + 2as, where u is the initial velocity of the body and v is its Ünal velocity.
equation of state An equation that relates the pressure p, volume V, and thermodynamic temperature T of an amount of substance n. The simplest is the ideal *gas law: pV = nRT, where R is the universal gas constant. Applying only to ideal gases, this equation takes no account of the volume occupied by the gas molecules (according to this law if the pressure is inÜnitely great the volume becomes zero), nor does it take into account any forces between molecules. A more accurate equation of state would therefore be (p + k)(V – nb) = nRT, where k is a factor that reÛects the decreased pressure on the walls of the container as a result of the attractive forces between particles, and nb is the volume occupied by the particles themselves when the pressure is inÜnitely high. In the van der Waals equa-
equation of time tion of state, proposed by the Dutch physicist J. D. van der Waals (1837–1923), k = n2a/V2, where a is a constant. This equation more accurately reÛects the behaviour of real gases; several others have done better but are more complicated.
equation of time The length of time that must be added to the mean solar time, as shown on a clock, to give the apparent solar time, as shown by a sundial. The amount varies during the year, being a minimum of –14.2 minutes in February and a maximum of +16.4 minutes in October. It is zero on four days (April 15/16, June 14/15, Sept. 1/2, Dec. 25/26). The difference arises as a result of two factors: the eccentricity of the earth's orbit and the inclination of the ecliptic to the celestial equator. equator 1. The great circle around the earth that lies in a plane perpendicular to the earth's axis. It is equidistant from the two geographical poles. 2. The magnetic equator is a line of zero magnetic dip (see ) that is close to the geographical equator but lies north of it in Africa and south of it in America. 3. The celestial equator is the circle formed on the *celestial sphere by the extension of the earth's equatorial plane. 4. (in cell biology) See . equilibrium A state in which a system has its energy distributed in the statistically most probable manner; a state of a system in which forces, inÛuences, reactions, etc., balance each other out so that there is no net change. A body is in static equilibrium if the resultants of all forces and all couples acting on it are both zero; it may be at rest and will certainly not be accelerated. Such a body at rest is in stable equilibrium if after a slight displacement it returns to its original position – for a body whose weight is the only downward force this will be the case if the vertical line through its centre of gravity always passes through its base. If a slight displacement causes the body to move to a new position, then the body is in unstable equilibrium. A body is said to be in thermal equilibrium if no net heat exchange is taking place within it or between it and its surroundings. A system is in *chemical equilibrium when a reaction and its reverse are proceeding at equal rates (see also  ). These are examples of dynamic equi-
librium, in which activity in one sense or direction is in aggregate balanced by comparable reverse activity.
equilibrium constant For a reversible reaction of the type xA + yB ˆ zC + wD chemical equilibrium occurs when the rate of the forward reaction equals the rate of the back reaction, so that the concentrations of products and reactants reach steady-state values. It can be shown that at equilibrium the ratio of concentrations [C]z[D]w/[A]x[B]y is a constant for a given reaction and Üxed temperature, called the equilibrium constant Kc (where the c indicates concentrations have been used). Note that, by convention, the products on the right-hand side of the reaction are used on the top line of the expression for equilibrium constant. This form of the equilibrium constant was originally introduced in 1863 by C. M. Guldberg and P. Waage using the law of *mass action. They derived the expression by taking the rate of the forward reaction kf[A]x[B]y and that of the back reaction kb[C]z[D]w Since the two rates are equal at equilibrium, the equilibrium constant Kc is the ratio of the rate constants kf/kb. The principle that the expression is a constant is known as the equilibrium law or law of chemical equilibrium. The equilibrium constant shows the position of equilibrium. A low value of Kc indicates that [C] and [D] are small compared to [A] and [B]; i.e. that the back reaction predominates. It also indicates how the equilibrium shifts if concentration changes. For example, if [A] is increased (by adding A) the equilibrium shifts towards the right so that [C] and [D] increase, and Kc remains constant. For gas reactions, partial pressures are used rather than concentrations. The symbol Kp is then used. Thus, in the example above Kp = pCzpDw/pAxpBy It can be shown that, for a given reaction Kp = Kc(RT)∆ν, where ∆ν is the difference in stoichiometric coefÜcients for the reaction (i.e. z + w – x – y). Note that the units of Kp and Kc depend on the numbers of molecules appearing in the stoichiometric equation. The value of the equilibrium constant depends
on the temperature. If the forward reaction is exothermic, the equilibrium constant decreases as the temperature rises; if endothermic it increases (see also ' H' ). The expression for the equilibrium constant can also be obtained by thermodynamics; it can be shown that the standard equilibrium constant KŠ is given by exp(–∆GŠ/RT), where ∆GŠ is the standard Gibbs free energy change for the complete reaction. Strictly, the expressions above for equilibrium constants are true only for ideal gases (pressure) or inÜnite dilution (concentration). For accurate work *activities are used.
equilibrium law See  . equinox 1. Either of the two points on the *celestial sphere at which the *ecliptic intersects the celestial equator. The sun appears to cross the celestial equator from south to north at the vernal equinox and from north to south at the autumnal equinox. 2. Either of the two instants at which the centre of the sun appears to cross the celestial equator. In the northern hemisphere the vernal equinox occurs on or about March 21 and the autumnal equinox on or about Sept. 23. In the southern hemisphere the dates are reversed. See    . equipartition of energy The theory, proposed by Ludwig Boltzmann and given some theoretical support by James Clerk Maxwell, that the energy of gas molecules in a large sample under thermal *equilibrium is equally divided among their available *degrees of freedom, the average energy for each degree of freedom being kT/2, where k is the *Boltzmann constant and T is the thermodynamic temperature. The proposition is not generally true if *quantum considerations are important, but is frequently a good approximation. equivalence point The point in a titration at which reaction is complete. See . equivalent proportions See  . equivalent weight The mass of an element or compound that could combine with or displace one gram of hydrogen (or eight grams of oxygen or 35.5 grams of chlorine) in a chemical reaction. The equivalent weight represents the 'combining power' of
the substance. For an element it is the relative atomic mass divided by the valency. For a compound it depends on the reaction considered.
erbium Symbol Er. A soft silvery metallic element belonging to the *lanthanoids; a.n. 68; r.a.m. 167.26; r.d. 9.006 (20°C); m.p. 1529°C; b.p. 2863°C. It occurs in apatite, gadolinite, and xenotine from certain sources. There are six natural isotopes, which are stable, and twelve artiÜcial isotopes are known. It has been used in alloys for nuclear technology as it is a neutron absorber; it is being investigated for other potential uses. It was discovered by Carl Mosander (1797–1858) in 1843.
erecting prism A glass prism used in optical instruments to convert an inverted image into an erect image, as in prismatic binoculars. erg A unit of work or energy used in the c.g.s. system and deÜned as the work done by a force of 1 dyne when it acts through a distance of 1 centimetre. 1 erg = 10–7 joule. ergocalciferol See  D. ergonomics The study of the engineering aspects of the relationship between workers and their working environment. ergosphere The region immediately around a *black hole. The hole's rotation drags the space–time continuum round with it, so that frames of reference are not stationary with reference to the remainder of the universe. The ergosphere's outer boundary is called the stationary limit. ergosterol A *sterol occurring in fungi, bacteria, algae, and plants. It is converted into vitamin D2 by the action of ultraviolet light. Eris A *small solar system body made of rock and ice that is the largest *dwarf planet so far known and the ninth largest body orbiting the sun. At an estimated 2400 km or more in diameter, it is at least 5% larger than *Pluto and 27% more massive. Eris was discovered in January 2005 by a team at Mount Palomar led by Michael E. Brown (1965– ). It is a *scattered disc object, orbiting the sun once every 557.43 years at a mean distance of 10.12 × 109 km. The eccentricity of its orbit is about 0.44 and when at its farthest point
Erlangen Programme from the sun (14.6 × 109 km), it is three times as far away as Neptune. One natural satellite, Dysnomia, orbits Eris at an estimated distance of nearly 37 400 km in a period of 15.77 days.
Erlangen Programme The view, put forward by the German mathematician Felix Klein (1849–1925) in 1872 when he became a faculty member of the University of Erlangen, that each type of geometry can be characterized by a group of transformations. There are many physical examples of this, including the Lorentz group of Minkowski space–time and the space groups of crystals. However, not all types of geometry Üt into this view readily; in particular, Riemannian geometry, and hence curved space–time, do not Üt unless the Erlangen Programme is given a wider interpretation. It has been suggested that such a broader interpretation might lead to important insights into the foundations of fundamental physics. erosion The wearing away of the land surface by natural agents that involves the transport of rock debris. These natural agents include moving waters (e.g. rivers, ocean waves), ice (e.g. glaciers), wind, organisms, and gravity. See also  . erratic A fragment of rock, often unlike the rocks around it, that has been displaced from its original location by the action of a glacier or, more rarely, an iceberg. Erratics may have been moved as little as several metres to more than 800 km. They vary in size from small pebbles to massive boulders, and may be found on the surface or embedded in *boulder clay. They provide geologists with information about the movement of ice sheets. erythroblast Any of the cells in the *myeloid tissue of red bone marrow that develop into erythrocytes (red blood cells). Erythroblasts have a nucleus and are at Ürst colourless, but Üll with *haemoglobin as they develop. In mammals the nucleus disappears. erythrocyte (red blood cell) The most numerous type of blood cell, which contains the red pigment *haemoglobin and is responsible for oxygen transport. Mammalian erythrocytes are disc-shaped and lack a nucleus; those of other vertebrates are oval and nucleated. In man the number of erythrocytes in the blood varies between 4.5 and 5.5 million per cubic millimetre. They survive
for about four months and are then destroyed in the spleen and liver. See also . Compare .
Esaki diode See  . ESCA See  . escapement A device in a clock or watch that controls the transmission of power from the spring or falling weight to the hands. It is usually based on a balance wheel or pendulum. It thus allows energy to enter the mechanism in order to move the hands round the face, overcome friction in the gear trains, and maintain the balance wheel or pendulum in continuous motion. escape velocity The minimum speed needed by a space vehicle, rocket, etc., to escape from the gravitational Üeld of the earth, moon, or other celestial body. The gravitational force between a rocket of mass m and a celestial body of mass M and radius r is MmG/r2 (see N'   ). Therefore the gravitational potential energy of the rocket with respect to its possible position very far from the celestial body on which it is resting can be shown to be –GmM/r, assuming (by convention) that the potential energy is zero at an inÜnite distance from the celestial body. If the rocket is to escape from the gravitational Üeld it must have a kinetic energy that exceeds this potential energy, i.e. the kinetic energy mv2/2 must be greater than MmG/r, or v > √(2MG/r). This is the value of the escape velocity. Inserting numerical values for the earth and moon into this relationship gives an escape velocity from the earth of 11 200 m s–1 and from the moon of 2370 m s–1. Escherichia coli (E. coli) A species of Gram-negative aerobic bacteria that is found in the intestine and is also widely used in microbiological and genetics research. The motile rod-shaped cells ferment lactose and are usually harmless commensals, although certain strains are pathogenic and can cause a severe form of food poisoning. Studies of E. coli laboratory cultures have revealed much about the genetics of prokaryotes; the species is also frequently used in genetic engineering, particularly as a host for *gene cloning and the expression of recombinant foreign genes in culture. ESI See  . esker An elongated, steep-sided ridge of debris left behind by meltwater streams
etaerio
Ûowing in or under a slow-moving glacier. The ridge may be straight or, more often, sinuous in shape. It may be up to 50 m high and 700 m wide. In very cold regions, some eskers have ice cores. The debris is stratiÜed in layers and generally consists of rounded particles of gravel and sand, with some Ünegrain deposits. See also .
ESR See - . essential amino acid An *amino acid that an organism is unable to synthesize in sufÜcient quantities. It must therefore be present in the diet. In humans the essential amino acids are histidine, lysine, threonine, methionine, isoleucine, leucine, valine, phenylalanine, and tryptophan. These are required for protein synthesis and deÜciency leads to retarded growth and other symptoms. Most of the amino acids required by man are also essential for all other multicellular animals and for most protozoans. essential element Any of a number of elements required by living organisms to ensure normal growth, development, and maintenance. Apart from the elements found in organic compounds (i.e. carbon, hydrogen, oxygen, and nitrogen), plants, animals, and microorganisms all require a range of elements in inorganic forms in varying amounts, depending on the type of organism. The major elements, present in tissues in relatively large amounts (greater than 0.005%), are calcium, phosphorus, potassium, sodium, chlorine, sulphur, and magnesium (see also ). The trace elements occur at much lower concentrations and thus requirements are much less. The most important are iron, manganese, zinc, copper, iodine, cobalt, selenium, molybdenum, chromium, and silicon (see also ). Each element may fulÜl one or more of a variety of metabolic roles. Sodium, potassium, and chloride ions are the chief electrolytic components of cells and body Ûuids and thus determine their electrical and osmotic status. Calcium, phosphorus, and magnesium are all present in bone. Calcium is also essential to nerve and muscle activity, while phosphorus is a key constituent of the chemical energy carriers (e.g. *ATP) and the nucleic acids. Sulphur is needed primarily for amino acid synthesis (in plants and microorganisms). The trace elements may serve as *cofactors or as constituents of complex molecules, e.g. iron in
haem and cobalt in vitamin B12. See also  .
essential fatty acids (omega fatty acids) *Fatty acids that must normally be present in the diet of certain animals, including humans. Essential fatty acids belong to the n-3 (or omega-3) and n-6 (or omega-6) classes of polyunsaturated fatty acids and include *arachidonic (omega-6), *linoleic (omega-6), and *linolenic (omega-3) acids; all possess double bonds at the same two positions along their hydrocarbon chain and so can act as precursors of *prostaglandins. DeÜciency can cause dermatosis, weight loss, irregular oestrus, etc. Both omega-3 and omega-6 acids should be consumed in the diet, ideally in roughly the same amounts. essential oil A natural oil with a distinctive scent secreted by the glands of certain aromatic plants. *Terpenes are the main constituents. Essential oils are extracted from plants by steam distillation, extraction with cold neutral fats or solvents (e.g. alcohol), or pressing and used in perfumes, Ûavourings, and medicine. Examples are citrus oils, Ûower oils (e.g. rose, jasmine), and oil of cloves. esteriÜcation A reaction of an alcohol with an acid to produce an ester and water; e.g. CH3OH + C6H5COOH ˆ CH3OOCC6H5 + H2O The reaction is an equilibrium and is normally slow, but can be speeded up by addition of a strong acid catalyst. The ester can often be distilled off so that the reaction can proceed to completion. The reverse reaction is ester hydrolysis or *saponiÜcation. See also . esters Organic compounds formed by reaction between alcohols and acids (see illustration overleaf). Esters formed from carboxylic acids have the general formula RCOOR′. Examples are ethyl ethanoate, CH3COOC2H5, and methyl propanoate, C2H5COOCH3. Esters containing simple hydrocarbon groups are volatile fragrant substances used as Ûavourings in the food industry. Triesters, molecules containing three ester groups, occur in nature as oils and fats. See also .
etaerio A cluster of fruits formed from the
ethanal
300 O C
C CH3
C2H5 + H2O
propanoic acid methyl propanoate
Ester formation.
unfused carpels of a single Ûower. For example, the anemone has an etaerio of *achenes, larkspur an etaerio of *follicles, and blackberry an etaerio of *drupes.
ethanal (acetaldehyde) A colourless highly Ûammable liquid aldehyde, CH3CHO; r.d. 0.78; m.p. –121°C; b.p. 20.8°C. It is made from ethene by the *Wacker process and used as a starting material for making many organic compounds. The compound polymerizes if dilute acid is added to give ethanal trimer (or paraldehyde), which contains a six-membered ring of alternating carbon and oxygen atoms with a hydrogen atom and a methyl group attached to each carbon atom. It is used as a drug for inducing sleep. Addition of dilute acid below 0°C gives ethanal tetramer (or metaldehyde), which has a similar structure to the trimer but with an eight-membered ring. It is used as a solid fuel in portable stoves and in slug pellets. ethanamide (acetamide) A colourless solid crystallizing in the form of long white crystals with a characteristic smell of mice, CH3CONH2; r.d. 1.159; m.p. 82.3°C; b.p. 221.25°C. It is made by the dehydration of ammonium ethanoate or by the action of ammonia on ethanoyl chloride, ethanoic anhydride, or ethyl ethanoate. ethane A colourless Ûammable gaseous hydrocarbon, C2H6; m.p. –183°C; b.p. –89°C. It is the second member of the *alkane series of hydrocarbons and occurs in natural gas. ethanedioic acid See  . ethane-1,2-diol (ethylene glycol; glycol) A colourless viscous hygroscopic liquid, CH2OHCH2OH; m.p. –11.5°C; b.p. 198°C. It is made by hydrolysis of epoxyethane (from ethene) and used as an antifreeze and a raw material for making *polyesters (e.g. Terylene). ethanoate (acetate) A salt or ester of ethanoic acid (acetic acid). ethanoic acid (acetic acid) A clear vis-
cous liquid or glassy solid *carboxylic acid, CH3COOH, with a characteristically sharp odour of vinegar; r.d. 1.049; m.p. 16.6°C; b.p. 117.9°C. The pure compound is called glacial ethanoic acid. It is manufactured by the oxidation of ethanol or by the oxidation of butane in the presence of dissolved manganese(II) or cobalt(II) ethanoates at 200°C, and is used in making ethanoic anhydride for producing cellulose ethanoates. It is also used in making ethenyl ethanoate (for polyvinylacetate). The compound is formed by the fermentation of alcohol and is present in vinegar, which is made by fermenting beer or wine. 'Vinegar' made from ethanoic acid with added colouring matter is called 'nonbrewed condiment'. In living organisms it combines with *coenzyme A to form acetyl coenzyme A, which plays a crucial role in energy metabolism.
ethanoic anhydride (acetic anhydride) A pungent-smelling colourless liquid, (CH3CO)2O, b.p. 139.5°C. It is used in organic synthesis as an *ethanoylating agent (attacking an –OH or –NH group) and in the manufacture of aspirin and cellulose plastics. It hydrolyses in water to give ethanoic acid. ethanol (ethyl alcohol) A colourless water-soluble *alcohol, C2H5OH; r.d. 0.789 (0°C); m.p. –117.3°C; b.p. –78.3°C. It is the active principle in intoxicating drinks, in which it is produced by fermentation of sugar using yeast C6H12O6 → 2C2H5OH + 2CO2 The ethanol produced kills the yeast and fermentation alone cannot produce ethanol solutions containing more than 15% ethanol by volume. Distillation can produce a constant-boiling mixture containing 95.6% ethanol and 4.4% water. Pure ethanol (absolute alcohol) is made by removing this water by means of drying agents. The main industrial use of ethanol is as a solvent although at one time it was a major starting point for making other chemicals. For this it was produced by fermentation of
molasses. Now ethene has replaced ethanol as a raw material and industrial ethanol is made by hydrolysis of ethene.
ethanoylating agent (acetylating agent) A chemical reagent used to introduce an ethanoyl group (–COCH3) instead of hydrogen in an organic compound. Examples include *ethanoic anhydride and ethanoyl chloride (acetyl chloride, CH3COCl). ethanoyl chloride (acetyl chloride) A colourless liquid acyl chloride (see  ), CH3COCl, with a pungent smell; r.d. 1.105; m.p. –112.15°C; b.p. 50.9°C. It is made by reacting ethanoic acid with a halogenating agent such as phosphorus(III) chloride, phosphorus(V) chloride, or sulphur dichloride oxide and is used to introduce ethanoyl groups into organic compounds containing –OH, –NH2, and –SH groups. See . ethanoyl group (acetyl group) The organic group CH3CO–. ethene (ethylene) A colourless Ûammable gaseous hydrocarbon, C2H4; m.p. –169°C; b.p. –103.7°C. It is the Ürst member of the *alkene series of hydrocarbons. It is made by cracking hydrocarbons from petroleum and is now a major raw material for making other organic chemicals (e.g. ethanal, ethanol, ethane-1,2-diol). It can be polymerized to *polyethene. It occurs naturally in plants, in which it acts as a growth substance (see  ): its best-known effect is the stimulation of fruit ripening. ethenone See . ethenyl ethanoate (vinyl acetate) An unsaturated organic ester, CH2:CHOOCCH3; r.d. 0.9; m.p. –100°C; b.p. 73°C. It is made by catalytic reaction of ethanoic acid and ethene and used to make polyvinylacetate. ether 1. (aether) A hypothetical medium once believed to be necessary to support the propagation of electromagnetic radiation. It is now regarded as unnecessary and in modern theory electromagnetic radiation can be propagated through empty space. The existence of the ether was Ürst called into question as a result of the *Michelson–Morley experiment. 2. See ; . ethers Organic compounds containing the group –O– in their molecules. Examples are dimethyl ether, CH3OCH3, and diethyl ether, C2H5OC2H5 (see ). They are
volatile highly Ûammable compounds made by dehydrating alcohols using sulphuric acid.
ethology The study of the biology of *animal behaviour. Central to the ethologist's approach is the principle that animal behaviour (like physical characteristics) is subject to evolution through natural selection. Ethologists therefore seek to explain how the behaviour of animals, both as individuals and social groups, in their natural environment may contribute to the survival of the maximum number of its relatives and offspring. This involves recognizing the stimuli that are important in nature (see  ) and how innate predispositions interact with *learning in the development of behaviour (see ). Studies of this sort were pioneered by Konrad Lorenz and Niko Tinbergen and have led to the modern disciplines of sociobiology and behavioural ecology. ethoxyethane (diethyl ether; ether) A colourless Ûammable volatile *ether, C2H5OC2H5; r.d. 0.71; m.p. –116°C; b.p. 34.5°C. It can be made by *Williamson's synthesis. It is an anaesthetic and useful organic solvent. ethyl 3-oxobutanoate (ethyl acetoacetonate) A colourless liquid ester with a pleasant odour, CH3COCH2COOC2H5; r.d. 1.03; m.p. 85), such as bread, rice, and potatoes, cause blood glucose to rise rapidly following ingestion, whereas the increase is lower and more prolonged in foods with a low GI (< 60), such as apples, beans, and yoghurt. Lower GI foods are recommended to optimize control of blood glucose for diabetics. glycan See . glyceraldehyde 3-phosphate (GALP) A triose phosphate, CHOCH(OH)CH2OPO3H2, that is an intermediate in the *Calvin cycle (see also ) and glycolysis. glycerate 3-phosphate A phosphorylated three-carbon monosaccharide that is an intermediate in the *Calvin cycle of photosynthesis and also in *glycolysis. It was for-
glyceride
merly known as 3-phosphoglycerate or phosphoglyceric acid (PGA).
glyceride (acylglycerol) A fatty-acid ester of glycerol. EsteriÜcation can occur at one, two, or all three hydroxyl groups of the glycerol molecule producing mono-, di-, and triglycerides respectively. *Triglycerides are the major constituent of fats and oils found in living organisms. Alternatively, one of the hydroxyl groups may be esteriÜed with a phosphate group forming a phosphoglyceride (see ) or to a sugar forming a glycolipid.
glycerine See . glycerol (glycerine; propane-1,2,3,-triol) A trihydric alcohol, HOCH2CH(OH)CH2OH. Glycerol is a colourless sweet-tasting viscous liquid, miscible with water but insoluble in ether. It is widely distributed in all living organisms as a constituent of the *glycerides, which yield glycerol when hydrolysed. Glycerol itself is used as an *antifreeze molecule by certain organisms. glycerophospholipid See . glycine A sweet-tasting *amino acid that, besides being a component of proteins, is the main inhibitory neurotransmitter for fast synapses in the spinal cord of vertebrates. glycobiology The study of carbohydrates and carbohydrate complexes, especially *glycoproteins. glycogen (animal starch) A *polysaccharide consisting of a highly branched polymer of glucose occurring in animal tissues, espeglucose
cially in liver and muscle cells. It is the major store of carbohydrate energy in animal cells and is present as granular clusters of minute particles.
glycogenesis The conversion of glucose to glycogen, which is stimulated by insulin from the pancreas. Glycogenesis occurs in skeletal muscles and to a lesser extent in the liver. Glucose that is taken up by cells is phosphorylated to glucose 6-phosphate; this is converted successively to glucose 1-phosphate, uridine diphosphate glucose, and Ünally to glycogen. See also . Compare . glycogenolysis The conversion of glycogen to glucose, which occurs in the liver and is stimulated by glucagon from the pancreas and adrenaline from the adrenal medulla. These hormones activate an enzyme that phosphorylates glucose molecules in the glycogen chain to form glucose 1-phosphate, which is converted to glucose 6-phosphate. This is then converted to glucose by a phosphatase enzyme. In skeletal muscle glycogen is degraded to glucose 6-phosphate, which is then converted into pyruvate and used in ATP production during glycolysis and the Krebs cycle. However, pyruvate can also be converted, in the liver, to glucose; thus muscle glycogen is indirectly a source of blood glucose. Compare . glycol See -,-. glycolipid See . glycolysis (Embden–Meyerhof pathway) The series of biochemical reactions in which
(a hexose: 6 carbon atoms)
ATP phosphorylation ATP fructose 1,6–diphosphate
glyceraldehyde 3–phosphate ATP glycerate 3–phosphate
glyceraldehyde 3–phosphate (a triose phosphate: 3 carbon atoms) ATP glycerate 3–phosphate
ATP pyruvate
Glycolysis. The principal stages.
glucose is broken down to pyruvate with the release of usable energy in the form of *ATP (see illustration). One molecule of glucose undergoes two phosphorylation reactions and is then split to form two triosephosphate molecules. Each of these is converted to pyruvate. The net energy yield is two ATP molecules per glucose molecule. In *aerobic respiration pyruvate then enters the *Krebs cycle. Alternatively, when oxygen is in short supply or absent, the pyruvate is converted to various products by *anaerobic respiration. Other simple sugars, e.g. fructose and galactose, and glycerol (from fats) enter the glycolysis pathway at intermediate stages. Compare .
glycoprotein A carbohydrate linked covalently to a protein. Formed in the Golgi apparatus in the process of *glycosylation, glycoproteins are important components of plasma membranes, in which they extend throughout the *lipid bilayer. They are also constituents of body Ûuids, such as mucus, that are involved in lubrication. Many of the hormone receptors on the surfaces of cells are glycoproteins. Glycoproteins produced by viruses attach themselves to the surface of the host cell, where they act as markers for the receptors of leucocytes. Viral glycoproteins can also act as target molecules and help viruses to detect certain types of host cell; for example, a glycoprotein on the surface of *HIV enables the virus to Ünd and infect white blood cells. glycosaminoglycan Any one of a group of polysaccharides that contain *amino sugars (such as glucosamine). Formerly known as mucopolysaccharides, they include *hyaluronic acid and chondroitin (see ), which provide lubrication in joints and form part of the matrix of cartilage. The three-dimensional structure of these molecules enables them to trap water, which CH2OH
glycosidic bond (glycosidic link) The type of chemical linkage between the monosaccharide units of disaccharides, oligosaccharides, and polysaccharides, which is formed by the removal of a molecule of water (i.e. a *condensation reaction). The bond is normally formed between the carbon-1 on one sugar and the carbon-4 on the other (see illustration). An α-glycosidic bond is formed when the –OH group on carbon-1 is below the plane of the glucose ring and a β-glycosidic bond is formed when it is above the plane. Cellulose is formed of glucose molecules linked by 1-4 β-glycosidic bonds, whereas starch is composed of 1-4 α-glycosidic bonds. glycosylation The process in which a carbohydrate is joined to another molecule, such as a protein to form a *glycoprotein or to a lipid to form a glycolipid (see ). Glycosylation occurs in the rough endoplasmic reticulum and the *Golgi apparatus of cells. glyphosate N-(phosphonomethyl)glycine: a herbicide, marketed as Roundup, that kills a wide range of plants but shows little persistence in soil and has low toxicity to animals. If applied to the leaves it is rapidly translocated to the rest of the plant, CH2OH
OH OH OH glucose
Glycosidic bond formation.
CH2OH O
OH glucose
glycoside Any one of a group of compounds consisting of a pyranose sugar residue, such as glucose, linked to a noncarbohydrate residue (R) by a *glycosidic bond: the hydroxyl group (–OH) on carbon-1 of the sugar is replaced by –OR. Glycosides are widely distributed in plants; examples are the *anthocyanin pigments and the cardiac glycosides, such as digoxin and ouabain, which are used medicinally for their stimulant effects on the heart.
O condensation
forms a gel and gives glycosaminoglycans their elastic properties.
+ H2O
1–4 α-glycosidic bond maltose
OH OH water
Gnathostomata and hence can penetrate the roots of even hardy perennials. It works by blocking the synthesis of aromatic amino acids, so that treated plants are unable to manufacture proteins and other key metabolites. Certain crops, notably soya bean, have been genetically engineered to give them resistance to glyphosate. These 'Roundup-ready' crops, which can be sprayed with the herbicide without being affected, are now widely grown in North America and elsewhere.
Gnathostomata A subphylum or superclass of chordates consisting of all vertebrates that possess jaws. It contains six extant classes: *Chondrichthyes (cartilaginous Üshes), *Osteichthyes (bony Üshes), *Amphibia, *Reptilia, *Aves (birds), and *Mammalia. See also C. gneiss A coarse-grained rock that is characterized by compositional banding of metamorphic origin. It consists chieÛy of irregular granular bands of quartz and feldspar alternating with thin undulating bands of micas and amphiboles. Gneisses are formed during high-grade regional metamorphism; those derived from sedimentary origins are known as paragneisses and those from igneous origins as orthogneisses. gnotobiotic Designating germ-free conditions, especially those in which experimental animals are inoculated with known strains of microorganisms. goblet cell A goblet-shaped cell, found in the epithelium of the intestine and respiratory system in mammals and in the epidermis of Üsh, that secretes *mucus. Goblet cells have a wide top and constricted base and possess glycoprotein-containing vesicles. Gödel's theorem A fundamental result in mathematics stating that, in a given mathematical structure, some propositions cannot be proved to be true or false, i.e. the propositions are undecidable, using only the axioms of that mathematical structure. The theorem was proved by the Austrian mathematician Kurt Gödel (1906–78) in 1931. The relevance of this result to fundamental physics has been extensively debated. goethite A yellow-brown mineral, FeO.OH, crystallizing in the orthorhombic system. It is formed as a result of the oxidation and hydration of iron minerals or as a direct precipitate from marine or fresh water (e.g. in swamps and bogs). Most *limonite is
composed largely of cryptocrystalline goethite. Goethite is mined as an ore of iron.
gold Symbol Au. A soft yellow malleable metallic *transition element; a.n. 79; r.a.m. 196.967; r.d. 19.32; m.p. 1064.43°C; b.p. 2807±2°C. Gold has a face-centred-cubic crystal structure. It is found as the free metal in gravel or in quartz veins, and is also present in some lead and copper sulphide ores. It also occurs combined with silver in the telluride sylvanite, (Ag,Au)Te2. It is used in jewellery, dentistry, and electronic devices. Chemically, it is unreactive, being unaffected by oxygen. It reacts with chlorine at 200°C to form gold(III) chloride. It forms a number of complexes with gold in the +1 and +3 oxidation states.
Gold, Thomas See H, S F. golden ratio A visually satisfying proportion found by dividing a line in such a way that the ratio of the smaller part to the larger part is the same as that of the larger part to the whole. It is equal to about 8:13 or about 1:1.618034, and can be expressed exactly as 1:½(1 + √5). See also F . Goldschmidt process A method of extracting metals by reducing the oxide with aluminium powder, e.g. Cr2O3 + 2Al → 2Cr + Al2O3 The reaction can also be used to produce molten iron (see ). It was discovered by the German chemist Hans Goldschmidt (1861–1923). Goldstone's theorem The theorem in relativistic quantum Üeld theory that if there is an exact continuous symmetry of the *Hamiltonian or *Lagrangian deÜning the system, and this is not a symmetry of the *vacuum state (i.e. there is *broken symmetry), then there must be at least one spinzero massless particle called a Goldstone boson. In the quantum theory of many-body systems Goldstone bosons are *collective excitations. An important exception to Goldstone's theorem is provided in *gauge theories with the Higgs mechanism, whereby the Goldstone bosons gain mass and become *Higgs bosons. The theorem is named after Jeffrey Goldstone. Golgi, Camillo (1843–1926) Italian cytologist, who experimented with cells and tissues while working as a physician. He later be-
came a professor at Pavia University. He devised a method of staining cells using silver salts, which enabled him to study nerve cells. He is best known for his discovery of the cell organelle now called the *Golgi apparatus. For his work on the structure of the nervous system he shared the 1906 Nobel Prize for physiology or medicine with Santiago Ramón y Cajal (1852–1934).
Golgi apparatus (Golgi complex) An assembly of vesicles and folded membranes within the cytoplasm of eukaryotic *cells that modiÜes proteins and packages them and other materials (e.g. polysaccharides) for delivery to the plasma membrane for secretion or to destinations within the cell. Proteins arrive in vesicles following their assembly in the *endoplasmic reticulum; after processing in the Golgi apparatus, they are sorted into Golgi vesicles, for secretion, storage, or transport to lysosomes. The apparatus is named after its discoverer, Camillo Golgi. gonad Any of the usually paired organs in animals that produce reproductive cells (gametes). The most important gonads are the male *testis, which produces spermatozoa, and the female *ovary, which produces ova (egg cells). The gonads also produce hormones that control secondary sexual characteristics. gonadotrophin (gonadotrophic hormone) Any of several hormones, secreted by the mammalian anterior *pituitary gland, that stimulate reproductive activity of the testes or ovaries (the gonads). Pituitary gonadotrophins include *follicle-stimulating hormone and *luteinizing hormone. Chorionic gonadotrophin is a hormone produced by the placenta of higher mammals that maintains the *corpus luteum. The presence of large amounts of human chorionic gonadotrophin (hCG) in the urine of women is an indication of pregnancy. Gondwanaland See  . Gooch crucible A porcelain dish with a perforated base over which a layer of asbestos is placed, used for Ültration in gravimetric analysis. It is named after US chemist Frank Gooch (1852–1929). Gopher A computer program used on a computer connected to the *Internet that carries out routine tasks of collecting information for the user from services attached to the Internet. Gopher presents the user with a directory of material accessible at a particu-
G protein lar point, a set of documents that can be searched using keywords, or a document containing text or other forms of material that the computer can display. Gopher is simpler but rather less Ûexible than the *World Wide Web.
Gouy balance A method of measuring magnetic susceptibility. The sample is suspended from a balance, with the bottom part of the sample between the poles of an electromagnet. When the magnetic Üeld is switched on, the sample experiences a Üeld gradient which causes an apparent change in weight. In particular, paramagnetic substances show an increase in weight, which, after correction for a smaller diamagnetic contribution, can be used to calculate the paramagnetic part of the susceptibility. This can be used to calculate the number of unimpaired electrons in the sample. Magnetic measurements of this type are widely used to investigate the electronic structures of metal complexes. The Evans balance is a portable version of the Gouy balance using permanent magnets and giving a direct readout. Other methods of measuring magnetic susceptibility include magnetic resonance techniques and the use of a SQID (superconducting quantum interference device). The Gouy balance is named after the French scientist Louis Georges Gouy (1854–1926). governor A device that maintains a motor or engine at a constant speed despite variations in the load, using the principle of negative feedback. A common method uses a set of Ûying balls that reduce the fuel intake as the speed increases. The balls, attached by Ûexible steel strips to a collar capable of moving vertically up and down a rotating shaft, move outwards as the speed increases. The collar rises as the balls Ûy out and is coupled to a lever that controls the fuel intake. G protein Any one of a group of proteins that relay signals in mammalian cells. They occur on the inner surface of the plasma membrane and transmit signals from receptors on the outer surface of the cell to intracellular components. G proteins are activated by binding to GTP and become inactive when they bind to GDP (see ). The cholera toxin exerts its effects by changing the G protein in the epithelial cells of the intestine so that it is continually activated, which causes an abnormal increase in cellular adenylate cyclase levels. One consequence of this is that sodium ions are ac-
G-protein-coupled receptor tively pumped into the intestine, causing water to follow by osmosis: the result is diarrhoea and dehydration.
G-protein-coupled receptor (GPCR) Any of a superfamily of proteins that are located in cell membranes and act as receptors by relaying signals from the exterior to the interior of the cell via associated G proteins. The latter then activate second messengers, which regulate the activity of the cell. A wide range of substances, including peptide hormones, acetylcholine, noradrenaline, gamma-aminobutyric acid, and glutamate, can bind as ligands to the various types of GPCR, at binding sites in the extracellular region of the receptor molecule. Ligand binding changes the shape of the receptor allowing the G protein to bind to it, which activates the G protein by causing GTP to replace bound GDP. GPS See   . GraaÜan follicle (ovarian follicle) The Ûuid-Ülled cavity that surrounds and protects the developing egg cell in the ovary of a mammal. After the release of the ovum it develops into a *corpus luteum. It is named after the Dutch anatomist Reinier de Graaf (1641–73). graben A long elongated block of rock or crust that sinks between a pair of parallel near-vertical faults, when they move slightly outwards. The higher bordering areas are called horsts. grad See  .
of joining the tissues.) Grafting of plant tissues is a horticultural practice used to propagate plants, especially certain bushes and fruit trees, artiÜcially. A shoot or bud of the desired variety (the scion) is grafted onto a rootstock of either a common or a wild related species (the stock). The scion retains its desirable characteristics (e.g. Ûower form or fruit yield) and supplies the stock with food made by photosynthesis. The stock supplies the scion with water and mineral salts and affects only the size and vigour of the scion. Animal and human grafts are used to replace faulty or damaged parts of the body. An autograft is taken from one part of the body and transferred to another part of the same individual, e.g. a skin graft used for severe burns. An allograft (homograft) is taken from one individual (the donor) and implanted in another of the same species (the recipient), the process being known as transplantation, e.g. a heart or kidney transplant. In such cases the graft may be regarded by the body as foreign (a state of incompatibility): an *immune response follows and the graft is rejected (see also ).
graft copolymer See . graft hybrid A type of plant *chimaera that may be produced when a part of one plant (the scion) is grafted onto another plant of a different genetic constitution (the stock). Shoots growing from the point of union of the graft contain tissues from both the stock and the scion.
gradient 1. The slope of a line. In Cartesian coordinates, a straight line y = mx + c, has a gradient m. For a curve, y = f(x), the gradient at a point is the derivative dy/dx at that point, i.e. the slope of the tangent to the curve at that point. 2. See  .
Graham, Thomas (1805–69) Scottish chemist, who became professor of chemistry at Glasgow University in 1830, moving to University College, London, in 1837. His 1829 paper on gaseous diffusion introduced *Graham's law. He went on to study diffusion in liquids, leading in 1861 to the deÜnition of *colloids.
gradient operator (grad) The *operator ∇ = i ∂/∂x + j ∂/∂y + k ∂/∂z, where i, j, and k are unit vectors in the x, y, and z directions. Given a scalar function f and a unit vector n, the *scalar product n.∇f is the rate of change of f in the direction of n. See also ; .
Graham's law The rates at which gases diffuse is inversely proportional to the square roots of their densities. This principle is made use of in the diffusion method of separating isotopes. The law was formulated in 1829 by Thomas Graham.
graft An isolated portion of living tissue that is joined to another tissue, either in the same or a different organism, the consequent growth resulting in fusion of the tissues. (The word is also used for the process
gram Symbol g. One thousandth of a kilogram. The gram is the fundamental unit of mass in *c.g.s. units and was formerly used in such units as the gram-atom, grammolecule, and gram-equivalent, which have now been replaced by the *mole.
Gram's stain A staining method used to differentiate bacteria. The bacterial sample is smeared on a microscope slide, stained with a violet dye, treated with acetonealcohol (a decolourizer), and Ünally counterstained with a red dye. Gram-positive bacteria retain the Ürst dye, appearing blueblack under the microscope. In Gramnegative bacteria, the acetone-alcohol washes out the violet dye and the counterstain is taken up, the cells appearing red. It is named after the Danish bacteriologist Hans Gram (1853–1938), who Ürst described the technique (since modiÜed) in 1884. grand uniÜed theory (GUT) A theory that attempts to combine the strong, weak, and electromagnetic interactions into a single *gauge theory with a single symmetry group. There are a number of different theories, most of which postulate that the interactions merge at high energies into a single interaction (the standard model emerges from the GUT as a result of *broken symmetry). The energy above which the interactions are the same is around 1015 GeV, which is much higher than those obtainable with existing accelerators. One prediction of GUTs is the occurrence of *proton decay. Some also predict that the neutrino has nonzero mass. There is no evidence for proton decay at present, although there is some evidence that neutrinos have very small nonzero masses. See also  . granite An extremely hard light-coloured acid igneous rock consisting mainly of plagioclase feldspar and quartz (average 25%), with some biotite (mica), hornblende, or other coloured mineral. It generally results from the slow solidiÜcation of molten *magma, giving it a coarse grain size. Some granites result from metamorphism of preexisting rocks. granulocyte Any white blood cell (see ) that contains granular material (secretory vessels) and *lysosomes in its cytoplasm. *Neutrophils and *basophils are examples of granulocytes. Compare . granum (pl. grana) A stack of platelike bodies (thylakoids), many of which are found in plant *chloroplasts (each chloroplast contains about 50 grana). Grana bear the light-receptive pigment chlorophyll and contain the enzymes responsible for the
light-dependent reactions of *photosynthesis.
grape sugar See . graph A diagram that illustrates the relationship between two variables. It usually consists of two perpendicular axes, calibrated in the units of the variables and crossing at a point called the origin. Points are plotted in the spaces between the axes and the points are joined to form a curve. See also C ;  . graphene A single sheet of carbon atoms arranged in hexagons. Graphene can be regarded as a single plane of graphite and is of interest because it has some unusual and potentially useful electrical properties. graphite See . graphite-moderated reactor See  . graph theory The area of mathematics that deals with *graphs and their properties. It has important applications in *topology and in the construction of certain types of *algorithms, as used in some computer programs. graptolites A group of extinct marine colonial animals that were common in the Palaeozoic era. Graptolites are generally regarded as being related to colonial softbodied marine invertebrates called pterobranchs. They had chitinous outer skeletons in the form of simple or branched stems, the individual polyps occupying minute cups (thecae) along these stems. Fossils of these skeletons are found in Palaeozoic rocks of all continents; they are particularly abundant in Ordovician and Silurian rock strata, for which they are used as *index fossils. At the end of the Silurian many graptolites became extinct but a few groups continued into the early Carboniferous. grass-green bacteria See C. grassland A major terrestrial *biome in which the dominant plants are species of grass; the rainfall is insufÜcient to support extensive growth of trees, which are also suppressed by grazing animals. Tropical grassland (savanna), which covers much of Africa south of the Sahara, has widely spaced trees, such as acacias and baobabs, and supports large herds of grazing animals and
graticule their predators. Temperate grasslands, such as the steppes of Asia, the prairies of North America, and the pampas of South America, have few trees and are largely used for agriculture.
graticule (in optics) A network of Üne wires or a scale in the eyepiece of a telescope or microscope or on the stage of a microscope, or on the screen of a cathode-ray oscilloscope for measuring purposes. grating See  .
gravimetric analysis A type of quantitative analysis that depends on weighing. For instance, the amount of silver in a solution of silver salts could be measured by adding excess hydrochloric acid to precipitate silver chloride, Ültering the precipitate, washing, drying, and weighing. gravitation See N'   . gravitational collapse A phenomenon predicted by the general theory of *relativity in which matter collapses as a consequence of gravitational attraction until it becomes a compact object such as a *white dwarf, *neutron star, or *black hole. The type of object depends on the initial mass. The process of gravitational collapse is important in *astrophysics as it gives rise to such phenomena as *supernova explosions and gamma-ray bursts (see - ). gravitational constant Symbol G. The constant that appears in *Newton's law of gravitation; it has the value 6.672 59(85) × 10–11 N m2 kg–2. G is usually regarded as a universal constant although, in some models of the universe, it is proposed that it decreases with time as the universe expands. gravitational Üeld The region of space surrounding a body that has the property of *mass. In this region any other body that has mass will experience a force of attraction. The ratio of the force to the mass of the second body is the gravitational field strength. gravitational interaction See  . gravitational lens An object that deÛects light by gravitation as described by the general theory of *relativity; it is analogous to a lens in *optics. The prediction of a gravitational lensing effect in general relativity theory has been conÜrmed in observations on *quasars. In 1979 a 'double' quasar was
discovered, due to the multiple image of a single quasar caused by gravitational lensing by a galaxy, or cluster of galaxies, along the line of sight between the observer and the quasar. The images obtained by gravitational lensing can be used to obtain information about the mass distribution of the galaxy or cluster of galaxies.
gravitational mass See . gravitational shift See . gravitational waves 1. (in physics) Waves propagated through a *gravitational Üeld. The prediction that an accelerating mass will radiate gravitational waves (and lose energy) comes from the general theory of *relativity. Many attempts have been made to detect waves from space directly using large metal detectors. The theory suggests that a pulse of gravitational radiation (as from a supernova explosion or *black hole) causes the detector to vibrate, and the disturbance is detected by a transducer. The interaction is very weak and extreme care is required to avoid external disturbances and the effects of thermal noise in the detecting system. So far, no accepted direct observations have been made. However, indirect evidence of gravitational waves has come from observations of a pulsar in a binary system with another star. 2. (in oceanography) Water surface waves transmitted primarily because of the weight of the water in the crests, which causes them to collapse. Ocean waves are of this type.
• The home page for the Laser Interferometer Gravitational-Wave Observatory (LIGO)
graviton A hypothetical particle or quantum of energy exchanged in a gravitational interaction (see  ). Such a particle has not been observed but is postulated to make the gravitational interaction consistent with quantum mechanics. It would be expected to travel at the speed of light and have zero rest mass and charge, and spin 2. gravitropism See . gravity The phenomenon associated with the gravitational force acting on any object that has mass and is situated within the earth's *gravitational Üeld. The weight of a body (see ) is equal to the force of gravity acting on the body. According to Newton's second law of motion F = ma, where F
is the force producing an acceleration a on a body of mass m. The weight of a body is therefore equal to the product of its mass and the acceleration due to gravity (g), which is now called the *acceleration of free fall. By combining the second law of motion with *Newton's law of gravitation (F = GM1M2/d2) it follows that: g = GM/d2, where G is the *gravitational constant, M is the mass of the earth, and d is the distance of the body from the centre of the earth. For a body on the earth's surface g = 9.806 65 m s–2. A force of gravity also exists on other planets, moons, etc., but because it depends on the mass of the planet and its diameter, the strength of the force is not the same as it is on earth. If Fe is the force acting on a given mass on earth, the force Fp acting on the same mass on another planet will be given by: Fp = Fede2Mp/Medp2, where Mp and dp are the mass and diameter of the planet, respectively. Substituting values of Mp and dp for the moon shows that the force of gravity on the moon is only 1/6 of the value on earth.
• The home page of the NASA/JPL Gravity Recovery and Climate experiment to make precise measurements of the earth's gravitational field
gray Symbol Gy. The derived SI unit of absorbed *dose of ionizing radiation (see  ). It is named after the British radiobiologist L. H. Gray (1905–65). grazing The consumption of vegetation, usually on *grassland, by animals, particularly cattle and sheep. Overgrazing can lead to *desertiÜcation. grazing incidence telescope A type of astronomical telescope designed to work at X-ray and gamma-ray wavelengths. Its mirror consists of a paraboidal annulus of metal, which deÛects incoming photons striking it at an acute angle. The deÛected high-energy photons are detected by a spark chamber. Great Attractor A huge concentration of mass, equivalent to about a million galaxies, beyond the Hydra and Centaurus constellations. Our own Galaxy and others near to it are heading towards the Great Attractor at a rate of about 600 kilometres per second. great circle Any circle on a sphere formed by a plane that passes through the centre of the sphere. The equator and the meridians
greenhouse gas of longitude are all great circles on the earth's surface.
green algae See C. green Ûuorescent protein (GFP) A naturally Ûuorescent protein obtained from the jellyÜsh Aequorea victoria and used as a marker for identifying cells containing recombinant DNA or for localizing speciÜc proteins in cells. It absorbs blue light and emits a green Ûuorescence, and hence the abundance and location of GFP in cells can be visualized microscopically under ultraviolet light. Several colour variants of GFP are now available. greenhouse effect An effect occurring in the atmosphere because of the presence of certain gases (greenhouse gases) that absorb infrared radiation. Light and ultraviolet radiation from the sun are able to penetrate the atmosphere and warm the earth's surface. This energy is re-radiated as infrared radiation, which, because of its longer wavelength, is absorbed by such substances as carbon dioxide. The greenhouse effect is a natural phenomenon, without which the earth's climate would be much more hostile to life. However, emissions of carbon dioxide from human activities (e.g. farming, industry, and transport) have increased markedly in the last 150 years or so. The overall effect is that the average temperature of the earth and its atmosphere is increasing (see  ). The effect is similar to that occurring in a greenhouse, where light and longwavelength ultraviolet radiation can pass through the glass into the greenhouse but the infrared radiation is absorbed by the glass and part of it is re-radiated into the greenhouse. The greenhouse effect is seen as a major environmental hazard. Average increases in temperature are likely to change weather patterns and agricultural output. It is already causing the polar ice caps to melt, with a corresponding rise in sea level. Carbon dioxide, from coal-Üred power stations and car exhausts, is the main greenhouse gas. Other contributory pollutants are nitrogen oxides, ozone, methane, and *chloroÛuorocarbons. Many countries have now agreed targets to limit emissions of greenhouse gases, e.g. by switching to renewable energy sources. See also . greenhouse gas See  .
greenockite greenockite A mineral form of cadmium sulphide, CdS.
coverer, the French chemist Victor Grignard (1871–1935).
Green's function One of a set of functions that are used for solving *differential equations with *boundary conditions. It was named after the British mathematician George Green (1793–1841).
grooming The actions of an animal of rearranging fur or feathers and cleaning the body surface by biting, scratching, licking, etc., which is important for removing parasites and spreading oils over the body surface. In many mammals, especially primates, grooming between individuals (allogrooming) has an important role in maintaining social cohesion.
green vitriol See (II) . Gregorian telescope See . Greisen–Zatsepin–Kuzmin limit See GZK .
grey matter Part of the tissue that makes up the central nervous system of vertebrates. It is brown-grey in colour, consisting largely of nerve *cell bodies, *synapses, and *dendrites. The grey matter is the site of coordination between nerves of the central nervous system. Compare  . grid 1. (in electricity) The system of overhead wires or underground cables by which electrical power is distributed from power stations to users. The grid is at a high voltage, up to 750 kV in some countries. 2. (in electronics) See  . 3. (in cartography) A network of horizontal and vertical lines on a map that provide a means of locating a speciÜc point. Griess test A test for nitrates or nitrites, once widely used to detect the possible existence of gunshot residue. Grignard reagents A class of organometallic compounds of magnesium, with the general formula RMgX, where R is an organic group and X a halogen atom (e.g. CH3MgCl, C2H5MgBr, etc.). They actually have the structure R2Mg.MgCl2, and can be made by reacting a haloalkane with magnesium in ether; they are rarely isolated but are extensively used in organic synthesis, when they are made in one reaction mixture. Grignard reagents have a number of reactions that make them useful in organic synthesis. With methanal they give a primary alcohol CH3MgCl + HCHO → CH3CH2OH Other aldehydes give a secondary alcohol CH3CHO + CH3MgCl → (CH3)2CHOH With alcohols, hydrocarbons are formed CH3MgCl + C2H5OH → C2H5CH3 Water also gives a hydrocarbon CH3MgCl + H2O → CH4 The compounds are named after their dis-
ground state The lowest stable energy state of a system, such as a molecule, atom, or nucleus. See  . ground substance The matrix of connective tissue, in which various cells and Übres are embedded. The ground substance of cartilage consists of *chondrin. See  . ground tissues All the plant tissues formed by the *apical meristems except the epidermis and vascular tissue. The principal ground tissues are the *cortex, *pith, and primary *medullary rays, and they consist chieÛy of *parenchyma. Collenchyma is a form of ground tissue less frequently observed. It consists of living cells with additional cellulose thickening in the walls, giving them additional strength, and is most commonly found in the stem cortex. groundwater 1. (subterranean water) Water that occurs below the surface of the ground, as opposed to that at the surface. 2. (phreatic water) Water that occurs below the surface in soil or rocks that are saturated, either in cavities and pores (through which it can Ûow) or below the water table. Groundwater has an erosive action in permeable rocks, such as limestone, in which it can form underground rivers and caves. ground wave A radio wave that travels in approximately a straight line between points on the earth's surface. For transmission over longer distances sky waves have to be involved. See  . group 1. (in physics) A set of elements A, B, C, etc., for which there exists a law of composition, referred to as 'multiplication'. Any two elements can be combined to give a 'product' AB. (1) Every product of two elements is an element of the set.
(2) The operation is associative, i.e. A(BC) = (AB)C. (3) The set has an element I, called the identity element, such that IA = AI = A for all A in the set. (4) Each element of the set has an inverse A–1 belonging to the set such that AA–1 = A–1A = I. Although the law of combination is called 'multiplication' this does not necessarily have its usual meaning. For example, the set of integers forms a group if the law of composition is addition. Two elements A, B of a group commute if AB = BA. If all the elements of a group commute with each other the group is said to be Abelian. If this is not the case the group is said to be non-Abelian. The distinction between Abelian and non-Abelian groups is of fundamental importance in *gauge theories. The interest of group theory in physics and chemistry is in analysing symmetry. Discrete groups have a Ünite number of elements, such as the symmetries involved in rotations and reÛections of molecules, which give rise to point groups. Continuous groups have an inÜnite number of elements where the elements are continuous. An example of a continuous group is the set of rotations about a Üxed axis. The rotation group thus formed underlies the *quantum theory of *angular momentum, which has many applications to *atoms and *nuclei. More abstract and more general continuous groups describe fundamental interactions by gauge theories. 2. (in chemistry) See  .
group 0 elements See  . group 1 elements A group of elements in the *periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and Francium (Fr). They are known as the *alkali metals. Formerly, they were classiÜed in group I, which consisted of two subgroups: group IA (the main group) and group IB. Group IB consisted of the *coinage metals, copper, silver, and gold, which comprise group 11 and are usually considered with the *transition elements. group 2 elements A group of elements in the *periodic table: beryllium (Be), magnesium (Hg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). They are known as the *alkaline-earth metals. Formerly, they were classiÜed in group II, which consisted of two subgroups: group IIA (the main group, see - ) and
group 13 elements group IIB. Group IIB consisted of the three metals zinc (Zn), cadmium (Cd), and mercury (Hg), which have two s-electrons outside Ülled d-subshells. Moreover, none of their compounds have unÜlled d-levels, and the metals are regarded as nontransition elements. They now form group 12 and are sometimes called the zinc group. Zinc and cadmium are relatively electropositive metals, forming compounds containing divalent ions Zn2+ or Cd2+. Mercury is more unreactive and also unusual in forming mercury(I) compounds, which contain the ion Hg22+.
groups 3–12 See  . group 13 elements A group of elements in the *periodic table: boron (B), aluminium (Al), gallium (Ga), indium (In), and thallium (Tl), which all have outer electronic conÜgurations ns2np1 with no partly Ülled inner levels. They are the Ürst members of the p-block. The group differs from the alkali metals and alkaline-earth metals in displaying a considerable variation in properties as the group is descended. Formerly, they were classiÜed in group III, which consisted of two subgroups: group IIIB (the main group) and group IIIA. Group IIIA consisted of scandium (Sc), yttrium (Yt), and lanthanum (La), which are generally considered with the *lanthanoids, and actinium (Ac), which is classiÜed with the *actinoids. Scandium and yttrium now belong to group 3 (along with lutetium and lawrencium). Boron has a small atomic radius and a relatively high ionization energy. In consequence its chemistry is largely covalent and it is generally classed as a metalloid. It forms a large number of volatile hydrides, some of which have the uncommon bonding characteristic of *electron-deÜcient compounds. It also forms a weakly acidic oxide. In some ways, boron resembles silicon (see  ). As the group is descended, atomic radii increase and ionization energies are all lower than for boron. There is an increase in polar interactions and the formation of distinct M3+ ions. This increase in metallic character is clearly illustrated by the increasing basic character of the hydroxides: boron hydroxide is acidic, aluminium and gallium hydroxides are amphoteric, indium hydroxide is basic, and thallium forms only the oxide. As the elements of group 13 have a vacant porbital they display many electron-acceptor properties. For example, many boron compounds form adducts with donors such as
group 14 elements
ammonia and organic amines (acting as Lewis acids). A large number of complexes of the type [BF4]–, [AlCl4]–, [InCl4]–, [TlI4]– are known and the heavier members can expand their coordination numbers to six as in [AlF6]3– and [TlCl6]3–. This acceptor property is also seen in bridged dimers of the type Al2Cl6. Another feature of group 13 is the increasing stability of the monovalent state down the group. The electron conÜguration ns2np1 suggests that only one electron could be lost or shared in forming compounds. In fact, for the lighter members of the group the energy required to promote an electron from the s-subshell to a vacant p-subshell is small. It is more than compensated for by the resulting energy gain in forming three bonds rather than one. This energy gain is less important for the heavier members of the group. Thus, aluminium forms compounds of the type AlCl in the gas phase at high temperatures. Gallium similarly forms such compounds and gallium(I) oxide (Ga2O) can be isolated. Indium has a number of known indium(I) compounds (e.g. InCl, In2O, In3I[InIIICl6]). Thallium has stable monovalent compounds. In aqueous solution, thallium(I) compounds are more stable than the corresponding thallium(III) compounds. See - .
group 14 elements A group of elements in the *periodic table: carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb), which all have outer electronic conÜgurations ns2np2 with no partly Ülled inner levels. Formerly, they were classiÜed in group IV, which consisted of two subgroups: IVB (the main group) and group IVA. Group IVA consisted of titanium (Ti), zirconium (Zr), and hafnium (Hf), which now form group 4 and are generally considered with the *transition elements. The main valency of the elements is 4, and the members of the group show a variation from nonmetallic to metallic behaviour in moving down the group. Thus, carbon is a nonmetal and forms an acidic oxide (CO2) and a neutral oxide. Carbon compounds are mostly covalent. One allotrope (diamond) is an insulator, although graphite is a fairly good conductor. Silicon and germanium are metalloids, having semiconductor properties. Tin is a metal, but does have a nonmetallic allotrope (grey tin). Lead is deÜnitely a metal. Another feature of the group is the tendency to form divalent compounds as the size of the atom increases.
Thus carbon has only the highly reactive carbenes. Silicon forms analogous silylenes. Germanium has an unstable hydroxide (Ge(OH)2), a sulphide (GeS), and halides. The sulphide and halides disproportionate to germanium and the germanium(IV) compound. Tin has a number of tin(II) compounds, which are moderately reducing, being oxidized to the tin(IV) compound. Lead has a stable lead(II) state. See  . In general, the reactivity of the elements increases down the group from carbon to lead. All react with oxygen on heating. The Ürst four form the dioxide; lead forms the monoxide (i.e. lead(II) oxide, PbO). Similarly, all will react with chlorine to form the tetrachloride (in the case of the Ürst four) or the dichloride (for lead). Carbon is the only one capable of reacting directly with hydrogen. The hydrides all exist from the stable methane (CH4) to the unstable plumbane (PbH4).
group 15 elements A group of elements in the *periodic table: nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi), which all have outer electronic conÜgurations ns2np3 with no partly Ülled inner levels. Formerly, they were classiÜed in group V, which consisted of two subgroups: group VB (the main group) and group VA. Group VA consisted of vanadium (V), niobium (Nb), and tantalum (Ta), which are generally considered with the *transition elements: The lighter elements (N and P) are nonmetals; the heavier elements are metalloids. The lighter elements are electronegative in character and have fairly large ionization energies. Nitrogen has a valency of 3 and tends to form covalent compounds. The other elements have available d-sublevels and can promote an s-electron into one of these to form compounds with the V oxidation state. Thus, they have two oxides P2O3, P2O5, Sb2O3, Sb2O5, etc. In the case of bismuth, the pentoxide Bi2O5 is difÜcult to prepare and unstable – an example of the increasing stability of the III oxidation state in going from phosphorus to bismuth. The oxides also show how there is increasing metallic (electropositive) character down the group. Nitrogen and phosphorus have oxides that are either neutral (N2O, NO) or acidic. Bismuth trioxide (Bi2O3) is basic. Bismuth is the only member of the group that forms a wellcharacterized positive ion Bi3+.
group 16 elements A group of elements in the *periodic table: oxygen (O), sulphur (S), selenium (Se), tellurium (Te), and polonium (Po), which all have outer electronic conÜgurations ns2np4 with no partly Ülled inner levels. They are also called the chalcogens. Formerly, they were classiÜed in group VI, which consisted of two subgroups: group VIB (the main group) and group VIA. Group VIA consisted of chromium (Cr), molybdenum (Mo), and tungsten (W), which now form group 6 are generally classiÜed with the *transition elements. The conÜgurations are just two electrons short of the conÜguration of a noble gas and the elements are characteristically electronegative and almost entirely nonmetallic. Ionization energies are high, (O 1314 to Po 813 kJ mol–1) and monatomic cations are not known. Polyatomic cations do exist, e.g. O2+, S82+, Se82+, Te42+. Electronegativity decreases down the group but the nearest approach to metallic character is the occurrence of 'metallic' allotropes of selenium, tellurium, and polonium along with some metalloid properties, in particular, marked photoconductivity. The elements of group 16 combine with a wide range of other elements and the bonding is largely covalent. The elements all form hydrides of the type XH2. Apart from water, these materials are all toxic foulsmelling gases; they show decreasing thermal stability with increasing relative atomic mass of X. The hydrides dissolve in water to give very weak acids (acidity increases down the group). Oxygen forms the additional hydride H2O2 (hydrogen peroxide), but sulphur forms a range of sulphanes, such as H2S2, H2S4, H2S6. Oxygen forms the Ûuorides O2F2 and OF2, both powerful Ûuorinating agents; sulphur forms analogous Ûuorides along with some higher Ûuorides, S2F2, SF2, SF4, SF6, S2F10. Selenium and tellurium form only the higher Ûuorides MF4 and MF6; this is in contrast to the formation of lower valence states by heavier elements observed in groups 13, 14, and 15. The chlorides are limited to M2Cl2 and MCl4; the bromides are similar except that sulphur only forms S2Br2. All metallic elements form oxides and sulphides and many form selenides. group 17 elements A group of elements in the *periodic table: Ûuorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They are known as the *halogens. Formerly, they were classiÜed in group VII,
growth hormone which consisted of two subgroups: group VIIB (the main group) and group VIIA. Group VIIA consisted of the elements manganese (Mn), technetium (Te), and rhenium (Re), which now form group 7 and are usually considered with the transition elements.
group 18 elements A group of elements in the *periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Formerly classiÜed as group 0 elements, they are usually referred to as the *noble gases. Grover's algorithm An algorithm, invented in 1996 by the Indian–American computer scientist Lov Grover (1961– ), for searching an unsorted database on a quantum computer. It is a more efÜcient method of searching than a traditional algorithm, i.e. one for a conventional computer. See also S' . growth An increase in the dry weight or volume of an organism through cell division and cell enlargement. Growth may continue throughout the life of the organism, as occurs in woody plants, or it may cease at maturity, as in humans and other mammals. See also  ;  . growth factor Any of various chemicals, particularly polypeptides, that have a variety of important roles in the stimulation of new cell growth and cell maintenance. They bind to the cell surface on receptors. SpeciÜc growth factors can cause new cell proliferation (epidermal growth factor, insulin-like growth factor (IGF), haemopoietic growth factor – see  ) and the migration of cells (Übroblast growth factor) and play a role in wound healing (plateletderived growth factor; PDGF). Some growth factors act in the embryonic stage of development; for example, nerve growth factor (see ) stimulates the growth of axons and dendrites from developing sensory and sympathetic neurons. Some growth factors or their receptors are involved in the abnormal regulation of growth seen in cancer when produced in excessive amounts or permanently activated. growth hormone (GH; somatotrophin) A hormone, secreted by the mammalian pituitary gland, that stimulates protein synthesis and growth of the long bones in the legs and arms. It also promotes the breakdown and use of fats as an energy source, rather than
growth ring
glucose. Production of growth hormone is greatest during early life. Its secretion is controlled by the opposing actions of two hormones from the hypothalamus: growth hormone releasing hormone (somatoliberin), which promotes its release; and *somatostatin, which inhibits it. Overproduction of human growth hormone (hGH) results in gigantism in childhood and *acromegaly in adults; underproduction results in dwarÜsm. Bovine somatotrophin (BST) has been used to increase milk and meat production in cattle.
GTP (guanosine triphosphate) See .
growth ring (annual ring) Any of the rings that can be seen in a cross-section of a woody stem (e.g. a tree trunk). It represents the *xylem formed in one year as a result of Ûuctuating activity of the vascular *cambium. In temperate climates pale soft spring wood, characterized by large xylem vessels, is formed in spring and early summer. Growth slows down in late summer and a darker dense autumn wood with smaller xylem vessels is formed (see illustration). The age of a tree can be determined by counting the rings. Under certain circumstances two or more growth rings may form in one year, giving rise to false annual rings.
guanosine A nucleoside consisting of one guanine molecule linked to a D-ribose sugar molecule. The derived nucleotides, guanosine mono-, di-, and triphosphate (GMP, GDP, and GTP, respectively), participate in various metabolic reactions.
growth substance See  .
gum 1. Any of a variety of substances obtained from plants. Typically they are insoluble in organic solvents but form gelatinous or sticky solutions with water. Most gums are complex polysaccharides. Commercially im-
GSC (gas–solid chromatography) See  . G-series See  .
guanidine A crystalline basic compound HN:C(NH2)2, related to urea. guanine A *purine derivative. It is one of the major component bases of *nucleotides and the nucleic acids *DNA and *RNA. guano An accumulation of the droppings of birds, bats, or seals, usually formed by a long-established colony of animals. It is rich in plant nutrients, and some deposits are extracted for use as fertilizer.
guard cell See . guild (in ecology) A group of species within a community that exploit the same resources in a similar way. Thus, for example, different species of snakes and lizards may belong to a guild by virtue of occupying the same types of underground shelters, and different species of seed-eating birds may constitute a guild. gullet See .
vascular cambium
spring wood
growth ring year 3
cortex pith
medullary ray
cork autumn wood cork cambium spring wood
Growth ring. Transverse section through a three-year-old woody stem to show the growth rings.
portant examples are gum arabic and gum tragacanth. Gum arabic (or gum acacia) is obtained from various acacia trees; it is used in the manufacture of confectionery, cosmetics, linctuses and other medicinal products, and gummed labels. Gum tragacanth, extracted from trees of the genus Astragalus, forms a thick *mucilage in water; it is used in the manufacture of pills and confectionery and as a sauce thickener. Gum resins are mixtures of gums and natural resins. Gums are produced by the young xylem vessels of some plants (mainly trees) in response to wounding or pruning. The exudate hardens when it reaches the plant surface and thus provides a temporary protective seal while the cells below divide to form a permanent repair. Excessive gum formation is a symptom of some plant diseases. 2. See .
guncotton See  . gun metal A type of bronze usually containing 88–90% copper, 8–10% tin, and 2–4% zinc. Admiralty gunmetal, which is used in shipbuilding, contains 88% copper, 10% tin, and 2% zinc. Because it was easy to cast it was originally used to make cannons; it is still used for bearings and other parts that require high resistance to wear and corrosion. gunpowder An explosive consisting of a mixture of potassium nitrate, sulphur, and charcoal. It was invented by the Chinese, probably in the 10th century, although the English monk Roger Bacon (1214–92) is often credited with its discovery. For many centuries it was the explosive used in Ürearms; it is no longer used for this purpose, although it is the basis of many Üreworks. gut See  . GUT See   . guttation See . guyot (tablemount) A Ûat-topped circular underwater mountain. Guyots occur in deep water (usually below 2000 m) and are thought to be conical volcanic peaks levelled off by the action of water currents. They consist mainly of *basalt and tend to occur in groups, similar to island arcs. GWS model Glashow–Weinberg–Salam model. See  . gymnosperm Any plant whose ovules and the seeds into which they develop are borne
gyroscope unprotected, rather than enclosed in ovaries, as are those of the Ûowering plants (the term gymnosperm means naked seed). In traditional systems of classiÜcation such plants were classiÜed as the Gymnospermae, a class of the Spermatophyta, but they are now divided into separate phyla: *Coniferophyta (conifers), *Cycadophyta (cycads), Ginkgophyta (ginkgo), and Gnetophyta (e.g. Welwitschia).
gynoecium (gynaecium) The female sex organs (*carpels) of a Ûower. Compare . gypsum A monoclinic mineral form of hydrated *calcium sulphate, CaSO4.2H2O. It occurs in Üve varieties: rock gypsum, which is often red stained and granular; gypsite, an impure earthy form occurring as a surface deposit; alabaster, a pure Üne-grained translucent form; satin spar, which is Übrous and silky; and selenite, which occurs as transparent crystals in muds and clays. It is used in the building industry and in the manufacture of cement, rubber, paper, and plaster of Paris. gyrocompass A *gyroscope that is driven continuously so that it can be used as a nonmagnetic compass. When the earth rotates the gyroscope experiences no torque if its spin axis is parallel to the earth's axis; if these axes are not parallel, however, the gyroscope experiences a sequence of restoring torques that tend to make it align itself with the earth's axis. The gyrocompass is therefore an accurate north-seeking device that is uninÛuenced by metallic or magnetic objects and it is also more consistent than the magnetic compass. It is therefore widely used on ships, aircraft, missiles, etc. gyromagnetic ratio Symbol γ. The ratio of the angular momentum of an atomic system to its magnetic moment. The inverse of the gyromagnetic ratio is called the magnetomechanical ratio. gyroscope A disk with a heavy rim mounted in a double *gimbal so that its axis can adopt any orientation in space. When the disk is set spinning the whole contrivance has two useful properties: (1) Gyroscopic inertia, i.e. the direction of the axis of spin resists change so that if the gimbals are turned the spinning disk maintains the same orientation in space. This property forms the basis of the *gyrocompass and other navigational devices. (2) Precession, i.e. when a gy-
GZK limit
roscope is subjected to a torque that tends to alter the direction of its axis, the gyroscope turns about an axis at right angles both to the axis about which the torque was applied and to its main axis of spin. This is a consequence of the need to conserve *angular momentum. In the gyrostabilizer for stabilizing a ship, aircraft, or platform, three gyroscopes are kept spinning about mutually perpendicular axes so that any torque tending to alter the orientation of the whole device affects one of the gyroscopes and thereby activates a servomechanism that restores the original orientation.
GZK limit (Greisen–Zatsepin–Kuzmin limit) A limit on the energy of cosmic rays imposed by the interaction between the cosmic rays and the cosmic microwave background radiation. This limit, found by Kenneth Greisen, Vadim Kuzmin, and Georgiy Zatsepin in 1966 using the theory of the *inverse Compton effect, is about 6 × 1019 eV. Cosmic rays with higher energies have been detected but it has been established that these come from relatively near sources and so have not had a chance to be slowed down by interaction with the background radiation.
H Haber, Fritz (1868–1934) German chemist who worked at the Karlsruhe Technical Institute, where he developed the *Haber process in 1908. As a Jew, he left Germany in 1933 to go into exile in Britain, working in Cambridge at the Cavendish Laboratory. For his Haber process, he was awarded the 1918 Nobel Prize for chemistry. Haber process An industrial process for producing ammonia by reaction of nitrogen with hydrogen: N2 + 3H2 ˆ 2NH3 The reaction is reversible and exothermic, so that a high yield of ammonia is favoured by low temperature (see L C' ). However, the rate of reaction would be too slow for equilibrium to be reached at normal temperatures, so an optimum temperature of about 450°C is used, with a catalyst of iron containing potassium and aluminium oxide promoters. The higher the pressure the greater the yield, although there are technical difÜculties in using very high pressures. A pressure of about 250 atmospheres is commonly employed. The process is of immense importance for the Üxation of nitrogen for fertilizers. It was developed in 1908 by Fritz Haber and was developed for industrial use by Carl Bosch (1874–1940), hence the alternative name Haber–Bosch process. The nitrogen is obtained from liquid air. Formerly, the hydrogen was from *water gas and the water-gas shift reaction (the Bosch process) but now the raw material (called synthesis gas) is obtained by steam *reforming natural gas. habit See .
psychologically, but not physically, dependent on a drug.
Hadean The earliest eon in the history of the earth, from the time of the accretion of planetary material, around 4600 million years ago, to the date of the oldest known rocks – and hence the beginning of the geological record – about 3900 million years ago. The young earth was probably a rocky planet with a hot interior and a moist surface with oceans of liquid water. No evidence of life has been found. Compare A. hadron Any of a class of subatomic particles that interact by the strong interaction (see  ). The class includes protons, neutrons, and pions. Hadrons are believed to have an internal structure and to consist of quarks; they are therefore not truly elementary. Hadrons are either *baryons, which decay into protons and are believed to consist of three quarks, or *mesons, which decay into *leptons and photons or into proton pairs and are believed to consist of a quark and an antiquark. See  . haem (heme) An iron-containing molecule that binds with proteins as a *cofactor or *prosthetic group to form the haemoproteins. These are *haemoglobin, *myoglobin, and the *cytochromes. Essentially, haem comprises a *porphyrin with its four nitrogen atoms holding the iron(II) atom as a chelate. This iron can reversibly bind oxygen (as in haemoglobin and myoglobin) or (as in the cytochromes) conduct electrons by conversion between iron(II) and iron(III) species.
habitat The place in which an organism lives, which is characterized by its physical features or by the dominant plant types. Freshwater habitats, for example, include streams, ponds, rivers, and lakes. See also .
haemagglutination See .
habituation 1. A simple type of learning consisting of a gradual waning in the response of an animal to a continuous or repeated stimulus that is not associated with *reinforcement. 2. The condition of being
haematite A mineral form of iron(III) oxide, Fe2O3. It is the most important ore of iron and usually occurs in two main forms: as a massive red kidney-shaped ore (kidney ore) and as grey to black metallic
haematin test (Teichmann test) A test for blood using the presence of characteristic haematin crystals. It was introduced in 1853 by Ludwig Teichmann.
haematoxylin
crystals known as specular iron ore. Haematite is the major red colouring agent in rocks; the largest deposits are of sedimentary origin. In industry haematite is also used as a polishing agent (jeweller's rouge) and in paints.
oxygen-depleted tissues: oxygen is released and haemoglobin is reformed (see   ). The haem groups also bind other inorganic molecules, including carbon monoxide (to form *carboxyhaemoglobin).
haematoxylin A compound used in its oxidized form (haematein) as a blue dye in optical microscopy, particularly for staining smears and sections of animal tissue. It stains nuclei blue and is frequently used with *eosin as a counterstain for cytoplasm. Haematoxylin requires a mordant, such as iron alum, which links the dye to the tissue. Different types of haematoxylin can be made up depending on the mordant used, the method of oxidation, and the pH. Examples are Delafield's haematoxylin and Ehrlich's haematoxylin.
haemolysis The breakdown of red blood cells. It can be due to the action of diseasecausing microorganisms, poisons, antibodies in mismatched blood transfusions, or certain allergic reactions. It produces anaemia.
haemochromogen test (Takayama test) A test used to conÜrm the presence of blood. It is a microcrystal test exploiting the characteristic appearance of haemochromogen crystals observed under a microscope. The test was introduced in Japan in 1912 by Masao Takayama. haemocoel The body cavity of arthropods and molluscs, which is Ülled with blood. The haemocoel is an enlarged blastocoel (see ), which greatly reduces the coelom (this is restricted to the cavities of the gonads and excretory organs). The haemocoel can act as a *hydrostatic skeleton. haemocyanin (hemocyanin) Any of a group of copper-containing respiratory proteins found in solution in the blood of certain arthropods and molluscs. Haemocyanins contain two copper atoms that reversibly bind oxygen, changing between the colourless deoxygenated form (CuI) and the blue oxygenated form (CuII). In some species, haemocyanin molecules form giant polymers with molecular weights of several million. haemoglobin One of a group of globular proteins occurring widely in animals as oxygen carriers in blood. Vertebrate haemoglobin is contained in the red blood cells (erythrocytes). It comprises two pairs of polypeptide chains, known as α-chains and β-chains (forming the globin protein), with each chain folded to provide a binding site for a *haem group. Each of the four haem groups binds one oxygen molecule to form oxyhaemoglobin. Dissociation occurs in
haemophilia A hereditary sex-linked disease (see  ) in which there is a deÜciency or defect of *Factor VIII, causing the blood to clot very slowly. There may be prolonged bleeding following injury and, in severe cases, spontaneous bleeding into the joints and muscles. The disorder is due to a defective recessive allele of the Factor VIII gene, which is located on the X chromosome. Female carriers of the defective allele are unaffected, whereas all males who inherit a defective allele exhibit the disease. haemopoietic tissue The tissue that gives rise to blood cells in the process of haemopoiesis. The haemopoietic tissue of the embryo and fetal stage of vertebrates is the bone marrow, lymph nodes, yolk sac, liver, spleen, and thymus but after birth haemopoiesis occurs in the red bone marrow (see  ). The different types of *stem cells in haemopoietic tissue that give rise to erythrocytes and leucocytes are all originally derived from haemopoietic stem cells (or haemocytoblasts). The formation of the different types of blood cell is under the control of haemopoietic growth factors, which include hormones and *cytokines. hafnium Symbol Hf. A silvery lustrous metallic *transition element; a.n. 72; r.a.m. 178.49; r.d. 13.3; m.p. 2227±20°C; b.p. 4602°C. The element is found with zirconium and is extracted by formation of the chloride and reduction by the Kroll process. It is used in tungsten alloys in Ülaments and electrodes and as a neutron absorber. The metal forms a passive oxide layer in air. Most of its compounds are hafnium(IV) complexes; less stable hafnium(III) complexes also exist. The element was Ürst reported by Urbain in 1911, and its existence was Ünally established by
Dirk Coster (1889–1950) and George de Hevesey (1885–1966) in 1923.
Hahn, Otto (1879–1968) German chemist, who studied in London (with William *Ramsay) and Canada (with Ernest *Rutherford) before returning to Germany in 1907. In 1917, together with Lise *Meitner, he discovered protactinium. In the late 1930s he collaborated with Fritz Strassmann (1902– ) and in 1938 bombarded uranium with slow neutrons. Among the products was barium, but it was Meitner (now in Sweden) who the next year interpreted the process as *nuclear Üssion. In 1944 Hahn received the Nobel Prize for chemistry. hahnium See  . hair 1. A multicellular threadlike structure, consisting of many dead keratinized cells, that is produced by the epidermis in mammalian *skin. The section of a hair below the skin surface (the root) is contained within a *hair follicle, the base of which produces the hair cells. Hair assists in maintaining body temperature by reducing heat loss from the skin. Bristles and whiskers are specialized types of hair. 2. Any of various threadlike structures on plants, such as a *trichome. hair follicle A narrow tubular depression in mammalian skin containing the root of a *hair. It is lined with epidermal cells and extends down through the epidermis and dermis to its base in the subcutaneous tissue. The ducts of *sebaceous glands empty into hair follicles. half cell An electrode in contact with a solution of ions, forming part of a *cell. Various types of half cell exist, the simplest consisting of a metal electrode immersed in a solution of metal ions. Gas half cells have a gold or platinum plate in a solution with gas bubbled over the metal plate. The commonest is the *hydrogen half cell. Half cells can also be formed by a metal in contact with an insoluble salt or oxide and a solution. The *calomel half cell is an example of this. Half cells are commonly referred to as electrodes. half-life See ;  -. half sandwich See  . half-thickness The thickness of a speciÜed material that reduces the intensity
Halley, Edmund of a beam of radiation to half its original value.
half-wave plate See  . half-wave rectiÜer See . half-width The width of a spectral line measured at half its height. In some contexts, the term is used for half the width of the line measured at half its height. halide A compound of a halogen with another element or group. The halides of typical metals are ionic (e.g. sodium Ûuoride, Na+F–). Metals can also form halides in which the bonding is largely covalent (e.g. aluminium chloride, AlCl3). Organic compounds are also sometimes referred to as halides; e.g. the alkyl halides (see ) and the *acyl halides. Halides are named Ûuorides, chlorides, bromides, or iodides. halite (rock salt) Naturally occurring *sodium chloride (common salt, NaCl), crystallizing in the cubic system. It is chieÛy colourless or white (sometimes blue) when pure but the presence of impurities may colour it grey, pink, red, or brown. Halite often occurs in association with anhydrite and gypsum. Hall effect The production of an e.m.f. within a conductor or semiconductor through which a current is Ûowing when there is a strong transverse magnetic Üeld. The potential difference develops at right angles to both the current and the Üeld. It is caused by the deÛection of charge carriers by the Üeld and was Ürst discovered by Edwin Hall (1855–1938). The strength of the electric Üeld EH produced is given by the relationship EH = RHjB, where j is the current density, B is the magnetic Ûux density, and RH is a constant called the Hall coefficient. The value of RH can be shown to be 1/ne, where n is the number of charge carriers per unit volume and e is the electronic charge. The effect is used to investigate the nature of charge carriers in metals and semiconductors, in the Hall probe for the measurement of magnetic Üelds, and in magnetically operated switching devices. See also  H . Halley, Edmund (1656–1742) British astronomer and mathematician, who published a catalogue of southern stars in 1679, made improvements to barometers, and investigated the optics of *rainbows. In 1705
Halley's comet he calculated the orbit of *Halley's comet and in 1718 discovered the *proper motion of the stars.
Halley's comet A bright *comet with a period of 75–76 years. Its last visit to the inner solar system was in 1986. This comet was the Ürst short-period comet to be recognized. Its orbit was Ürst calculated in 1705 by Edmund Halley, after whom it is named. The comet moves around the sun in the opposite direction to the planets and is associated with two meteor showers, the Eta Aquarids (May) and the Orionids (October).
Hall–Heroult cell An electrolytic cell used industrially for the extraction of aluminium from bauxite. The bauxite is Ürst puriÜed by dissolving it in sodium hydroxide and Ültering off insoluble constituents. Aluminium hydroxide is then precipitated (by adding CO2) and this is decomposed by heating to obtain pure Al2O3. In the Hall–Heroult cell, the oxide is mixed with cryolite (to lower its melting point) and the molten mixture electrolysed using graphite anodes. The cathode is the lining of the cell, also of graphite. The electrolyte is kept in a molten state (about 850°C) by the current. Molten aluminium collects at the bottom of the cell and can be tapped off. Oxygen forms at the anode, and gradually oxidizes it away. The cell is named after the US chemist Charles Martin Hall (1863–1914), who discovered the process in 1886, and the French chemist Paul Heroult (1863–1914), who discovered it independently in the same year. hallucinogen A drug or chemical that causes alterations in perception (usually visual), mood, and thought. Common hallucinogenic drugs include *lysergic acid diethylamide (LSD) and mescaline. There is no common mechanism of action for this class of compounds although many hallucinogens are structurally similar to *neurotransmitters in the central nervous system, such as serotonin and the catecholamines. hallux The innermost digit on the hindlimb of a tetrapod vertebrate. In man it is the big toe and contains two phalanges. The hallux is absent in some mammals and in many birds it is directed backwards as an adaptation to perching. Compare . halo 1. A luminous ring that sometimes can be observed around the sun or the moon. It is caused by diffraction of their light by particles in the earth's atmosphere; the
radius of the ring is inversely proportional to the predominant particle radius. 2. (galactic halo) A region of a galaxy, especially a spiral galaxy, forming a spheroidal extension beyond the main aggregation of stars close to the galactic plane or the galactic bulge.
haloalkanes (alkyl halides) Organic compounds in which one or more hydrogen atoms of an alkane have been substituted by halogen atoms. Examples are chloromethane, CH3Cl, dibromoethane, CH2BrCH2Br, etc. Haloalkanes can be formed by direct reaction between alkanes and halogens using ultraviolet radiation. They are usually made by reaction of an alcohol with a halogenating agent. halocarbons Compounds that contain carbon and halogen atoms and (sometimes) hydrogen. The simplest are compounds such as tetrachloromethane (CCl4), tetrabromomethane (CBr4), etc. The *haloforms are also simple halocarbons. The *chloroÛuorocarbons (CFCs) contain carbon, chlorine, and Ûuorine. Similar to these are hydrochlorofluorocarbons (HCFCs), which contain carbon, chlorine, Ûuorine, and hydrogen, and the hydrofluorocarbons (HFCs), which contain carbon, Ûuorine, and hydrogen. The *halons are a class of halocarbons that contain bromine. haloform reaction A reaction for producing haloforms from methyl ketones. An example is the production of chloroform from propanone using sodium chlorate(I) (or bleaching powder): CH3COCH3 + 3NaOCl → CH3COCl3 + 3NaOH The substituted ketone then reacts to give chloroform (trichloromethane): CH3COCCl3 + NaOH → NaOCOCH3 + CHCl3 The reaction can also be used for making carboxylic acids, since RCOCH3 gives the product NaOCOR. It is particularly useful for aromatic acids as the starting ketone can be made by a Friedel–Crafts acylation. The reaction of methyl ketones with sodium iodate(I) gives iodoform (triiodomethane), which is a yellow solid with a characteristic smell. This reaction is used in the iodoform test to identify methyl ketones. It also gives a positive result with a secondary alcohol of the formula RCH(OH)CH3 (which is Ürst oxidized to a methylketone) or
with ethanol (oxidized to ethanal, which also undergoes the reaction).
haloforms The four compounds with formula CHX3, where X is a halogen atom. They are chloroform (CHCl3), and, by analogy, Ûuoroform (CHF3), bromoform (CHBr3), and iodoform (CHI3). The systematic names are trichloromethane, triÛuoromethane, etc. halogenating agent See . halogenation A chemical reaction in which a halogen atom is introduced into a compound. Halogenations are described as chlorination, Ûuorination, bromination, etc., according to the halogen involved. Halogenation reactions may take place by direct reaction with the halogen. This occurs with alkanes, where the reaction involves free radicals and requires high temperature, ultraviolet radiation, or a chemical initiator; e.g. C2H6 + Br2 → C2H5Br + HBr Halogenation of aromatic compounds can be effected by electrophilic substitution using an aluminium chloride catalyst: C6H6 + Cl2 → C6H5Cl + HCl Halogenation can also be carried out using compounds, such as phosphorus halides (e.g. PCl3) or sulphur dihalide oxides (e.g. SOCl2), which react with –OH groups. Such compounds are called halogenating agents. Addition reactions are also referred to as halogenations; e.g. C2H4 + Br2 → CH2BrCH2Br halogens (group 17 elements) A group of elements in the *periodic table (formerly group VIIB): Ûuorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). All have a characteristic electron conÜguration of noble gases but with outer ns2np5 electrons. The outer shell is thus one electron short of a noble-gas conÜguration. Consequently, the halogens are typical nonmetals; they have high electronegativities – high electron afÜnities and high ionization energies. They form compounds by gaining an electron to complete the stable conÜguration; i.e. they are good oxidizing agents. Alternatively, they share their outer electrons to form covalent compounds, with single bonds. All are reactive elements with the reactivity decreasing down the group. The electron afÜnity decreases down the group and other properties also show a change from Ûuorine
halon to astatine. Thus, the melting and boiling points increase; at 20°C, Ûuorine and chlorine are gases, bromine a liquid, and iodine and astatine are solids. All exist as diatomic molecules. The name 'halogen' comes from the Greek 'salt-producer', and the elements react with metals to form ionic halide salts. They also combine with nonmetals, the activity decreasing down the group: Ûuorine reacts with all nonmetals except nitrogen and the noble gases helium, neon, and argon; iodine does not react with any noble gas, nor with carbon, nitrogen, oxygen, or sulphur. The elements Ûuorine to iodine all react with hydrogen to give the acid, with the activity being greatest for Ûuorine, which reacts explosively. Chlorine and hydrogen react slowly at room temperature in the dark (sunlight causes a free-radical chain reaction). Bromine and hydrogen react if heated in the presence of a catalyst. Iodine and hydrogen react only slowly and the reaction is not complete. There is a decrease in oxidizing ability down the group from Ûuorine to iodine. As a consequence, each halogen will displace any halogen below it from a solution of its salt, for example: Cl2 + 2Br– → Br2 + 2Cl– The halogens also form a wide variety of organic compounds in which the halogen atom is linked to carbon. In general, the aryl compounds are more stable than the alkyl compounds and there is decreasing resistance to chemical attack down the group from the Ûuoride to the iodide. Fluorine has a valency of only 1, although the other halogens can have higher oxidation states using their vacant d-electron levels. There is also evidence for increasing metallic behaviour down the group. Chlorine and bromine form compounds with oxygen in which the halogen atom is assigned a positive oxidation state. Only iodine, however, forms positive ions, as in I+NO3–.
halon A compound obtained by replacing the hydrogen atoms of a hydrocarbon by bromine along with other halogen atoms (see ), for instance halon 1211 is bromochlorodiÛuoromethane (CF2BrCl) and halon 1301 is bromotriÛuoromethane (CF3Br). Halons are very stable and unreactive and are widely used in Üre extinguishers. There is concern that they are being broken down in the atmosphere to bromine, which reacts with ozone, leading to depletion of the *ozone layer, and their use is being curtailed.
halo nucleus Although more *chloroÛuorocarbons are present in the atmosphere, halons are between three and ten times more destructive of ozone.
halo nucleus A type of nucleus in which there are many more neutrons (or, more rarely, more protons) than are present in stable isotopes of that element. Sometimes, a few of the extra neutrons are only weakly bound to the rest of the nucleus and are relatively far from the centre of the nucleus. Halo nuclei are highly unstable; examples include beryllium–11 and carbon–19.
halophyte A plant that can tolerate a high concentration of salt in the soil. Such conditions occur in salt marshes and mudÛats. Halophytes possess some of the structural modiÜcations of *xerophytes; for example, many of them are *succulents. In addition, they are physiologically adapted to withstand the high salinity of the soil water: their root cells have a higher than normal concentration of solutes, which enables them to take up water by osmosis from the surrounding soil. Examples of halophytes are mangrove trees (see  ), thrift (Armeria), sea lavender (Limonium), and rice grass (Spartina). Compare ; . Hamilton, Sir William Rowan (1805– 65) Irish mathematician and physicist. Hamilton invented what is now known as the *Hamiltonian formulation of Newtonian mechanics. In doing so, he discovered that there is a very close analogy between Newtonian mechanics and geometrical (ray) optics. He also invented *quaternions in mathematics and found a number of applications of quaternions in physics. Hamiltonian Symbol H. A function used to express the energy of a system in terms of its momentum and positional coordinates. In simple cases this is the sum of its kinetic and potential energies. In Hamiltonian equations, the usual equations used in mechanics (based on forces) are replaced by equations expressed in terms of momenta. This method of formulating mechanics (Hamiltonian mechanics) was Ürst introduced by Sir William Rowan *Hamilton. haploid Describing a nucleus, cell, or organism with a single set of unpaired chromosomes. The haploid number is designated as n. Reproductive cells, formed as a result of *meiosis, are haploid. Fusion of two such
cells (see ) restores the normal (*diploid) number.
haplotype 1. (in genetics) a. A set of linked genes or other genetic markers that are generally inherited together as a unit. This occurs because during meiosis there is little or no *recombination with the corresponding region on the homologous chromosome, and hence shufÛing of alleles between the homologous regions is rare. The stretch of DNA containing a haplotype is called a haplotype block. b. The entire set of genes occurring on a single chromosome, or haploid set of chromosomes. Hence an individual has two haplotypes for each chromosome, one derived from its mother and one from its father. 2. (in medicine) a. The antigenic constitution of an individual (i.e. antigenic phenotype) resulting from the inheritance of a particular haploid combination of histocompatibility alleles at the HLA locus. b. The set of phenotypic features associated with either the paternal or maternal alleles inherited by an individual. hapticity Symbol η. The number of electrons in a ligand that are directly coordinated to a metal. haptotropism See . hard acid See HSAB . hard base See HSAB . hardening of oils The process of converting unsaturated esters of *fatty acids into (more solid) saturated esters by hydrogenation using a nickel catalyst. It is used in the manufacture of margarine from vegetable oils. hard ferromagnetic materials See  . hardness of water The presence in water of dissolved calcium or magnesium ions, which form a scum with soap and prevent the formation of a lather. The main cause of hard water is dissolved calcium hydrogencarbonate (Ca(HCO3)2), which is formed in limestone or chalk regions by the action of dissolved carbon dioxide on calcium carbonate. This type is known as temporary hardness because it is removed by boiling: Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(g) The precipitated calcium carbonate is the 'fur' (or 'scale') formed in kettles, boilers,
pipes, etc. In some areas, hardness also results from dissolved calcium sulphate (CaSO4) or calcium Ûuoride (CaF2). These compounds can not be removed by boiling (permanent hardness). Hard water causes problems in washing and by reducing the efÜciency of boilers, heating systems, and certain industrial processes. Various methods of water softening are used. In public supplies, temporary hardness can be removed by adding lime (calcium hydroxide), which precipitates calcium carbonate Ca(OH)2(aq) + Ca(HCO3)2(aq) → 2CaCO3(s) + 2H2O(l) This is known as the Clark process (or as clarking). It does not remove permanent hardness. Both temporary and permanent hardness can be treated by precipitating calcium carbonate by added sodium carbonate – hence its use as washing soda and in bath salts. Calcium (and other) ions can also be removed from water by ion-exchange using zeolites (e.g. Permutit). This method is used in small domestic water-softeners. Another technique is to complex the Ca2+ ions and prevent them reacting further. For domestic use polyphosphates (containing the ion P6O186–, e.g. Calgon) are added. Other sequestering agents are also used for industrial water. See also .
hard radiation Ionizing radiation of high penetrating power, usually gamma rays or short-wavelength X-rays. Compare  . hardware See . hardwood See . Hargreaves process See  . harmonic An oscillation having a frequency that is a simple multiple of a fundamental sinusoidal oscillation. The fundamental frequency of a sinusoidal oscillation is usually called the Ürst harmonic. The second harmonic has a frequency twice that of the fundamental and so on (see illustration). A taut string or column of air, as in a violin or organ, will sound upper harmonics at the same time as the fundamental sounds. This is because the string or column of air divides itself into sections, each section then vibrating as if it were a whole. The upper harmonics are also called overtones, but the second harmonic is the Ürst overtone, and so on. Musicians, however, often regard har-
monic and overtone as synonymous, not counting the fundamental as a harmonic.
1st harmonic (fundamental)
2nd harmonic (1st overtone)
3rd harmonic (2nd overtone)
Harmonics. harmonic mean See . harmonic motion See   . harmonic oscillator A system (in either *classical physics or *quantum mechanics) that oscillates with *simple harmonic motion. The harmonic oscillator is exactly soluble in both classical mechanics and quantum mechanics. Many systems exist for which harmonic oscillators provide very good approximations. An example in classical mechanics is a simple *pendulum, while at low temperatures atoms vibrating about their mean positions in molecules or crystal lattices can be regarded as good approximations to harmonic oscillators in quantum mechanics. Even if a system is not exactly a harmonic oscillator the solution of the harmonic oscillator is frequently a useful starting point for solving such systems using *perturbation theory. Compare  . harmonic series (harmonic progression) A series or progression in which the reciprocals of the terms have a constant difference between them, e.g. 1 + 1/2 + 1/3 + 1/4 … + 1/n. Harvard classiÜcation See  . harvesting 1. The processes involved in gathering in ripened crops (see ). 2. The collection of cells from cell cultures or of organs from donors for the purpose of transplantation (see ).
Harvey, William Harvey, William (1578–1657) English physician, who worked at St Bartholomew's Hospital, London, from 1609 and from 1618 was court physician. He is best known for discovering the *circulation of the blood, which he announced in 1628. hashish See .
hassium Symbol Hs. A radioactive transactinide element; a.n. 108. It was Ürst made in 1984 by Peter Armbruster and a team in Darmstadt, Germany. It can be produced by bombarding lead-208 nuclei with iron-58 nuclei. Only a few atoms have ever been produced. The name comes from the Latinized form of Hesse, the German state where it was Ürst synthesized.
haustorium A specialized structure of certain parasitic plants and fungi that penetrates the cells of the host plant to absorb nutrients. In parasitic fungi haustoria are formed from enlarged hyphae and in parasitic Ûowering plants, such as the dodder (Cuscuta), they are outgrowths of the stem. Haversian canals Narrow tubes within compact *bone containing blood vessels and nerves. They generally run parallel to the bone surface. Each canal surrounded by a series of rings of bone (lamellae) is known as a Haversian system. Haversian systems are joined to each other by bone material. They are named after Clopton Havers (1650–1702). Hawking, Stephen William (1942– ) British cosmologist and physicist, who in 1979 became the Lucasian Professor of mathematics at Cambridge University. Working with Roger Penrose (1931– ), who had shown how a singularity results from a *black hole, Hawking postulated that the original big bang must have come from a singularity (see - ). He also showed how black holes can emit particles by the *Hawking process. Hawking is the author of such popular science works as A Brief History of Time (1988) and The Universe in a Nutshell (2001). Hawking process Emission of particles by a *black hole as a result of quantummechanical effects. The process was Ürst suggested by Stephen Hawking. The gravitational Üeld of the black hole causes production of particle–antiparticle pairs in the
vicinity of the event horizon (the process is analogous to that of pair production). One member of each pair (either the particle or the antiparticle) falls into the black hole, while the other escapes. To an external observer it appears that the black hole is emitting radiation (Hawking radiation). The energy of the particles that fall in is negative and exactly balances the (positive) energy of the escaping particles. This negative energy reduces the mass of the black hole and the net result of the process is that the emitted particle Ûux appears to carry off the blackhole mass. It can be shown that the black hole radiates like a *black body, with the energy distribution of the particles obeying *Planck's radiation law for a temperature that is inversely proportional to the mass of the hole. For a black hole of the mass of the sun, this temperature is only about 10–7 K, so the energy loss is negligible. However, for a 'mini' black hole, such as might have been formed in the early universe, with a mass of order 1012 kg (and a radius of order 10–15 m), the temperature would be of order 1011 K and the hole would radiate copiously (at a rate of about 6 × 109 W) a Ûux of gamma rays, neutrinos, and electron–positron pairs. (The observed levels of cosmic gamma rays put strong constraints on the number of such 'mini' black holes, suggesting that there are too few of them to solve the *missing-mass problem.)
hazchem code (emergency action code; EAC) A code designed to be displayed when hazardous chemicals are transported or stored in bulk. It is used to help the emergency services to take action quickly in any accident. The code consists of a number followed by one or two letters. The number indicates the type of substance to be used in treating the accident (e.g. stream of water, Üne spray, foam, dry agent). The Ürst letter indicates the type of protective clothing needed along with information about the possibility of violent reaction on whether the substance should be contained or diluted. The second letter, where it exists, is letter E, indicating that people have to be evacuated from the neighbourhood of the incident. In the UK, the code is usually displayed as part of a panel, which includes an international UN number for the substance, a telephone number for specialist advice, the company name, and a symbol indicating the danger (e.g. a skull and crossbones for toxic substances).
HCFC (hydrochloroÛuorocarbon) See . h.c.p. Hexagonal close packing. See  . headspace The space above a sample held in a sealed container. Headspace analysis is used in forensic science to investigate the volatile constituents of a sample. health physics The branch of medical physics concerned with the protection of medical, scientiÜc, and industrial workers from the hazards of ionizing radiation and other dangers associated with atomic physics. Establishing the maximum permissible *dose of radiation, the disposal of radioactive waste, and the shielding of dangerous equipment are the principal activities in this Üeld. hearing The sense by which sound is deaorta
tected. In vertebrates the organ of hearing is the *ear. In higher vertebrates variation in air pressure caused by sound waves are ampliÜed in the outer and middle ears and transmitted to the inner ear, where sensory cells in the *cochlea register the vibrations. The resulting information is transmitted to the brain via the auditory nerve. The ear can distinguish between sounds of different intensity (*loudness) and frequency (*pitch).
heart A hollow muscular organ that, by means of regular contractions, pumps blood through the circulatory system. The vertebrate heart is composed of a specialized muscle (see  ). Mammals have a four-chambered heart consisting of two atria and two ventricles; the right and left sides are completely separate from each other so there is no mixing of oxygenated and deoxygenated blood (see illustration). Oxygenated blood from the pulmonary veins pulmonary artery
pulmonary vein vena cava (from upper body) left atrium right atrium valve vena cava (from lower body) bicuspid valve tricuspid valve tendon supporting valve
deoxygenated blood direction of blood flow
Heart. Structure of the mammalian heart.
oxygenated blood
enters the heart through the left atrium, passes to the left ventricle, and leaves the heart through the *aorta. Deoxygenated blood from the *venae cavae enters the right atrium and is pumped through the right ventricle to the pulmonary artery, which conveys it to the lungs for oxygenation. The tricuspid and bicuspid valves ensure that there is no backÛow of blood. The contractions of the heart are initiated and controlled by the sinoatrial node (see ); an average adult human heart contracts about 70 times per minute. See also  ;  . The hearts of other vertebrates are similar except in the number of atria and ventricles (there may be one or two) and in the degree of separation of oxygenated and deoxygenated blood. Invertebrates, however, show great variation in the form and functioning of the heart.
heartwood (duramen) The wood at the centre of a tree trunk or branch. It consists of dead *xylem cells heavily thickened with lignin and provides structural support. Many heartwood cells contain oils, gums, and resins, which darken the wood. Compare . heat The process of energy transfer from one body or system to another as a result of a difference in temperature. The energy in the body or system before or after transfer is also sometimes called heat. A body in equilibrium with its surroundings contains energy (the kinetic and potential energies of its atoms and molecules) but this is called internal energy, U, rather than heat. When such a body changes its temperature or phase there is a change in internal energy, ∆U, which (according to the Ürst law of *thermodynamics) is given by ∆U = Q – W, where Q is the heat absorbed by the body from the surroundings and W is the work done simultaneously on the surroundings. To use the word 'heat' for both U and Q is clearly confusing. Note also that certain physical quantities are described as *heat of atomization, *heat of combustion, etc. What is usually used is a standard molar *enthalpy change for the process under consideration. The units are kJ mol–1; a negative value indicates that energy is liberated. See also  ;  ;  . heat balance 1. A balance sheet showing all the heat inputs to a system (such as a chemical process, furnace, etc.) and all the
heat outputs. 2. The equilibrium that exists on the average between the radiation received from the sun by the earth and its atmosphere and that reradiated or reÛected by the earth and the atmosphere. In general, the regions of the earth nearer the equator than about 35°N or S receive more energy from the sun than they are able to reradiate, whereas those regions polewards of 35°N or S receive less energy than they lose. The excess of heat received by the low latitudes is carried to the higher latitudes by atmospheric and oceanic circulations.
heat capacity (thermal capacity) The ratio of the heat supplied to an object or specimen to its consequent rise in temperature. The specific heat capacity is the ratio of the heat supplied to unit mass of a substance to its consequent rise in temperature. The molar heat capacity is the ratio of the heat supplied to unit amount of a substance to its consequent rise in temperature. In practice, heat capacity (C) is measured in joules per kelvin, speciÜc heat capacity (c) in J K–1 kg–1, and molar heat capacity (Cm) in J K–1 mol–1. For a gas, the values of c and Cm are commonly given either at constant volume, when only its *internal energy is increased, or at constant pressure, which requires a greater input of heat as the gas is allowed to expand and do work against the surroundings. The symbols for the speciÜc and molar heat capacities at constant volume are cv and Cv, respectively; those for the speciÜc and molar heat capacities at constant pressure are cp and Cp. heat death of the universe The condition of the universe when *entropy is maximized and all large-scale samples of matter are at a uniform temperature. In this condition no energy is available for doing work and the universe is Ünally unwound. The condition was predicted by Rudolf Clausius, who introduced the concept of entropy. Clausius's dictum that "the energy of the universe is constant, its entropy tends to a maximum" is a statement of the Ürst two laws of thermodynamics. These laws apply in this sense only to closed systems and, for the predicted heat death to occur, the universe must be a closed system. heat engine A device for converting heat into work. The heat is derived from the combustion of a fuel. In an *internal-combustion engine the fuel is burnt inside the engine, whereas in a *steam engine or steam turbine,
examples of external-combustion engines, the fuel is used to raise steam outside the engine and then some of the steam's internal energy is used to do work inside the engine. Engines usually work on cycles of operation, the most efÜcient of which would be the *Carnot cycle. This cannot be realized in practice, but the *Rankine cycle is approximated by some engines.
heat exchanger A device for transferring heat from one Ûuid to another without permitting the two Ûuids to contact each other. A simple industrial heat exchanger consists of a bundle of parallel tubes, through which one Ûuid Ûows, enclosed in a container, through which the other Ûuid Ûows in the opposite direction (a counter-current heat exchanger). heat of atomization The energy required to dissociate one mole of a given substance into atoms. heat of combustion The energy liberated when one mole of a given substance is completely oxidized. heat of crystallization The energy liberated when one mole of a given substance crystallizes from a saturated solution of the same substance. heat of dissociation The energy absorbed when one mole of a given substance is dissociated into its constituent elements. heat of formation The energy liberated or absorbed when one mole of a compound is formed in their *standard states from its constituent elements. heat of neutralization The energy liberated in neutralizing one mole of an acid or base. heat of reaction The energy liberated or absorbed as a result of the complete chemical reaction of molar amounts of the reactants. heat of solution The energy liberated or absorbed when one mole of a given substance is completely dissolved in a large volume of solvent (strictly, to inÜnite dilution). heat pump A device for transferring heat from a low temperature source to a high temperature region by doing work. It is essentially a refrigerator with a different emphasis. The working Ûuid is at one stage a vapour, which is compressed by a pump adi-
Heaviside–Lorentz units abatically so that its temperature rises. It is then passed to the radiator, where heat is given out to the surroundings (the space to be heated) and the Ûuid condenses to a liquid. It is then expanded into an evaporator where it takes up heat from its surroundings and becomes a vapour again. The cycle is completed by returning the vapour to the compressor. Heat pumps are sometimes adapted as dual-purpose space-heating-inwinter and air-conditioning-in-summer devices.
heat radiation (radiant heat) Energy in the form of electromagnetic waves emitted by a solid, liquid, or gas as a result of its temperature. It can be transmitted through space; if there is a material medium this is not warmed by the radiation except to the extent that it is absorbed. Although it covers the whole electromagnetic spectrum, the highest proportion of this radiation lies in the infrared portion of the spectrum at normal temperatures. See  . heat shield A specially prepared surface that prevents a spacecraft or capsule from overheating as it re-enters the earth's atmosphere. The surface is coated with a plastic impregnated with quartz Übres, which is heated by friction with air molecules as the craft enters the atmosphere, causing the outer layer to vaporize. In this way about 80% of the energy is reradiated and the craft is safeguarded from an excessive rise in temperature. heat-shock protein (HSP) See  . heat transfer The transfer of energy from one body or system to another as a result of a difference in temperature. The heat is transferred by *conduction (see also ), *convection, and radiation (see  ). Heaviside–Kennelly layer See ' . Heaviside–Lorentz units A system of units for electric and magnetic quantities based upon c.g.s. electrostatic and electromagnetic units. They are the rationalized forms of *Gaussian units and, like the latter, are widely used in particle physics and relativity in preference to the *SI units now employed for general purposes in physics. They are named after Oliver Heaviside (1850– 1925) and Hendrik Lorentz (1853–1928).
heavy-fermion system
heavy-fermion system A solid in which the electrons have a very high effective mass; i.e. they act as if they had masses several hundred times the normal mass of the electron. An example of a heavy-fermion system is the cerium–copper–silicon compound CeCuSi2. The electrons with high effective mass are f-electrons in narrow energy bands associated with strong many-body effects. Substances containing such electrons have unusual thermodynamic, magnetic, and superconducting properties, which are still not completely understood. The *superconductivity of such materials has a more complicated mechanism than that for metals described by the BCS theory, since the Cooper pairs are formed from *quasiparticles with very high effective masses rather than from electrons. heavy hydrogen See . heavy metal A metal with a high relative atomic mass. The term is usually applied to common transition metals, such as copper, lead, and zinc. These metals are a cause of environmental *pollution (heavy-metal pollution) from a number of sources, including lead in petrol, industrial efÛuents, and leaching of metal ions from the soil into lakes and rivers by acid rain. heavy spar A mineral form of *barium sulphate, BaSO4. heavy water (deuterium oxide) Water in which hydrogen atoms, 1H, are replaced by the heavier isotope deuterium, 2H (symbol D). It is a colourless liquid, which forms hexagonal crystals on freezing. Its physical properties differ from those of 'normal' water; r.d. 1.105; m.p. 3.8°C; b.p. 101.4°C. Deuterium oxide, D2O, occurs to a small extent (about 0.003% by weight) in natural water, from which it can be separated by fractional distillation or by electrolysis. It is useful in the nuclear industry because of its ability to reduce the energies of fast neutrons to thermal energies (see ) and because its absorption cross-section is lower than that of hydrogen and consequently it does not appreciably reduce the neutron Ûux. In the laboratory it is used for *labelling other molecules for studies of reaction mechanisms. Water also contains the compound HDO. hecto- Symbol h. A preÜx used in the metric system to denote 100 times. For example, 100 coulombs = 1 hectocoulomb (hC).
Heisenberg, Werner Karl (1901–76) German physicist, who became a professor at the University of Leipzig and, after World War II, at the Kaiser Wilhelm Institute in Göttingen. In 1923 he was awarded the Nobel Prize for physics for his work on *matrix mechanics. But he is best known for his 1927 discovery of the *uncertainty principle. Heisenberg uncertainty principle See  . heliacal rising The Ürst appearance of a star, a constellation, a planet, or the moon above the eastern horizon just before dawn following a period of invisibility during which the object had been rising in daylight. Heliacal risings of the same star or constellation happen one year apart. helicate A type of inorganic molecule containing a double helix of bipyridyl-derived molecules formed around a chain of up to Üve copper(I) ions. See also  . heliocentric universe A conception of the universe in which the sun is taken to be at its centre. Aristarchus of Samos (310–230 BC) was the Ürst formally to propose the heliocentric model, but it failed to predict what was actually observed and thus made little headway against the rival, more intuitive *geocentric universe, which held sway throughout the Middle Ages. Nicolaus Copernicus revived an essentially heliocentric view, which was upheld by Galileo against strong opposition from the church on the grounds that if the earth was not at the centre of the universe man's position in God's creation was diminished. In the modern view the sun is at the centre of the *solar system, but the sun, with its family of planets and other bodies, is just one of an enormous number of stars in the Galaxy, which is itself one of an enormous number of *galaxies. helioseismology The study of waves in the sun. The waves studied in helioseismology are not seismic waves, as in geophysics, but pressure waves generated by turbulence in the solar convection layer. These cause slight changes in the pattern of light from the sun and the pattern of pressure waves can be inferred from these changes, leading to information about conditions within the sun. heliosphere The vast bubble-like region enveloping the *solar system and carried with it through space, within which the *solar wind plays a dominant role.
heliotropism See . helium Symbol He. A colourless odourless gaseous nonmetallic element belonging to group 18 of the periodic table (see  ); a.n. 2; r.a.m. 4.0026; d. 0.178 g dm–3; m.p. –272.2°C (at 20 atm.); b.p. –268.93°C. The element has the lowest boiling point of all substances and can be solidiÜed only under pressure. Natural helium is mostly helium–4, with a small amount of helium–3. There are also two short-lived radioactive isotopes: helium–5 and –6. It occurs in ores of uranium and thorium and in some natural-gas deposits. It has a variety of uses, including the provision of inert atmospheres for welding and semiconductor manufacture, as a refrigerant for superconductors, and as a diluent in breathing apparatus. It is also used in Ülling balloons. Chemically it is totally inert and has no known compounds. It was discovered in the solar spectrum in 1868 by Joseph Lockyer (1836–1920).
Helmholtz, Hermann Ludwig Ferdinand von (1821–94) German physiologist and physicist. In 1850 he measured the speed of a nerve impulse and in 1851 invented the ophthalmoscope. In physics, he discovered the conservation of energy (1847), introduced the concept of *free energy, and invented *vortex dynamics. Helmholtz coils Two coaxial parallel Ûat coils with the same radius placed a distance apart that is equal to the radius. If the same current is Ûowing in both coils then the value of the magnetic Üeld strength is approximately uniform between the coils. Coils of this type are used to create Üelds and, in some cases, are employed to counter the effect of the earth's magnetic Üeld. Helmholtz coils are also used in magnetic measurement, with the coils connected to a *Ûuxmeter. If a small magnet is placed between the coils and then removed, the integrated signal on the Ûuxmeter is proportional to the magnet's magnetic moment. Helmholtz free energy See  . helper T cell See T . heme See . hemiacetals See . hemicellulose A *polysaccharide found in the cell walls of plants. The branched chains
of this molecule bind to cellulose microÜbrils, together with pectins, forming a network of cross-linked Übres.
hemihedral form The form of a crystal in which only half the number of faces required for the symmetry are present. Compare  . hemihydrate A crystalline hydrate containing two molecules of compound per molecule of water (e.g. 2CaSO4.H2O). hemiketals See . Hemiptera An order of insects comprising the true bugs. Hemipterans typically have oval Ûattened bodies with two pairs of wings, which are folded back across the abdomen at rest. The forewings are hardened, either at their bases only (in the suborder Heteroptera) or uniformly (in the suborder Homoptera). The mouthparts are modiÜed for piercing and sucking, with long slender stylets forming a double tube. Many bugs feed on plant sap and are serious agricultural pests, including aphids, leaf-hoppers, scale insects, mealy bugs, etc. Others are carnivorous, and the order contains many aquatic species, such as the water boatmen, which have legs adapted for swimming and the exchange of respiratory gases. henry Symbol H. The *SI unit of inductance equal to the inductance of a closed circuit in which an e.m.f. of one volt is produced when the electric current in the circuit varies uniformly at a rate of one ampere per second. It is named after Joseph Henry. Henry, Joseph (1797–1878) US physicist, who became professor of natural philosophy at Princeton in 1832. In 1829 he made an *electric motor, and used insulated windings to produce a powerful *electromagnet. A year later he discovered *electromagnetic induction (independently of *Faraday), and in 1832 he discovered self-induction (see ). In 1835 he invented the electric *relay. Henry's law At a constant temperature the mass of gas dissolved in a liquid at equilibrium is proportional to the partial pressure of the gas. The law, discovered in 1801 by the British chemist and physician William Henry (1775–1836), is a special case of the partition law. It applies only to gases that do not react with the solvent. heparin A glycosaminoglycan (muco-
hepatic polysaccharide) with *anticoagulant properties, occurring in vertebrate tissues, especially the lungs and blood vessels. Pharmaceutical preparations of heparin are administered to prevent or dissolve blood clots.
hepatic Of or relating to the liver. For example, the hepatic portal vein (see   ) and the hepatic artery supply blood to the liver, and the hepatic vein carries blood away from the liver. Hepaticae See H.
hepatic portal system The vein (hepatic portal vein) or veins that transport blood containing the absorbed products of digestion from the intestine directly to the liver. Hepatophyta (Marchantiophyta) A phylum comprising the liverworts – simple plants that lack vascular tissue and possess rudimentary rootlike organs (rhizoids). Liverworts occur in moist situations (including fresh water) and as epiphytes on other plants. Like the mosses (see B), liverworts show marked alternation of generations between haploid gamete-bearing forms (gametophytes) and diploid sporebearing forms (sporophytes), the latter being dependent on the former for nutrients, etc. The plant body (gametophyte) may be a thallus, growing closely pressed to the ground (thallose liverworts, e.g. Pellia), or it may bear many leaÛike lobes (leafy liverworts). It gives rise to leaÛess stalks bearing capsules (sporophytes). Spores formed in the capsules are released and grow to produce new plants. Liverworts were formerly placed in the class Hepaticae in the phylum Bryophyta, which now contains only the mosses. heptadecanoic acid (margaric acid) A white crystalline carboxylic acid with a linear chain of carbon atoms, C16H33COOH; m.p. 59–61°C; b.p. 227°C (100 mm Hg). It is present in certain natural fats. heptahydrate A crystalline hydrate that has seven moles of water per mole of compound. heptane A liquid straight-chain alkane obtained from petroleum, C7H16; r.d. 0.684; m.p. –90.6°C; b.p. 98.4°C. In standardizing *octane numbers, heptane is given a value zero. heptaoxodiphosphoric(V) acid See (V) .
heptavalent (septivalent) Having a valency of seven. herb 1. A *herbaceous plant, i.e. a seedbearing plant that does not form hard woody tissue. 2. A plant with medicinal or culinary uses. Culinary herbs are usually plants whose leaves are used for Ûavouring food. herbaceous Describing a plant that contains little permanent woody tissue. The aerial parts of the plant die back after the growing season. In *annuals the whole plant dies; in *biennials and herbaceous *perennials the plant has organs (e.g. bulbs or corms) that survive beneath the soil in unfavourable conditions. herbal cannabinoids See . herbicide See . Herbig–Haro object (HH object) A strange bright region in interstellar space. It contains gas and dust that is excited to emit electromagnetic radiation by a jet of gas, which may be the *bipolar outÛow from a young *T Tauri star or other protostar. herbivore An animal that eats vegetation, especially any of the plant-eating mammals, such as ungulates (cows, horses, etc.). Herbivores are characterized by having teeth adapted for grinding plants and alimentary canals specialized for digesting cellulose (see ). heredity The transmission of characteristics from parents to offspring via the chromosomes. The study of heredity (*genetics) was Ürst undertaken by Gregor Mendel, who derived a series of laws that govern heredity (see M' ). Hermann–Mauguin system (international system) A notation used to describe the symmetry of point groups. In contrast to the *SchoenÛies system, which is used for isolated molecules (e.g. in spectroscopy), the Hermann–Mauguin system is used in *crystallography. Some of the categories are the same as the SchoenÛies system. n is the same group as Cn. nmm is the same group as Cnv. There are two ms because of two distinct types of mirror plane containing the nfold axis. n22 is the same group as Dn. The other categories do not coincide with the SchoenÛies system.  is a group with an nfold rotation–inversion axis and includes C3h as 6 ¯, S4 as 4¯, S6 as 3¯, and S2 as 1¯. n/m is the same group as Cnh except that C3h is re¯. n2m is the same group as Dnd, garded as 6
Hertzsprung–Russell diagram
except that D3h is regarded as 62m. n/m 2/m 2/m, abbreviated to n/mmm, is the same group as Dnh, except that D3h is regarded as ¯6¯2m. (Unlike the SchoenÛies system, the Hermann–Mauguin system regards the three-fold axis as a special case.) As regards the cubic groups, Oh is denoted m3m (or ¯ 2/m), O is denoted 432, Th is denoted 4/m 3 m3 (or 2/m 3 ¯), Td is denoted ¯4¯3m, and T is denoted 23. In the Hermann–Mauguin system all the cubic groups have 3 as the second number because of the three-fold axis that occurs in all cubic groups. It is named after the German crystallographer Carl Hermann (1898–1961) and the French minerologist Charles-Victor Mauguin (1878–1958).
hermaphrodite (bisexual) 1. An animal, such as the earthworm, that has both male and female reproductive organs. 2. A plant whose Ûowers contain both stamens and carpels. This is the usual arrangement in most plants. Compare ; . heroin (diacetylmorphine) A *narcotic compound that is a synthetic derivative of morphine (see ). The compound is easily absorbed by the brain and is used as a powerful *analgesic. Highly addictive, it is abused by drug users. herpesvirus One of a group of complex DNA-containing viruses causing infections
in man and most other vertebrates that tend to recur. The group includes herpes simplex, the agent of cold sores; herpes varicella/ zoster, the virus causing chickenpox and shingles; Epstein–Barr (EB) virus, the causal agent of glandular fever and also implicated in the cancer Burkitt's lymphoma; and the *cytomegalovirus.
hertz Symbol Hz. The *SI unit of frequency equal to one cycle per second. It is named after Heinrich Hertz. Hertz, Heinrich Rudolf (1857–94) German physicist, who worked as an engineer before attending Berlin University. He is best known for his 1888 discovery of *radio waves, as predicted by James Clerk *Maxwell. The SI unit of frequency is named after him. Hertzsprung–Russell diagram (H–R diagram) A graphical representation of the absolute magnitude of stars (usually along the y-axis) plotted against the spectral class or colour index (x-axis): see illustration. The yaxis then represents the energy output of the star and the x-axis its surface temperature. The majority of stars on such a diagram fall on a band running from the top left to the bottom right of the graph. These are called main-sequence stars (the sun falls into this class). The few stars falling in the lower left portion are called *white dwarfs. The *giant
supergiants
absolute magnitude
0 giants +5 main sequence
white dwarfs +15 O 25 000
Hertzsprung–Russell diagram.
G 5000
spectral class M 3000 temperature (K)
hesperidium stars fall in a cluster above the main sequence and the *supergiants are above them. The diagram, which was Ürst devised in 1911 by Ejnar Hertzsprung (1873–1969) and in 1913 by Henry Russell (1897–1957), forms the basis of the theory of *stellar evolution.
hesperidium See .
Hess's law If reactants can be converted into products by a series of reactions, the sum of the heats of these reactions (with due regard to their sign) is equal to the heat of reaction for direct conversion from reactants to products. More generally, the overall energy change in going from reactants to products does not depend on the route taken. The law can be used to obtain thermodynamic data that cannot be measured directly. For example, the heat of formation of ethane can be found by considering the reactions: 2C(s) + 3H2(g) + 3½O2(g) → 2CO2(g) + 3H2O(l) The heat of this reaction is 2∆HC + 3∆HH, where ∆HC and ∆HH are the heats of combustion of carbon and hydrogen respectively, which can be measured. By Hess' law, this is equal to the sum of the energies for two stages: 2C(s) + 3H2(g) → C2H6(g) (the heat of formation of ethane, ∆Hf) and C2H6(g) + 3½O2 → 2CO2(g) + 3H2O(l) (the heat of combustion of ethane, ∆HE). As ∆HE can be measured and as ∆Hf + ∆HE = 2∆Hc + 3∆HH ∆Hf can be found. Another example is the use of the *Born–Haber cycle to obtain lattice energies. The law was Ürst put forward in 1840 by the Russian chemist Germain Henri Hess (1802–50). It is sometimes called the law of constant heat summation and is a consequence of the law of conservation of energy. hetero atom An odd atom in the ring of a heterocyclic compound. For instance, nitrogen is the hetero atom in pyridine. heterochromatin See . heterocyclic See . heterodont Describing animals that possess teeth of more than one type (i.e. *incisors, *canine teeth, *premolars, and *molars), each with a particular function.
Most mammals are heterodont. See . Compare .
heterodyne Denoting a device or method of radio reception in which *beats are produced by superimposing a locally generated radio wave on an incoming wave. In the *superheterodyne receiver the intermediate frequency is ampliÜed and demodulated. In the heterodyne wavemeter, a variablefrequency local oscillator is adjusted to give a predetermined beat frequency with the incoming wave, enabling the frequency of the incoming wave to be determined. heterogametic sex The sex that is determined by possession of two dissimilar *sex chromosomes (e.g. XY). In humans and many other mammals this is the male sex. The heterogametic sex produces reproductive cells (gametes) of two kinds, half containing an X chromosome and half a Y chromosome. Compare  . heterogeneous Relating to two or more phases, e.g. a heterogeneous *catalyst. Compare . heterolytic Üssion The breaking of a bond in a compound in which the two fragments are oppositely charged ions. For example, HCl → H+ + Cl–. Compare  . heteropolar bond See  . heteropoly compound See  . heteropolymer See . heterosis See  . heterotrophic nutrition A type of nutrition in which energy is derived from the intake and digestion of organic substances, normally plant or animal tissues. The breakdown products of digestion are used to synthesize the organic materials required by the organism. All animals obtain their food this way: they are heterotrophs. See also . Compare  . heterozygous Describing an organism or cell in which the *alleles at a given locus on *homologous chromosomes are different. The aspect of the feature displayed by the organism will be that determined by the *dominant allele. Heterozygous organisms, called heterozygotes, do not breed true. Compare . heuristic Denoting a method of solving a
hexose
problem for which no *algorithm exists. It involves trial and error, as in *iteration.
hexanoate (caproate) A salt or ester of hexanoic acid.
Heusler alloys Ferromagnetic alloys containing no ferromagnetic elements. The original alloys contained copper, manganese, and tin and were Ürst made by Conrad Heusler (19th-century mining engineer).
hexanoic acid (caproic acid) A liquid fatty acid, CH3(CH2)4COOH; r.d. 0.93; m.p. –2 to –1.5°C; b.p. 205°C. Glycerides of the acid occur naturally in cow and goat milk and in some vegetable oils.
Hewish, Antony See R, S M.
Hexapoda (Insecta) A subphylum (or class) of arthropods comprising about a million known species (many more are thought to exist). They are distributed worldwide in nearly all terrestrial habitats. Ranging in length from 0.5 to over 300 mm, an insect's body consists of a head, a thorax of three segments and usually bearing three pairs of legs and one or two pairs of wings, and an abdomen of eleven segments. The head possesses a pair of sensory *antennae and a pair of large *compound eyes, between which are three simple eyes (*ocelli). The *mouthparts are variously adapted for either chewing or sucking, enabling insects to feed on a wide range of plant and animal material. Insects owe much of their success to having a highly waterproof *cuticle (to resist desiccation) and wings – outgrowths of the body wall that confer the greater mobility of Ûight. Breathing occurs through a network of tubes (see ). Most insect species have separate sexes and undergo sexual reproduction. In some, this may alternate with asexual *parthenogenesis and in a few, males are unknown and reproduction is entirely asexual. The newly hatched young grow by undergoing a series of moults. In the more primitive groups, e.g. *Dermaptera, *Orthoptera, *Dictyoptera, and *Hemiptera, the young (called a *nymph) resembles the adult. More advanced groups, e.g. *Coleoptera, *Diptera, *Lepidoptera, and *Hymenoptera, undergo *metamorphosis, in which the young (called a *larva) is transformed into a quiescent *pupa from which the fully formed adult emerges. Insects are of vital importance in many ecosystems and many are of economic signiÜcance – as pests or disease vectors or beneÜcially as crop pollinators or producers of silk, honey, etc.
hexacyanoferrate(II) (ferrocyanide) A complex iron-containing anion, [Fe(CN6)]4–, used as a solution of its potassium salt as a test for ferric iron (iron(III)), with which it forms a dark blue precipitate of Prussian blue. The sodium salt is used as an anticaking agent in common salt. hexacyanoferrate(III) (ferricyanide) A complex iron-containing anion, [Fe(CN)6)]3–, used as a solution of its potassium salt as a test for ferrous iron (iron(II)), with which it forms a dark blue precipitate of Prussian blue. hexadecane (cetane) A colourless liquid straight-chain alkane hydrocarbon, C16H34, used in standardizing cetane ratings of Diesel fuel. hexadecanoate See . hexadecanoic acid See  . hexagonal close packing See  . hexagonal crystal See  . hexahydrate A crystalline compound that has six moles of water per mole of compound. hexamine (hexamethylene tetramine) A white crystalline compound, C6H12N4, made by the condensation of methanal with ammonia. It has been used as a solid fuel for camping stoves and as an antiseptic for treating urinary infections in medicine. It is used in the production of *cyclonite. hexanedioate (adipate) A salt or ester of hexanedioic acid. hexanedioic acid (adipic acid) A carboxylic acid, (CH2)4(COOH)2; r.d. 1.36; m.p. 153°C; b.p. 265°C (100 mmHg). It is used in the manufacture of *nylon 6,6. See also .
6-hexanelactam See .
• This section of the Natural History Museum's website highlights various insect groups and other aspects of entomology
hexanitrohexaazaisowurtzitane See HNIW.
hexose A *monosaccharide that has six carbon atoms in its molecules.
hexyl group hexyl group (hexyl radical) The organic group CH3CH2CH2CH2CH2CH2–, derived from hexane. HFC (hydroÛuorocarbon) See .
hibernation A sleeplike state in which some animals pass the winter months as a way of surviving food scarcity and cold weather. Various physiological changes occur, such as lowering of the body temperature and slowing of the pulse rate and other vital processes, and the animal lives on its reserve of body fat. Animals that hibernate include bats, hedgehogs, and many Üsh, amphibians, and reptiles. See also . Compare . hidden matter See  . hidden-variables theory A theory that denies that the speciÜcation of a physical system given by a state described by *quantum mechanics is a complete speciÜcation. A successful hidden-variables theory has never been constructed. There are important considerations preventing the construction of a simple hidden-variables theory, notably *Bell's theorem. The only type of hidden-variables theories that appear not to have been ruled out are non-local hidden-variables theories, i.e. theories in which hidden parameters can affect parts of the system in arbitrarily distant regions simultaneously. A hidden-variables theory that does not satisfy this deÜnition is called a local hidden-variables theory. hierarchy (in biology) A type of social organization in which individuals are ranked according to their status or dominance relative to other group members. This affects their behaviour in various ways, e.g. by determining their access to food or to mates. Many vertebrate animals and some invertebrates live in hierarchical social groups. Higgs boson A spin-zero particle with a nonzero mass, predicted by Peter Higgs (1929– ) to exist in certain *gauge theories, in particular in *electroweak theory (the Weinberg–Salam model). The Higgs boson has not yet been found but it is thought likely that it will be found by larger *accelerators in the next few years, especially since other associated features of the theory, including W and Z bosons, have been found. Higgs Üeld The symmetry-breaking Üeld associated with a *Higgs boson. The Higgs
Üeld can either be an elementary *scalar quantity or the Üeld associated with a *bound state of two *fermions. In the Weinberg–Salam model (see  ), the Higgs Üeld is taken to be a scalar Üeld. It is not known whether this assumption is correct, although attempts to construct electroweak theory involving bound states for the Higgs Üeld, known as technicolour theory, have not been successful. Higgs Üelds also occur in many-body systems, which can be expressed in terms of *quantum Üeld theory with Higgs bosons; an example is the BCS theory of *superconductivity, in which the Higgs Üeld is associated with a *Cooper pair, rather than an elementary scalar Üeld.
higher dimensions Dimensions of space or time that exist in addition to the three space dimensions and one time dimension. Higher dimensions can be very small, as in *Kaluza–Klein theory, or large, as in the *brane world. There is no observational evidence for the existence of higher dimensions but they do appear to be a necessary part of certain fundamental theories. high frequency (HF) A radio frequency in the range 3–30 megahertz; i.e. having a wavelength in the range 10–100 metres. high-performance liquid chromatography (HPLC) A sensitive technique for separating or analysing mixtures, in which the sample is forced through the chromatography column under pressure. high-speed steel A steel that will remain hard at dull red heat and can therefore be used in cutting tools for high-speed lathes. It usually contains 12–22% tungsten, up to 5% chromium, and 0.4–0.7% carbon. It may also contain small amounts of vanadium, molybdenum, and other metals. high-temperature superconductivity See . high tension (HT) A high potential difference, usually one of several hundred volts or more. Batteries formerly used in the anode circuits of radio devices using valves were usually called high-tension batteries to distinguish them from the batteries supplying the heating Ülaments. high-velocity star An old population II star (see  ) that does not orbit the galactic centre but travels far away from the plane of the Galaxy. Such stars
histone
move no faster than the stars near them but appear to do so because of their eccentric orbits.
Hilbert space A linear *vector space that can have an inÜnite number of dimensions. The concept is of interest in physics because the state of a system in *quantum mechanics is represented by a vector in Hilbert space. The dimension of the Hilbert space has nothing to do with the physical dimension of the system. The Hilbert space formulation of quantum mechanics was put forward by the Hungarian-born US mathematician John von Neumann (1903–57) in 1927. Other formulations of quantum mechanics, such as *matrix mechanics and *wave mechanics, can be deduced from the Hilbert space formulation. Hilbert space is named after the German mathematician David Hilbert (1862–1943), who invented the concept early in the 20th century. hilum A scar on the seed coat of a plant marking the point at which the seed was attached to the fruit wall by the *funicle. It is a feature that distinguishes seeds from fruits. hindbrain (rhombencephalon) One of the three sections of the brain of a vertebrate embryo. It develops to form the *cerebellum, *pons, and *medulla oblongata, which control and coordinate fundamental physiological processes (including respiration and circulation of blood). Compare ; . hindgut 1. The posterior part of the alimentary canal of vertebrates, comprising the posterior section of the colon. 2. The posterior section of the alimentary canal of arthropods. See also ; . hip girdle See  . hippocampus A part of the vertebrate brain consisting of two ridges, one over each of the two lateral *ventricles. It is highly developed in advanced mammals (primates and whales) and its function seems to be concerned with forming new memories – by encoding information that can be stored as long-term memory elsewhere in the brain. Hirudinea A class of freshwater and terrestrial annelid worms that comprises the leeches. They have suckers at both anterior and posterior ends but no bristles. Some are blood-sucking parasites of vertebrates and invertebrates but the majority are predators. histamine A substance that is released
during allergic reactions, e.g. hay fever. Formed from the amino acid histidine, histamine can occur in various tissues but is concentrated in connective tissue. It is one of the inÛammatory mediators released from mast cells in response to antigen binding to IgE antibodies on the surface of the mast cell. It causes dilation and increased permeability of small blood vessels, which results in such symptoms as localized swelling, itching, sneezing, and runny eyes and nose. The effects of histamine can be countered by the administration of *antihistamine drugs.
histidine See  . histiocyte See . histochemistry The study of the distribution of the chemical constituents of tissues by means of their chemical reactions. It utilizes such techniques as *staining, light and electron microscopy, *autoradiography, and *chromatography. histocompatibility The degree to which tissue from one organism will be tolerated by the immune system of another organism. For any animal, it is essential that its immune system can distinguish its own tissues from foreign cells or tissues, so that only the latter are attacked. This self-recognition is achieved principally by a set of marker molecules, called histocompatibility proteins (or histocompatibility antigens), which occur on the surfaces of cells. These proteins (in humans also called human leucocyte antigens, or HLAs) are encoded in vertebrates by a cluster of genes called the *major histocompatibility complex (MHC), which in humans includes the *HLA system. Each species has a unique set of histocompatibility proteins, and there is also wide variation within any given species. This explains why in human transplantation it is very difÜcult to match donor and recipient tissue exactly. MHC proteins also play a vital role in the immune responses of lymphocytes, notably by enabling *T cells to identify foreign antigens. histocompatibility protein See ; HLA . histology The microscopic study of the tissues of living organisms. The study of cells, a specialized branch of histology, is known as *cytology. histone Any of a group of water-soluble proteins found in association with the *DNA of plant and animal chromosomes. They
HIV contain a large proportion of the basic (positively charged) amino acids lysine, arginine, and histidine. They are involved in the condensation and coiling of chromosomes during cell division, and chemical modiÜcation of histones is a key aspect of suppressing or activating gene activity. Histones do not occur in vertebrate sperm cells (see ) or in bacteria.
HIV (human immunodeÜciency virus) The *retrovirus that causes *AIDS in humans. It has a speciÜc afÜnity for the helper *T cells of its host, binding to *CD4 antigens on the cell surface and thereby disabling these cells. The membrane envelope glycoproteins encasing the virus show great variability in their amino-acid sequences, hence the difÜculty of preparing an effective AIDS vaccine. Two varieties (serovars) are known: HIV-1 and HIV-2. The latter, which is less virulent, is found chieÛy in Africa. HIV is thought to have originated from chimpanzees in central Africa. HLA system (human leucocyte antigen system) A series of gene loci, forming part of the *major histocompatibility complex in humans, that encode a group of proteins that act as antigens and are important in determining the acceptance or rejection by the body of a tissue or organ transplant (see ). These antigens are one group of the so-called histocompatibility proteins or antigens (see ). Two individuals with identical HLA types are said to be histocompatible. Successful transplantation requires a minimum number of HLA differences between the donor's and recipient's tissues. HMX (octogen; cyclotetramethylenetetranitramine) A colourless crystalline compound, C4H8N8O8; r.d. 1.9; m.p. 276–286°C. It has a structure similar to that of *cyclonite (RDX), but with an eightmembered ring rather than a six-membered ring. An extremely powerful explosive, it is used mainly for military purposes and as a rocket propellant. The name comes from 'High Molecular Weight RDX'. HNIW (hexanitrohexaazaisowurtzitane) A powerful explosive, C6N12O12. It has a three-dimensional bridged structure containing six N–NO2 groups. Also known as CL20, it is extremely sensitive and so far has not been produced in bulk. hnRNP See .
hoar frost A deposit of interlocking ice crystals formed on objects (such as branches of trees and hedgerows) that are exposed to air in which there is *supersaturation with water vapour. The formation of hoar frost is similar to the formation of dew with the difference that the temperature of the object on which the hoar frost forms is below freezing temperature, whereas this is not the case with dew. Hodgkin, Sir Alan See E, S J C. HOE See   . Hogness box See TATA . hole A vacant electron position in the lattice structure of a solid that behaves like a mobile positive *charge carrier with a negative *rest energy. See . holmium Symbol Ho. A soft silvery metallic element belonging to the *lanthanoids; a.n. 67; r.a.m. 164.93; r.d. 8.795 (20°C); m.p. 1474°C; b.p. 2695°C. It occurs in apatite, xenotime, and some other rare-earth minerals. There is one natural isotope, holmium– 165; eighteen artiÜcial isotopes have been produced. There are no uses for the element, which was discovered by Per Cleve (1840– 1905) and J. L. Soret in 1879.
Holocene (Recent) The most recent geological epoch of the *Neogene period, comprising roughly the past 11 500 years since the end of the *Pleistocene up to the present. It follows the Ünal glacial of the Pleistocene and thus is sometimes known as the Postglacial epoch. Some geologists consider the Holocene to be an interglacial phase of the Pleistocene that will be followed by another glacial. holocrine secretion See . holoenzyme A complex comprising an enzyme molecule and its *cofactor. Only in this state is an enzyme catalytically active. Compare . holographic hypothesis A key principle of *quantum gravity postulating that it is possible to obtain information about the bulk of a system, such as a black hole, from the surface of that system. This hypothesis was suggested as a general feature of quantum gravity in the mid-1990s and found
homeotic genes
to hold in superstring theory a few years later.
holographic optical element (HOE) An optical device consisting of a hologram, used to focus or deÛect electromagnetic radiation. Holographic optical elements are relatively easy to make and can function as lenses, mirrors, diffraction gratings, beam splitters, etc., in place of more traditional optical components. holography A method of recording and displaying a three-dimensional image of an object, usually using *coherent radiation from a *laser and photographic plates (see illustration). The light from a laser is divided so that some of it (the reference beam) falls directly on a photographic plate. The other part illuminates the object, which reÛects it back onto the photographic plate. The two beams form interference patterns on the plate, which when developed is called the hologram. To reproduce the image of the object, the hologram is illuminated by coherent light, ideally the original reference beam. The hologram produces two sets of diffracted waves; one set forms a virtual image coinciding with the original object position and the other forms a real image on the other side of the plate. Both are threedimensional. The method was invented by Dennis Gabor in 1948. More recent techniques can produce holograms visible in white light. hologram reference beam
semitransparent mirror laser
am be al n sig
hologram laser
virtual image
Holography. holohedral form The form of a crystal in which the full number of faces required for the symmetry are present. Compare  .
holophytic Describing organisms that feed like plants, i.e. that are photoautotrophic. See  . holotype See  . holozoic Describing organisms that feed by ingesting complex organic matter, which is subsequently digested and absorbed. See  ; . homeobox A nucleotide sequence containing about 180 base pairs, which are identical or very similar in many eukaryotic organisms, that encodes a series of amino acids known as a homeodomain. Present in many eukaryotic regulatory proteins, this sequence is an important region involved in the binding of regulatory proteins to the DNA molecule. A homeobox was Ürst identiÜed in the *homeotic genes of Drosophila and has since been found in the homeotic genes of many animals (including humans) and in plants. homeostasis The regulation by an organism of the chemical composition of its body Ûuids and other aspects of its *internal environment so that physiological processes can proceed at optimum rates. It involves monitoring changes in the external and internal environment by means of *receptors and adjusting the composition of the body Ûuids accordingly; *excretion and *osmoregulation are important in this process. Examples of homeostatic regulation are the maintenance of the *acid–base balance and body temperature (see ; ). homeotic genes A class of genes that play a central role in controlling the early development and differentiation of embryonic tissues in eukaryotic organisms. They include the Hox genes, which control the development of structures along the headto-tail (anteroposterior) axis of a wide range of animals. Homeotic genes code for proteins that bind DNA and regulate the expression of a wide range of other genes. This binding capability resides in a structural domain of the protein called a homeodomain, encoded by a nucleotide sequence (*homeobox) that is characteristic of homeotic genes. These genes were Ürst identiÜed in Drosophila fruit Ûies, through the occurrence of mutations that cause the transformation of one organ into another – the phenomenon of homeosis. In vertebrates there are four clusters of homeotic genes located on separate chromosomes.
hominid hominid Any member of the primate family Hominidae, which includes humans and their fossil ancestors (fossil hominids) in the genus *Homo. This family is now widely regarded as also including the extant great apes (chimpanzees, gorillas, orang-utan, formerly constituting the family Pongidae) and extinct groups, such as *Australopithecus and *Dryopithecus.
• Interactive documentary narrated by palaeontologist Donald Johanson and presented by the Institute of Human Origins, Arizona State University
Homo The genus of primates that includes modern humans (H. sapiens, the only living representative) and various extinct species. The oldest Homo fossils are those of H. habilis and H. rudolfensis, which Ürst appeared in Africa 2.2–2.4 million years ago. Both species used simple stone tools. H. habilis appears to have been 1–1.5 m tall and had more human-like features and a larger brain than *Australopithecus. H. erectus diverged from H. ergaster in Africa and subsequently spread to Asia between 1.8 and 1.5 million years ago. Fossils of H. erectus, which was formerly called Pithecanthropus (ape man), include Java man and Peking man. They are similar to present-day humans except that there was a prominent ridge above the eyes and no forehead or chin. They used crude stone tools and Üre. H. ergaster may also have given rise to H. heidelbergensis (represented by Heidelberg man and Boxgrove man). This species now contains all hominid specimens with a mixture of 'erectus-like' and 'modern' characters, dating from some 800 000 years ago to the emergence of H. sapiens between 130 000 and 90 000 years ago. Among them are the ancestors of both H. neanderthalensis (*Neanderthal man) and H. sapiens. See also C . homocyclic See . homodont Describing animals whose teeth are all of the same type. Most vertebrates except mammals are homodont. Compare . homogametic sex The sex that is determined by possession of two similar *sex chromosomes (e.g. XX). In humans and many other mammals this is the female sex. All the reproductive cells (gametes) produced by the homogametic sex have the same kind of sex chromosome (i.e. an X
chromosome). Compare  .
homogamy The condition in a Ûower in which the male and female reproductive organs mature at the same time, allowing selffertilization. Compare . homogeneous Relating to only one phase, e.g. a homogeneous mixture, a homogeneous *catalyst. Compare . homoiothermy The maintenance by an animal of its internal body temperature at a relatively constant value by using metabolic processes to counteract Ûuctuations in the temperature of the environment. Homoiothermy occurs in birds and mammals, which are described as *endotherms. The heat produced by their tissue metabolism and the heat lost to the environment are balanced by various means to keep body temperature constant: 36–38°C in mammals and 38–40°C in birds. The *hypothalamus in the brain monitors blood temperature and controls thermoregulation by both nervous and hormonal means. This produces both shortterm responses, such as shivering or sweating, and long-term adjustments to metabolism according to seasonal changes in climate (acclimatization). Endotherms generally possess insulating feathers or fur. Their relatively high internal temperature permits fast action of muscles and nerves and enables them to lead highly active lives even in cold climates. However, in certain animals, homoiothermy is abandoned during periods of *hibernation. Compare . homoleptic compound A chemical complex with only one type of ligand, as in nickel carbonyl or lead tetraethyl. homologous (in biology) Describing a character that is shared by a group of species because it is inherited from a common ancestor. Such characters, called homologies, are used in *cladistics to determine the evolutionary relationships of species or higher taxa. They are divided into two types: a shared derived homology is unique to a particular group and may be used to deÜne a *monophyletic group; a shared ancestral homology is not unique to the group, or may not be exhibited by all descendants of the ancestor in which it arose. Even though homologous features share the same evolutionary origin, they may have developed
horsetails
different functions. For example the wings of a bat, the Ûippers of a dolphin, and the arms of a human are homologous organs, having evolved from the paired pectoral Üns of a Üsh ancestor. Compare .
homologous chromosomes Chromosomes having the same pattern of genes along their lengths although the nature of the genes may differ (see ). In *diploid nuclei, pairs of homologous chromosomes form bivalents at the start of *meiosis (see ). One member of each pair comes from the female parent and the other from the male. homologous series A series of related chemical compounds that have the same functional group(s) but differ in formula by a Üxed group of atoms. For instance, the simple carboxylic acids: methanoic (HCOOH), ethanoic (CH3COOH), propanoic (C2H5COOH), etc., form a homologous series in which each member differs from the next by CH2. Successive members of such a series are called homologues. homolytic Üssion The breaking of a bond in a compound in which the fragments are uncharged free radicals. For example, Cl2 → Cl· + Cl·. Compare  . homoplasy The similarity of a particular character in two different, yet often related, groups of organisms that is not the result of common ancestry. Such a similarity may arise due to convergent or parallel evolution, and is therefore potentially misleading when examining shared characters in constructing phylogenetic trees (see ). For example, wings in bats and birds are a convergent, and therefore homoplasic, character. homopolar bond See  . homopolar generator A simple electric generator consisting of a metal disc rotating between two poles of a magnet. Contacts are made to the axle and the rim of the disc. A radial e.m.f. is produced. At constant rotational speed, the device produces a steady direct current and generators of this type are used in certain specialized applications. It can also be used as a simple motor if a direct current is supplied. A device of this type (known as the Barlow wheel) was invented in 1822 by the physicist Peter Barlow (1776–1862). This had a star-shaped wheel with the points of the star dipping into a pool of mercury to give the electrical con-
tact. A generator with a disc was used by Michael Faraday in his experiments, and the device is sometimes known as the Faraday disc.
homopolymer See . homozygous Describing an organism or cell in which the *alleles at a given locus on homologous chromosomes are identical (they may be either dominant or recessive). Homozygous organisms, which are called homozygotes, breed true when crossed with genetically identical organisms. Compare . Hooke, Robert (1635–1703) English physicist, who worked at Oxford University, where he assisted Robert *Boyle. Among his many achievements were the law of elasticity (see H' ), the watch balance wheel, and the compound *microscope. In 1665, using his microscope to study vegetable tissues, he saw 'little boxes', which he named 'cells'. Hooke's law The *stress applied to any solid is proportional to the *strain it produces within the elastic limit for that solid. The ratio of longitudinal stress to strain is equal to the Young modulus of elasticity (see  ). The law was Ürst stated by Robert Hooke in the form "Ut tensio, sic vis." hormone 1. A substance that is manufactured and secreted in very small quantities into the bloodstream by an *endocrine gland or a specialized nerve cell (see ) and regulates the growth or functioning of a speciÜc tissue or organ in a distant part of the body. For example, the hormone *insulin controls the rate and manner in which glucose is used by the body. Other hormones include the *sex hormones, *corticosteroids, *adrenaline, *thyroxine, and *growth hormone. 2. See  . hornblende Any of a group of common rock-forming minerals of the amphibole group with the generalized formula: (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2 Hornblendes consist mainly of calcium, iron, and magnesium silicate. horsepower (hp) An imperial unit of power originally deÜned as 550 foot-pound force per second; it is equal to 745.7 watts. horsetails See S.
horst horst See . host An organism whose body provides nourishment and shelter for a parasite (see ). A definitive (or primary) host is one in which an animal parasite becomes sexually mature; an intermediate (or secondary) host is one in which the parasite passes the larval or asexual stages of its life cycle. host–guest chemistry See  .
hot-wire instrument An electrical measuring instrument (basically an ammeter) in which the current to be measured is passed through a thin wire and causes its temperature to rise. The temperature rise, which is proportional to the square of the current, is measured by the expansion of the wire. Such instruments can be used for either direct current or alternating current.
display data on the *World Wide Web. It involves a set of rules for codes inserted into text Üles to indicate such things as special typefaces, paragraph breaks, illustration positions, and links to other *hypertext pages.
• The HTML (version 4.01) specification
HTTP Hypertext transport protocol: an application-level protocol with the lightness and speed necessary for distributed collaborative hypermedia information systems. It is generic, stateless, and object-oriented, with typing and negotiation of data representation, allowing systems to be built independently of the data being transferred. By extension of its request methods (commands), it can be used for many tasks, such as name servers and distributed objectmanagement systems. HTTP has been in use by the *World Wide Web since 1990.
Hox genes See  .
Hoyle, Sir Fred (1915–2001) British astronomer, who in 1958 became a professor of astronomy at Cambridge University. He is best known for his proposal in 1948, with Hermann Bondi (1919–2005) and Thomas Gold (1920–2004), of the *steady-state theory of the universe. He also carried out theoretical work on the formation of elements in stars.
Hubble, Edwin Powell (1889–1953) US astronomer, who worked at both the Yerkes Observatory and the Mount Wilson Observatory. Most of his studies involved nebulae and galaxies, which he classiÜed in 1926. In 1929 he established the *Hubble constant, which enabled him to estimate the age of the universe. The *Hubble space telescope is named after him.
HPLC See -  .
Hubble constant The rate at which the velocity of recession of the galaxies increases with distance as determined by the *redshift. The value is not agreed upon but current measurements indicate that it lies between 49 and 95 km s–1 per megaparsec. The reciprocal of the Hubble constant, the Hubble time, is a measure of the age of the universe, assuming that the expansion rate has remained constant. In fact, it is necessary to take account of the fact that the expansion of the universe is accelerating to get an accurate determination of its age.
HSAB principle A method of classifying Lewis acids and bases (see ) developed by Ralph Pearson in the 1960s. The acronym stands for 'hard and soft acids and bases'. It is based in empirical measurements of stability of compounds with certain ligands. Hard acids tend to complex with halide ions in the order F– > Cl– > Br– > I– Soft acids complex in the opposite order. Compounds that complex with hard acids are hard bases; ones that more readily form complexes with soft acids are called soft bases. In general, soft acids and bases are more easily polarized than hard acids and bases and consequently have more covalent character in the bond. The idea is an extension of the *type A and B metals concept to compounds other than metal complexes. HTML (hypertext markup language) The computer data format used to transfer and
• The HTTP (version 1.1) specification
Hubble space telescope An orbiting telescope managed by the US National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) and launched in April 1990, operating in the ultraviolet, visible, and near-infrared regions of the spectrum at high resolution free from the distorting effects of the earth's atmosphere. After Üve servicing missions (1993– 2009), the telescope continues to add to the wealth of spectacular images and other data
that it had already provided, expanding our understanding of the universe, especially its early phases.
Hückel rule See  . hue See . Hulse–Taylor pulsar A binary system discovered in 1974 consisting of a pulsar orbiting with a companion star. It was used to demonstrate, by measurements of a period of time, that the system was emitting gravitational waves. It is named after the US physicists Russell Hulse (1950– ) and Joseph Taylor (1941– ). human chorionic gonadotrophin (hCG) See . Human Genome Project A coordinated international project, begun in 1988, to map the entire human *genome so that the genes could be isolated and sequenced (see DNA ). It involved the production of a *DNA library. The haploid human genome contains about 3 × 109 nucleotide base pairs, but only 22 000 – 25 000 genes. The full draft sequence was completed in 2000 and published in February 2001, and the high-quality Ünished sequence was completed in April 2003, two years ahead of schedule. human growth hormone (hGH) See  . human immunodeÜciency virus See HIV. Hume-Rothery rules A set of empirical rules put forward in 1926 by the British metallurgist William Hume-Rothery (1899–1968) to describe how one metal dissolves in another metal. He found that one metallic element will not dissolve in another if the difference in their atomic radii is greater than 15 per cent or if the electronegativities of the two elements differ substantially. Also, a metal with a lower valency is more likely to dissolve in one with a higher valency, than vice versa. These rules were subsequently justiÜed by the quantum theory of electrons in solids. humerus The long bone of the upper arm of tetrapod vertebrates. It articulates with the *scapula (shoulder blade) at the *glenoid cavity and with the *ulna and *radius (via a *condyle) at the elbow. humidity The concentration of water vapour in the atmosphere. The absolute humidity is the mass of water vapour per unit
hurricane volume of air, usually expressed in kg m–3. Relative humidity is the ratio, expressed as a percentage, of the moisture in the air to the moisture it would contain if it were saturated at the same temperature and pressure. Specific humidity is also sometimes used: this is the mass of water vapour in the atmosphere per unit mass of air. See also .
humoral Relating to the blood or other body Ûuids. For example, humoral immunity is immunity conferred by the antibodies present in the blood, lymph, and tissue Ûuids (see ). humus The dark-coloured amorphous colloidal material that constitutes the organic component of soil. It is formed by the decomposition of plant and animal remains and excrement (see ) and has a complex and variable chemical composition. Being a colloid, it can hold water and therefore improves the water-retaining properties of soil; it also enhances soil fertility and workability. Acidic humus (mor) is found in regions of coniferous forest, where the decay is brought about mainly by fungi. Alkaline humus (mull) is typically found in grassland and deciduous forest: it supports an abundance of microorganisms and small animals (e.g. earthworms). Hund coupling cases See . Hund's rules Empirical rules in atomic *spectra that determine the lowest energy level for a conÜguration of two equivalent electrons (i.e. electrons with the same n and l quantum numbers), in a many-electron atom. (1) The lowest energy state has the maximum *multiplicity consistent with the *Pauli exclusion principle. (2) The lowest energy state has the maximum total electron *orbital angular momentum quantum number, consistent with rule (1). These rules were put forward by the German physicist Friedrich Hund (1896–1997) in 1925. hunting The oscillation of a gauge needle or engine speed about a mean value. In a rotating mechanism, set to operate at a constant speed, pulsation above and below the set speed can occur, especially if the speed is controlled by a governor. It can be corrected using a damping device. hurricane 1. A *tropical cyclone with surface wind speeds in excess of 64 knots (117 km/h) that occurs in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and eastern and central North PaciÜc Ocean
Hutton, James
(east of the dateline). (Tropical cyclones that occur in the North PaciÜc Ocean are known as typhoons.) Hurricanes cause widespread damage over land areas and are a considerable hazard to shipping. In order for a hurricane to form, an extensive area of ocean (at least 5° of latitude from the equator) must have a surface temperature in excess of 27°C. At the earth's surface a hurricane appears as a roughly circular vortex with the eye at the centre, in which pressure is very low. Winds spiral around this core at great speed in an anticlockwise direction in the northern hemisphere. A hurricane is usually accompanied by deep clouds and torrential rain over a wide area. 2. See B  .
Hutton, James (1726–97) British geologist, born in Scotland. He studied law, medicine, and industrial chemistry before returning to Edinburgh in 1768 to devote himself to geology. He advocated the theory that contemporary geological processes are the same as those that have always prevailed. This theory was not generally accepted until popularized by John Playfair (1748–1819) and championed by Charles *Lyell. Huxley, Sir Andrew See E, S J C. Huygens, Christiaan (1629–95) Dutch astronomer and physicist. Born in the Hague, he made several visits to Paris between 1655 and 1681 and in 1666 became a founding member of the French Academy of Sciences. In 1657 he designed the Ürst pendulum clock and wrote a description of it. He had already made improvements to astronomical telescopes, with which he discovered Saturn's satellite Titan in 1655 and the true shape of Saturn's rings in 1659. His greatest achievement, however, was the wave theory of light, announced in 1690 (see H' ). Huygens' construction (Huygens' principle) Every point on a wavefront may itself be regarded as a source of secondary waves. Thus, if the position of a wavefront at any instant is known, a simple construction enables its position to be drawn at any subsequent time. The construction was Ürst used by Christiaan Huygens. hyaline cartilage See . hyaluronic acid A *glycosaminoglycan (mucopolysaccharide) that is part of the
*ground substance of connective tissue. Hyaluronic acid is a major component of articular cartliage, helping in tissue repair and the lubrication of joints. It also plays a role in cell movement and proliferation.
hyaluronidase Any of a family of enzymes that break down *hyaluronic acid, thereby decreasing its viscosity and increasing the permeability of connective tissue. They are used in medicine to increase the absorption and diffusion of drugs administered by injection or application. hybrid The offspring of a mating in which the parents differ in at least one characteristic. The term is usually used of offspring of widely different parents, e.g. different varieties or species. Hybrids between different animal species are usually sterile. See also  . hybridization 1. The production of one or more *hybrid organisms by the mating of genetically different parents. 2. The production of hybrid cells. See   (somatic cell hybridization). 3. See DNA . hybridoma A type of hybrid cell that is produced by the fusion of a tumour cell (a myeloma cell) with a normal antibody-producing *B cell (see  ). The resulting hybrid cell line is able to produce large amounts of normal antibody, which is described as monoclonal (see  ) as it results from a cloned cell line. hybrid orbital See . hybrid vigour (heterosis) The increased vigour displayed by the offspring from a cross between genetically different parents. Hybrids from crosses between different crop varieties (F1 hybrids) are often stronger and produce better yields than the original varieties. Mules, the offspring of mares crossed with donkeys, have greater strength and resistance to disease and a longer lifespan than either parent. hydathode A pore found in the *epidermis of the leaves of certain plants. Like *stomata, hydathodes are surrounded by two crescent-shaped cells but these, unlike guard cells, do not regulate the size of the aperture. Hydathodes secrete water under conditions in which *transpiration is inhibited; for example, when the atmosphere is very humid. This process of water loss is called guttation.
hydracid See  . hydrate A substance formed by combination of a compound with water. See   . hydrated alumina See  . hydrated aluminium hydroxide See  . hydration See . hydraulic press A device in which a force (F1) applied to a small piston (A1) creates a pressure (p), which is transmitted through a Ûuid to a larger piston (A2), where it gives rise to a larger force (F2): see illustration. This depends on Pascal's principle that the pressure applied anywhere in an enclosed Ûuid is transmitted equally in all directions. The principle of the hydraulic press is widely used in jacks, vehicle brakes, presses, and earth-moving machinery, usually with oil as the working Ûuid.
monia reacts with sodium(I) chlorate (sodium hypochlorite) to give NH2Cl, which then undergoes further reaction with ammonia to give N2H4. Industrial production must be carefully controlled to avoid a side reaction leading to NH4Cl. The compound is a weak base giving rise to two series of salts, those based on N2H5+, which are stable in water (sometimes written in the form N2H4.HCl rather than N2H5+Cl–), and a less stable and extensively hydrolysed series based on N2H62+. Hydrazine is a powerful reducing agent and reacts violently with many oxidizing agents, hence its use as a rocket propellant. It reacts with aldehydes and ketones to give *hydrazones.
hydrazoic acid See  . hydrazones Organic compounds containing the group =C:NNH2, formed by condensation of substituted hydrazines with with aldehydes and ketones (see illustration). Phenylhydrazones contain the group =C:NNHC6H5.
• Information about IUPAC nomenclature F1
hydride A chemical compound of hydrogen and another element or elements. Nonmetallic hydrides (e.g. ammonia, methane, water) are covalently bonded. The alkali metals and alkaline earths (*s-block elements) form salt-like hydrides containing the hydride ion H–, which produce hydrogen on reacting with water. Hydride-forming *transition elements form interstitial hydrides, with the hydrogen atoms 'trapped' within the gaps in the lattice of metal atoms. Complex hydrides, such as *lithium tetrahydroaluminate(III), have hydride ions as *ligands; many are powerful reducing agents.
Hydraulic press. hydraulics The study of water or other Ûuids at rest or in motion, particularly with respect to their engineering uses. The study is based on the principles of *hydrostatics and *hydrodynamics. hydrazine A colourless liquid or white crystalline solid, N2H4; r.d. 1.01 (liquid); m.p. 1.4°C; b.p. 113.5°C. It is very soluble in water and soluble in ethanol. Hydrazine is prepared by the Raschig synthesis in which am-
hydriodic acid An acid made by dissolving *hydrogen iodide in water. On standing in air it is slowly oxidized to iodine (making the solution brown); it Ünds use as a reducing agent. hydrobromic acid See  . R″
R″ N
+ R′
H hydrazine
C R′ hydrazone
Hydrazones. Formation of a hydrazone from a ketone; the same reaction occurs with an aldehyde (R′ = H). If R″ = C6H5, the product is phenylhydrazone.
hydrocarbons hydrocarbons Chemical compounds that contain only carbon and hydrogen. A vast number of hydrocarbon compounds exist, the main types being the *alkanes, *alkenes, *alkynes, and *arenes. hydrochloric acid See  . hydrochloride See  . hydrochloroÛuorocarbon (HCFC) See . hydrocortisone See . hydrocyanic acid See  .
hydrodynamics The study of the motion of incompressible Ûuids and the interaction of such Ûuids with their boundaries. hydroelectric power Electric power generated by a Ûow of water. A natural waterfall provides a source of energy, in the form of falling water, which can be used to drive a water *turbine. This turbine can be coupled to a generator to provide electrical energy. Hydroelectric generators can be arranged to work in reverse so that during periods of low power demand current can be fed to the generator, which acts as a motor. This motor drives the turbine, which then acts as a pump. The pump then raises water to an elevated reservoir so that it can be used to provide extra power at peak-load periods. hydroÛuoric acid See  . hydroÛuorocarbon (HFC) See . hydrogen Symbol H. A colourless odourless gaseous chemical element; a.n. 1; r.a.m. 1.008; d. 0.0899 g dm–3; m.p. –259.14°C; b.p. –252.87°C. It is the lightest element and the most abundant in the universe. It is present in water and in all organic compounds. There are three isotopes: naturally occurring hydrogen consists of the two stable isotopes hydrogen–1 (99.985%) and *deuterium. The radioactive *tritium is made artiÜcially. The gas is diatomic and has two forms: orthohydrogen, in which the nuclear spins are parallel, and parahydrogen, in which they are antiparallel. At normal temperatures the gas is 25% parahydrogen. In the liquid it is 99.8% parahydrogen. The main source of hydrogen is steam *reforming of natural gas. It can also be made by the Bosch process (see H ) and by electrolysis of water. The main use is in the Haber process for making
ammonia. Hydrogen is also used in various other industrial processes, such as the reduction of oxide ores, the reÜning of petroleum, the production of hydrocarbons from coal, and the hydrogenation of vegetable oils. Considerable interest has also been shown in its potential use in a 'hydrogen fuel economy' in which primary energy sources not based on fossil fuels (e.g. nuclear, solar, or geothermal energy) are used to produce electricity, which is employed in electrolysing water. The hydrogen formed is stored as liquid hydrogen or as metal *hydrides. Chemically, hydrogen reacts with most elements. It was discovered by Henry Cavendish in 1776.
hydrogen acceptor See  . hydrogenation 1. A chemical reaction with hydrogen; in particular, an addition reaction in which hydrogen adds to an unsaturated compound. Nickel is a good catalyst for such reactions. 2. The process of converting coal to oil by making the carbon in the coal combine with hydrogen to form hydrocarbons. See F–T ; B . hydrogen azide (hydrazoic acid; azoimide) A colourless liquid, HN3; r.d. 1.09; m.p. –80°C; b.p. 37°C. It is highly toxic and a powerful reductant, which explodes in the presence of oxygen and other oxidizing agents. It is prepared by the reaction of sodium amide and sodium nitrate at 175°C followed by distillation of a mixture of the resulting sodium azide and a dilute acid. See also . hydrogen bomb See  . hydrogen bond A type of electrostatic interaction between electronegative atoms (Ûuorine, nitrogen, or oxygen) in one molecule and hydrogen atoms bound to electronegative atoms in another molecule. It is a strong dipole–dipole attraction caused by the electron-withdrawing properties of the electronegative atom. Thus, in the water molecule the oxygen atom attracts the electrons in the O–H bonds. The hydrogen atom has no inner shells of electrons to shield the nucleus, and there is an electrostatic interaction between the hydrogen proton and a lone pair of electrons on an oxygen atom in a neighbouring molecule. Each oxygen atom
has two lone pairs and can make hydrogen bonds to two different hydrogen atoms. The strengths of hydrogen bonds are about one tenth of the strengths of normal covalent bonds. Hydrogen bonding does, however, have signiÜcant effects on physical properties. Thus it accounts for the unusual properties of *water and for the relatively high boiling points of H2O, HF, and NH3 (compared with H2S, HCl, and PH3). It is also of great importance in living organisms. Hydrogen bonding occurs between bases in the chains of DNA (see  ). It also occurs between the C=O and N–H groups in proteins, and is responsible for maintaining the secondary structure.
hydrogen cyanide (hydrocyanic acid; prussic acid) A colourless liquid or gas, HCN, with a characteristic odour of almonds; r.d. 0.699 (liquid at 22°C); m.p. –14°C; b.p. 26°C. It is an extremely poisonous substance formed by the action of acids on metal cyanides. Industrially, it is made by catalytic oxidation of ammonia and methane with air and is used in producing acrylate plastics. Hydrogen cyanide is a weak acid (Ka = 2.1 × 10–9 mol dm–3). With organic carbonyl compounds it forms *cyanohydrins. hydrogen electrode See   .
0.177nm
hydrogen chloride A colourless fuming gas, HCl; m.p. –114.8°C; b.p. –85°C. It can be prepared in the laboratory by heating sodium chloride with concentrated sulphuric acid (hence the former name spirits of salt). Industrially it is made directly from the elements at high temperature and used in the manufacture of PVC and other chloro compounds. It is a strong acid and dissociates fully in solution (hydrochloric acid).
Hydrogen bonds. Shown as dotted lines between water molecules.
hydrogen bromide A colourless gas, HBr; m.p. –88.5°C; b.p. –67°C. It can be made by direct combination of the elements using a platinum catalyst. It is a strong acid dissociating extensively in solution (hydrobromic acid). hydrogencarbonate (bicarbonate) A salt of *carbonic acid in which one hydrogen atom has been replaced; it thus contains the hydrogencarbonate ion HCO3–. hydrogen carrier (hydrogen acceptor) A molecule that accepts hydrogen atoms or ions, becoming reduced in the process (see –). The *electron transport system, whose function is to generate energy in the form of ATP during respiration, involves a series of hydrogen carriers, including *NAD and *FAD, which pass on the hydrogen (derived from the breakdown of glucose) to the next carrier in the chain.
hydrogen Ûuoride A colourless liquid, HF; r.d. 0.99; m.p. –83°C; b.p. 19.5°C. It can be made by the action of sulphuric acid on calcium Ûuoride. The compound is an extremely corrosive Ûuorinating agent, which attacks glass. It is unlike the other hydrogen halides in being a liquid (a result of *hydrogen-bond formation). It is also a weaker acid than the others because the small size of the Ûuorine atom means that the H–F bond is shorter and stronger. Solutions of hydrogen Ûuoride in water are known as hydrofluoric acid. hydrogen half cell (hydrogen electrode) A type of *half cell in which a metal foil is immersed in a solution of hydrogen ions and hydrogen gas is bubbled over the foil. The standard hydrogen electrode, used in measuring standard *electrode potentials, uses a platinum foil with a 1.0 M solution of hydrogen ions, the gas at 1 atmosphere pressure, and a temperature of 25°C. It is written Pt(s)|H2(g), H+(aq), the effective reaction being H2 → 2H+ + 2e. hydrogenic Describing an atom or ion that has only one electron; for example, H, He+, Li2+, C5+. Hydrogenic atoms (or ions) do not involve electron–electron interactions and are easier to treat theoretically. hydrogen iodide A colourless gas, HI;
hydrogen ion m.p. –51°C; b.p. –35.38°C. It can be made by direct combination of the elements using a platinum catalyst. It is a strong acid, dissociating extensively in solution (hydroiodic acid), and a reducing agent.
hydrogen ion See ; H .
hydrogen molecule ion The simplest type of molecule (H2+), consisting of two hydrogen nuclei and one electron. In the *Born–Oppenheimer approximation, in which the nuclei are regarded as being Üxed, the *Schrödinger equation for the hydrogen molecule ion can be solved exactly. This enables ideas and approximation techniques concerned with chemical bonding to be tested quantitatively. hydrogen peroxide A colourless or pale blue viscous unstable liquid, H2O2; r.d. 1.44; m.p. –0.41°C; b.p. 150.2°C. As with water, there is considerable hydrogen bonding in the liquid, which has a high dielectric constant. It can be made in the laboratory by adding dilute acid to barium peroxide at 0°C. Large quantities are made commercially by electrolysis of KHSO4.H2SO4 solutions. Another industrial process involves catalytic oxidation (using nickel, palladium, or platinum with an anthraquinone) of hydrogen and water in the presence of oxygen. Hydrogen peroxide readily decomposes in light or in the presence of metal ions to give water and oxygen. It is usually supplied in solutions designated by volume strength. For example, 20-volume hydrogen peroxide would yield 20 volumes of oxygen per volume of solution. Although the *peroxides are formally salts of H2O2, the compound is essentially neutral. Thus, the acidity constant of the ionization H2O2 + H2O ˆ H3O+ + HO2– is 1.5 × 10–12 mol dm–3. It is a strong oxidizing agent, hence its use as a mild antiseptic and as a bleaching agent for cloth, hair, etc. It has also been used as an oxidant in rocket fuels. hydrogen spectrum The atomic spectrum of hydrogen is characterized by lines corresponding to radiation quanta of sharply deÜned energy. A graph of the frequencies at which these lines occur against the ordinal number that characterizes their position in the series of lines, produces a smooth curve indicating that they obey a formal law. In 1885 Johann Balmer (1825–98) discovered the law having the form:
1/λ = R(1/n12 – 1/n22) This law gives the so-called Balmer series of lines in the visible spectrum in which n1 = 2 and n2 = 3,4,5,…, λ is the wavelength associated with the lines, and R is the *Rydberg constant. In the Lyman series, discovered by Theodore Lyman (1874–1954), n1 = 1 and the lines fall in the ultraviolet. The Lyman series is the strongest feature of the solar spectrum as observed by rockets and satellites above the earth's atmosphere. In the Paschen series, discovered by Louis Paschen (1865– 1947), n1 = 3 and the lines occur in the far infrared. The Brackett series (n1 = 4), Pfund series (n1 = 5), and Humphreys series (n1 = 6) also occur in the far infrared.
• A translation of Balmer's 1885 paper on the spectral lines of hydrogen in Annalen der Physik und Chemie
hydrogensulphate (bisulphate) A salt containing the ion HSO4– or an ester of the type RHSO4, where R is an organic group. It was formerly called hydrosulphate. hydrogen sulphide (sulphuretted hydrogen) A gas, H2S, with an odour of rotten eggs; r.d. 1.54 (liquid); m.p. –85.5°C; b.p. –60.7°C. It is soluble in water and ethanol and may be prepared by the action of mineral acids on metal sulphides, typically hydrochloric acid on iron(II) sulphide (see K' ). Solutions in water (known as hydrosulphuric acid) contain the anions HS– and minute traces of S2– and are weakly acidic. Acid salts (those containing the HS– ion) are known as hydrogensulphides (formerly hydrosulphides). In acid solution hydrogen sulphide is a mild reducing agent. Hydrogen sulphide has an important role in traditional qualitative chemical analysis, where it precipitates metals with insoluble sulphides (in acid solution: Cu, Pb, Hg, Cd, Bi, As, Sb, Sn). The formation of a black precipitate with alkaline solutions of lead salts may be used as a test for hydrogen sulphide but the characteristic smell is usually sufÜcient. Hydrogen sulphide is exceedingly poisonous (more toxic than hydrogen cyanide). The compound burns in air with a blue Ûame to form sulphur(IV) oxide (SO2); solutions of hydrogen sulphide exposed to the air undergo oxidation but in this case only to elemental sulphur. North Sea gas contains some hydrogen sulphide (from S-proteins in
hydrophily
plants) as do volcanic emissions. See also C .
hydrogensulphite (bisulphite) A salt containing the ion –HSO3 or an ester of the type RHSO3, where R is an organic group. hydrography The study of the waters of the earth's surface, including the oceans, seas, lakes, and rivers. It involves the measurement of these features and the presentation of the information on hydrographic charts. hydroiodic acid See  . hydrolase Any of a class of enzymes that catalyse the addition of water to, or the removal of water from, a molecule. Hydrolases play an important role in the construction and breakdown of storage materials, such as starch. hydrological cycle (water cycle) The circulation of water between the atmosphere, land, and oceans on the earth (see illustration). Water evaporates from water bodies on earth to form water vapour in the atmosphere. This may condense to form clouds and be returned to the earth's surface as rainfall, hail, snow, etc. Some of this precipitation is returned to the atmosphere directly through evaporation or transpiration by plants; some Ûows off the land surface as overland Ûow, eventually to be returned to the oceans via rivers; and some inÜltrates the ground to Ûow underground forming groundwater storage. hydrology The scientiÜc study of terrestrial water, in particular inland water before
its discharge into the oceans or evaporation into the atmosphere. It includes the study of the occurrence and movement of water and ice on or under the earth's surface. The science has many important applications, for example in Ûood control, irrigation, domestic and industrial uses, and hydroelectric power.
hydrolysis A chemical reaction of a compound with water. For instance, salts of weak acids or bases hydrolyse in aqueous solution, as in Na+CH3COO– + H2O ˆ Na+ + OH– + CH3COOH The reverse reaction of *esteriÜcation is another example. See also . hydromagnesite A mineral form of basic *magnesium carbonate, 3MgCO3.Mg(OH)2. 3H2O. hydrometer An instrument for measuring the density or relative density of liquids. It usually consists of a glass tube with a long bulb at one end. The bulb is weighted so that the device Ûoats vertically in the liquid, the relative density being read off its calibrated stem by the depth of immersion. hydronium ion See  . hydrophilic Having an afÜnity for water. See . hydrophily A rare form of pollination in which pollen is carried to a Ûower by water. It occurs by one of two methods. In Canadian pondweed (Elodea canadensis) the male Ûowers break off and Ûoat downstream until they contact the female Ûowers. In Zostera, a
precipitation evapotranspiration from vegetation
overland flow water table
infiltrati o
evaporation from land, lakes, and water courses
evaporation from ocean
grou n
Hydrological cycle.
dwate r
hydrophobic marine species, the Ülamentous pollen grains are themselves carried in the water. Compare ; .
hydrophobic Lacking afÜnity for water. See .
hydrophyte Any plant that lives either in very wet soil or completely or partially submerged in water. Structural modiÜcations of hydrophytes include the reduction of mechanical and supporting tissues and vascular tissue, the absence or reduction of a root system, and specialized leaves that may be either Ûoating or Ünely divided, with little or no cuticle. Examples of hydrophytes are waterlilies and certain pondweeds. Compare ; ; . hydroponics A commercial technique for growing certain crop plants in culture solutions rather than in soil. The roots are immersed in an aerated solution containing the correct proportions of essential mineral salts. The technique is based on various water culture methods used in the laboratory to assess the effects of the absence of certain mineral elements on plant growth. hydroquinone See -,-. hydrosol A sol in which the continuous phase is water. See . hydrosphere The water on the surface of the earth. Some 74% of the earth's surface is covered with water, 97% (or some 1021 kilograms) of which is in the oceans. Icecaps and glaciers contain about 3 × 1019 kg, rivers about 1015 kg, lakes and inland seas about 2 × 1017 kg, and groundwater (down to 4000 metres) about 8 × 1019 kg. Water in the atmosphere contains only about 1016 kg. hydrostatics The study of liquids at rest, with special reference to storage tanks, dams, bulkheads, and hydraulic machinery. hydrostatic skeleton The system of support found in soft-bodied invertebrates, which relies on the incompressibility of Ûuids contained within the body cavity. For example, in earthworms the coelomic Ûuid is under pressure within the coelom and therefore provides support for internal organs. hydrosulphate See . hydrosulphide See  . hydrosulphuric acid See  . hydrotropism The growth of a plant part
in response to water. Roots, for example, grow towards water in the soil. See .
hydroxide A metallic compound containing the ion OH– (hydroxide ion) or containing the group –OH (hydroxyl group) bound to a metal atom. Hydroxides of typical metals are basic; those of *metalloids are amphoteric. hydroxoacid A type of acid in which the acidic hydrogen is on a hydroxyl group attached to an atom that is not attached to an oxo (=O) group. An example is Si(OH4) + H2O → Si(OH)3(O)– + H3O+. Compare . hydroxonium ion See  . 4-hydroxybutanoic acid (gammahydroxybutyric acid; GHB) A naturally occurring carboxylic acid, HO(CH2)3COOH, found in small amounts in most living things. It is used as a therapeutic drug to treat insomnia, depression, and alcoholism. GHB, as it is commonly known, is also used as an illegal club drug and as a date-rape drug. hydroxycerussite See (II)  . hydroxylamine A colourless solid, NH2OH, m.p. 33°C. It explodes on heating and may be employed as an oxidizing agent or reducing agent. Hydroxylamine is made by the reduction of nitrates or nitrites, and is used in the manufacture of nylon. With aldehydes and ketones it forms *oximes.
hydroxyl group The group –OH in a chemical compound. 2-hydroxypropanoic acid See  . 5-hydroxytryptamine See . hygrometer An instrument for measuring *humidity in the atmosphere. The mechanical type uses an organic material, such as human hair, which expands and contracts with changes in atmospheric humidity. The expansion and contraction is used to operate a needle. In the electric type, the change in resistance of a hygroscopic substance is used as an indication of humidity. In dew-point hygrometers a polished surface is reduced in temperature until water vapour from the atmosphere forms on it. The temperature of this dew point enables the relative humidity
of the atmosphere to be calculated. In the wet-and-dry bulb hygrometer, two thermometers are mounted side by side, the bulb of one being surrounded by moistened muslin. The thermometer with the wet bulb will register a lower temperature than that with a dry bulb owing to the cooling effect of the evaporating water. The temperature difference enables the relative humidity to be calculated. Only the dew-point hygrometer can be operated as an absolute instrument; all the others must ultimately be calibrated against this.
hygroscopic Describing a substance that can take up water from the atmosphere. See also . hymen A fold of mucous membrane that covers the opening of the vagina at birth. It normally perforates at puberty, to allow the Ûow of menstrual blood, but if the opening is small it may be ruptured during the Ürst occasion of sexual intercourse. Hymenoptera An order of insects that includes the ants, bees, wasps, ichneumon Ûies, and sawÛies. Hymenopterans generally have a narrow waist between thorax and abdomen. The smaller hindwings are interlocked with the larger forewings by a row of tiny hooks on the leading edges of the hindwings. Some species are wingless. The mouthparts are typically adapted for biting, although some advanced forms (e.g. bees) possess a tubelike proboscis for sucking liquid food, such as nectar. The long slender *ovipositor can serve for sawing, piercing, or stinging. Metamorphosis occurs via a pupal stage to the adult form. *Parthenogenesis is common in the group. Ants and some bees and wasps live in colonies, often comprising numerous individuals divided into *castes and organized into a coordinated and complex society. The colony of the honeybee (Apis mellifera), for example, consists of workers (sterile females), *drones (fertile males), and usually a single fertile female – the queen. The sole concern of the queen is egg laying. She determines the gender of the egg by either withholding or releasing stored sperm. Unfertilized eggs become males; fertilized eggs become females. The workers fulÜl a variety of tasks, including nursing the developing larvae, building the wax cells (combs) of the hive, guarding the colony, and foraging for nectar and pollen. The single function of the
larger drones is to mate with the young queen on her nuptial Ûight.
hyoid arch The second of seven bony Vshaped arches that support the gills of primitive vertebrates. In advanced vertebrates, part of the hyoid arch has evolved to form the stapes (one of the *ear ossicles). The rest of it forms the hyoid bone, which supports the tongue. hyper- A preÜx denoting over, above, high; e.g. hypersonic, hyperpolarization, hypertonic. hyperbola A *conic with eccentricity e > 1. It has two branches (see graph). For a hyperbola centred at the origin, the transverse axis runs along the x-axis between the vertices and has length 2a. The conjugate axis runs along the y-axis and has length 2b. There are two foci on the x-axis at (ae, 0) and (–ae, 0). The latus rectum, the chords through the foci perpendicular to the transverse axis, have length 2b2/a. The equation of the hyperbola is: x2/a2 – y2/b2 = 1, and the asymptotes are y = ± bx/a. transverse axis
conjugate axis
a (–ae, 0)
latus rectum x
Hyperbola. hyperbolic functions A set of functions, sinh, cosh, and tanh, that have similar properties to *trigonometric functions but are related to the hyperbola in the manner that trigonometric functions are related to the circle. The hyperbolic sine (sinh) of the angle x is deÜned by: sinhx = ½(ex – e–x). Similarly, coshx = ½(ex + e–x) tanhx = (ex – e–x)/(ex + e–x) Hyperbolic secant (sech), cosecant (cosech), and cotangent (coth) are the reciprocals of cosh, sinh, and tanh, respectively.
hypercharge A quantized property of *baryons (see  ) that
hyperÜne structure
provides a formal method of accounting for the nonoccurrence of certain expected decays by means of the strong interaction (see  ). Hypercharge is in some respects analogous to electric charge but it is not conserved in weak interactions. Nucleons have a hypercharge of +1, and the *pion has a value of 0.
hyperÜne structure See  .
hypermetropia (hyperopia) Long-sightedness, in which the lens of the eye is unable to accommodate sufÜciently to throw the image of near objects onto the retina. It is caused usually by shortness of the eyeball rather than any fault in the lens system. Spectacles with converging lenses are required to focus the image onto the surface of the retina. lens retina nearby object image formed behind retina
converging lens
hypersonic Denoting a velocity in excess of Mach 5 (see M ). Hypersonic flight is Ûight at hypersonic speeds in the earth's atmosphere. hypertension See  . hypertext A technique by which textual documents can be created and viewed on a computer screen so that one or more documents can be browsed in any order by the selection of key words or phrases by the user. The selected text leads (by underlying searches through associated Üles, indexes, etc.) to the display of another part of the document, or of some other document. Hypermedia is an extension of this technique enabling links to be made between text, images, sounds, etc. See also W W W. hypertext markup language See HTML. hypertonic solution A solution that has a higher osmotic pressure than some other solution. Compare  .
hypersensitivity Increased or abnormal sensitivity to compounds, which can elicit a speciÜc immune response accompanied by tissue damage. Hypersensitivity reactions include *allergies and *anaphylaxis.
sharp image on retina eye
Hypermetropia. hypernetted chain approximation An approximation used in the theory of Ûuids relating functions that give the correlation between two particles to radial distribution functions. hypernova An explosive event in which a large star collapses with the formation of a *black hole. An event of this type would be even more violent than a *supernova (in which a star collapses to form a neutron star). Hypernovae may be the cause of gamma-ray bursts. hyperon A shortlived *elementary particle; it is classiÜed as a *baryon and has a nonzero *strangeness. hyperplasia Increase in the size of a tissue or organ due to an increase in the number of its component cells. Compare .
hypertrophy An increase in the size of a tissue or organ due to an increase in the size of its component cells. Hypertrophy often occurs in response to an increased workload in an organ, which may result from malfunction or disease. Compare . hyperventilation An increase in the amount of air taken into the lungs caused by an increase in the depth or rate of breathing. See also . hypha A delicate Ülament in fungi many of which may form either a loose network (*mycelium) or a tightly packed interwoven mass of *pseudoparenchyma, as in the fruiting body of mushrooms. Hyphae may be branched or unbranched and may or may not possess cross walls. The cell wall consists of either fungal cellulose or *chitin. The cell wall is lined with cytoplasm, which often contains oil globules and glycogen, and there is a central vacuole. The hyphae produce enzymes that in parasitic fungi digest the host tissue, and in saprotrophic fungi digest dead organic matter. hypo- A preÜx denoting under, below, low; e.g. hypogyny, hyponasty, hypotonic.
hypochlorite See . hypochlorous acid See (I) . hypocotyl The region of the stem beneath the stalks of the seed leaves (*cotyledons) and directly above the young root of an embryo plant. It grows rapidly in seedlings showing *epigeal germination and lifts the cotyledons above the soil surface. In this region (the transition zone) the arrangement of vascular bundles in the root changes to that of the stem. Compare . hypodermis (exodermis) The outermost layer of cells in the plant *cortex, lying immediately below the epidermis. These cells are sometimes modiÜed to give additional structural support or to store food materials or water. After the loss of the *piliferous layer of the root the hypodermis takes over the protective functions of the epidermis. hypogeal 1. Describing seed germination in which the seed leaves (cotyledons) remain below ground. Examples of hypogeal germination are seen in oak and runner bean. Compare . 2. Describing fruiting bodies that develop underground, such as trufÛes and peanuts. hypolimnion The lower layer of water in a lake. See . Compare . hypophosphorus acid See  . hypophysis See  . hyposulphite See . hyposulphurous acid See  . hypotenuse The longest side of a rightangle triangle. hypothalamus Part of the vertebrate brain that is derived from the *forebrain and located on the ventral surface below the *thalamus and the *cerebrum. The hypothalamus regulates a wide variety of physiological processes, including maintenance of body temperature, water balance, sleeping, and feeding, via both the *autonomic nervous system (which it controls) and the *neuroendocrine system. Its endocrine functions are largely mediated by the *pituitary gland. The pituitary responds to releasing hormones produced by the hypothalamus, which in this way indirectly controls hormone production in other glands.
hypothesis See , ,  . hypothesis test In *statistics, a method of assessing how plausible a null hypothesis or statement is by comparing it with a test sample. For example, a null hypothesis will be accepted in a test carried out at the 10% signiÜcance level if the value of the sample is what can be expected from 90% of all random samples. hypotonic solution A solution that has a lower osmotic pressure than some other solution. Compare  . hypsometer A device for calibrating thermometers at the boiling point of water. As the boiling point depends on the atmospheric pressure, which in turn depends on the height above sea level, the apparatus can be used to measure height above sea level. hysteresis A phenomenon in which two physical quantities are related in a manner that depends on whether one is increasing or decreasing in relation to the other. The repeated measurement of *stress against *strain, with the stress Ürst increasing and then decreasing, will produce for some specimens a graph that has the shape of a closed loop. This is known as a hysteresis cycle. The most familiar hysteresis cycle, however, is produced by plotting the magnetic Ûux density (B) within a ferromagnetic material against the applied magnetic Üeld strength (H). If the material is initially unmagnetized at O it will reach saturation at P as H is increased. As the Üeld is reduced and again increased the loop PQRSTP is formed (see graph). The area of this loop is proportional to the energy loss (hysteresis loss) occurring P
B Q
R O
Hysteresis.
hysteresis during the cycle. The value of B equal to OQ is called the remanance (or retentivity) and is the magnetic Ûux density remaining in the material after the saturating Üeld has been reduced to zero. This is a measure of the tendency of the magnetic domain patterns (see ) to remain distorted even after
the distorting Üeld has been removed. The value of H equal to OR is called the coercive force (or coercivity) and is the Üeld strength required to reduce the remaining Ûux density to zero. It is a measure of the difÜculty of restoring the symmetry of the domain patterns.
I IAA (indoleacetic acid) See . ice See . ice age A period in the earth's history during which ice advanced towards the equator and a general lowering of temperatures occurred. The last major ice age, that of the Pleistocene epoch (sometimes known as the Ice Age), ended about 10 000 years ago. At least four major ice advances (glacials) occurred during the Pleistocene; these were separated by interglacials during which the ice retreated and temperatures rose. At present it is not known if the earth is between ice ages or is in an interglacial of the Pleistocene Ice Age. It has been established that ice ages also occurred during the Precambrian (over 500 million years ago) and during the Permo-Carboniferous (about 250 million years ago). ice point The temperature at which there is equilibrium between ice and water at standard atmospheric pressure (i.e. the freezing or melting point under standard conditions). It was used as a Üxed point (0°) on the Celsius scale, but the kelvin and the International Practical Temperature Scale are based on the *triple point of water. ichthyosaur Any extinct aquatic reptile belonging to the order Ichthyosauria, which lived during the Mesozoic era (250–90 million years ago). The earliest forms resembled lizards with Ûippers and probably swam in an eel-like fashion. Their successors evolved to become progressively more Üshlike, with thickened bodies, Ünlike fore and hind limbs, and crescent-shaped tails. Fossilized skeletal remains show that the eyes were unusually large and adapted for hunting in poor visibility. Ichthyosaurs typically lived on a diet of squid, and some species attained lengths of over 15 m. iconoscope A form of television camera tube (see ) in which the beam of light from the scene is focused on to a thin mica plate. One side of the plate is faced with a thin metallic electrode, the other side being coated with a mosaic of small globules of a photoemissive material. The light beam
falling on the mosaic causes photoemission of electrons, creating a pattern of positive charges in what is effectively an array of tiny capacitors. A high-velocity electron beam scans the mosaic, discharging each capacitor in turn through the metallic electrode. The resulting current is fed to ampliÜcation circuits, the current from a particular section of the mosaic depending on the illumination it has received. In this way the optical information in the light beam is converted into an electrical signal.
ICSH See  . ideal crystal A single crystal with a perfectly regular lattice that contains no impurities, imperfections, or other defects. ideal gas (perfect gas) A hypothetical gas that obeys the *gas laws exactly. An ideal gas would consist of molecules that occupy negligible space and have negligible forces between them. All collisions made between molecules and the walls of the container or between molecules and other gas molecules would be perfectly elastic, because the molecules would have no means of storing energy except as translational kinetic energy. ideal solution See R' . identical twins (monozygotic twins) Two individuals that develop from a single fertilized egg cell by its division into two genetically identical parts. Each part eventually gives rise to a separate individual and these twins have identical DNA sequences. However, differences in patterns of methylation and acetylation of their DNA can result in differences in gene expression, which may explain observed differences in looks, etc. Compare  . identity Symbol ≡. A statement of equality that applies for all values of the unknown quantity. For example, 5y ≡ 2y + 3y. idiogram See . idiosyncrasy An abnormal reaction to a drug or other foreign substance shown by an individual, which is usually genetically determined. An individual that shows im-
IE munological idiosyncrasy is said to be hypersensitive to a particular substance, agent, etc.
IE Ionization energy. See  . Ig See . IGF (insulin-like growth factor) See  .
igneous rocks A group of rocks formed from the crystallization of magma (molten silicate liquid). Igneous rocks form one of the three major rock categories (see also  ;  ). According to the depth at which the magma solidiÜes, igneous rocks may be classiÜed as plutonic, hypabyssal, or volcanic. Plutonic rocks solidify slowly at great depth, typically forming large intrusive masses (e.g. batholiths and stocks), and generally have the coarsest texture. Examples of plutonic rocks are granite, syenite, diorite, and gabbro. Volcanic (extrusive) rocks are formed from magma that has been poured out onto the earth's surface; these rocks (e.g. basalt, andesite) are characteristically Üne-grained or glassy as a result of their rapid cooling. Hypabyssal rocks (e.g. diorite), which cool at shallower depths than the plutonic, are intermediate in character and mediumgrained. They commonly occur in the form of small intrusions, such as dykes and sills. Igneous rocks may also be classiÜed chemically according to their silica content as acidic (over 66% silica), intermediate (55–66%), basic (45–55%), or ultrabasic (under 45%). ignition temperature 1. The temperature to which a substance must be heated before it will burn in air. 2. The temperature to which a *plasma has to be raised in order that nuclear fusion will occur. ileum The portion of the mammalian *small intestine that follows the *jejunum and precedes the *large intestine. It is a site of digestion and absorption. The internal lining of the ileum bears numerous small outgrowths (see ), which increase its absorptive surface area. ilium The largest of the three bones that make up each half of the *pelvic girdle. The ilium bears a Ûattened wing of bone that is attached by ligaments to the sacrum (see  ). See also ; . illuminance (illumination) Symbol E. The
energy in the form of visible radiation reaching a surface per unit area in unit time; i.e. the luminous Ûux per unit time. It is measured in *lux (lumens per square metre).
image A representation of a physical object formed by a lens, mirror, or other optical instrument. If the rays of light actually pass through the image, it is called a real image. If a screen is placed in the plane of a real image it will generally become visible. If the image is seen at a point from which the rays appear to come to the observer, but do not actually do so, the image is called a virtual image. No image will be formed on a screen placed at this point. Images may be upright or inverted and they may be magnified or diminished. image converter An electronic device in which an image formed by invisible radiation (usually gamma rays, X-rays, ultraviolet, or infrared) is converted into a visible image. Commonly the invisible radiation is focused on to a photocathode, which emits electrons when it is exposed to the radiation. These electrons fall on a Ûuorescent anode screen, after acceleration and focusing by a system of electron lenses. The Ûuorescent screen produces a visible image. The device is used in Ûuoroscopes, infrared telescopes, ultraviolet microscopes, and other devices. imaginary number A number that is a multiple of √–1, which is denoted by i; for example √–3 = i√3. See also  . imago The adult sexually mature stage in the life cycle of an insect after metamorphosis. imbibition The uptake of water by substances that do not dissolve in water, so that the process results in swelling of the substance. Imbibition is a property of many biological substances, including cellulose (and other constituents of plant cell walls), starch, and some proteins. It occurs in dry seeds before they germinate and – together with osmosis – is responsible for the uptake of water by growing plant cells. imides Organic compounds containing the group –CO.NH.CO.– (the imido group).
imido group See . imines Compounds containing the group –NH– in which the nitrogen atom is part of a
ring structure, or the group =NH, in which the nitrogen atom is linked to a carbon atom by a double bond. In either case, the group is referred to as an imino group.
imino group See . immersion objective An optical microscope objective in which the front surface of the lens is immersed in a liquid on the cover slip of the microscope specimen slide. Cedar-wood oil (for an oil-immersion lens) or sugar solution is frequently used. It has the same refractive index as the glass of the cover slip, so that the object is effectively immersed in it. The presence of the liquid increases the effective aperture of the objective, thus increasing the resolution. immune response The reaction of the body to foreign or potentially dangerous substances (*antigens), particularly diseaseproducing microorganisms. See . immunity The ability of an animal to resist infection or to counter the harmful effects of toxins produced by infecting organisms. Immunity depends on the presence in the body of a range of defensive cells and substances, notably *antibodies and white blood cells (*lymphocytes), which produce an immune response. Innate (inherited or natural) immunity is the body's Ürst line of defence and plays a vital role in controlling invading organisms during the early stages of an infection. Phagocytic macrophages can engulf bacteria and other microorganisms and induce them to secrete an array of *cytokines. These substances attract additional immune cells to the site and initiate the process of *inÛammation. The *complement system of defensive proteins is also activated. The innate response plays a crucial role in promoting adaptive (or acquired) immunity. Dendritic cells ingest foreign material, such as bacteria and virus particles (see ), and present it to T cells as part of the immune response; adaptive immunity can persist throughout the lifetime of the individual. Active immunity arises when the body produces antibodies against an invading foreign substance (antigen), either through infection or *immunization. Humoral immunity is when B lymphocytes produce free antibodies that circulate in the bloodstream (see B ); cell-mediated immunity is caused by the action of T lymphocytes (see T ).
Passive immunity is induced by injection of serum taken from an individual already immune to a particular antigen; it can also be acquired by the transfer of maternal antibodies to offspring via the placenta or breast milk (see ). Active immunity tends to be long-lasting; passive immunity is short-lived. See also .
• A series of colour slides illustrating the working of the immune system, produced by the US National Cancer Institute
immunization The production of *immunity in an individual by artiÜcial means. Active immunization (vaccination) involves the introduction, either orally or by injection (inoculation), of specially treated bacteria, viruses, or their toxins to stimulate the production of *antibodies (see ). Passive immunization is induced by the injection of preformed antibodies. immunoassay Any of various techniques that measure the amount of a particular substance by virtue of its binding antigenically to a speciÜc antibody. In solid-phase immunoassay the speciÜc antibody is attached to a solid supporting medium, such as a PVC sheet. The sample is added and any test antigens will bind to the antibody. A second antibody, speciÜc for a different site on the antigen, is added. This carries a radioactive or Ûuorescent label, enabling its concentration, and thus that of the test antigen, to be determined by comparison with known standards. Variations on this technique include *ELISA and *Western blotting. immunoglobulin (Ig) One of a group of proteins (*globulins) in the body that act as *antibodies. They are produced by specialized white blood cells called *B cells and are present in blood serum and other body Ûuids. There are several classes (e.g. IgE, IgG, and IgM) having different functions. immunosuppression The suppression of an *immune response. Immunosuppression is necessary following organ transplants in order to prevent the host rejecting the grafted organ (see ); it is artiÜcially induced by radiation or chemical agents that inhibit cell division of *lymphocytes. Immunosuppression occurs naturally in certain diseases, notably *AIDS. impact printer See . impedance Symbol Z. The quantity that
imperfect fungi measures the opposition of a circuit to the passage of a current and therefore determines the amplitude of the current. In a d.c. circuit this is the resistance (R) alone. In an a.c. circuit, however, the *reactance (X) also has to be taken into account, according to the equation: Z2 = R2 + X2, where Z is the impedance. The complex impedance is given by Z = R + iX, where i = √–1. The real part of the complex impedance, the resistance, represents the loss of power according to *Joule's law. The ratio of the imaginary part, the reactance, to the real part is an indication of the difference in phase between the voltage and the current.
imperfect fungi See D.
Imperial units The British system of units based on the pound and the yard. The former f.p.s. system was used in engineering and was loosely based on Imperial units; for all scientiÜc purposes *SI units are now used. Imperial units are also being replaced for general purposes by metric units. implant Any substance, device, or tissue that is inserted into the body. For example, drug implants and heart pacemakers are typically inserted under the skin. implantation (nidation) (in embryology) The embedding of a fertilized mammalian egg into the wall of the uterus (womb) where it will continue its development. After fertilization in the fallopian tube the egg passes into the womb in the form of a ball of cells (blastocyst). Its outer cells destroy cells of the uterine wall, forming a cavity into which the blastocyst sinks. implosion An inward collapse of a vessel, especially as a result of evacuation. imprinting 1. (in behaviour) A specialized form of learning in which young animals, during a particularly sensitive period in their early development, learn to recognize and approach some large moving object nearby. In nature this is usually the mother, though simple models or individuals of a different species (including humans) may sufÜce. Imprinting was Ürst described by Konrad *Lorenz, working with young ducks and geese. See  (Feature). 2. (in genetics) See  . impulse 1. (in physiology) (nerve impulse) The signal that travels along the length of a *nerve Übre and is the means by which information is transmitted through
the nervous system. It is marked by the Ûow of ions across the membrane of the *axon caused by changes in the permeability of the membrane, producing a reduction in potential difference that can be detected as the *action potential. The strength of the impulse produced in any nerve Übre is constant (see -- ). 2. (in physics) Symbol J. The product of a force F and the time t for which it acts. If the force is variable, the impulse is the integral of Fdt from t0 to t1. The impulse of a force acting for a given time interval is equal to the change in momentum produced over that interval, i.e. J = m(v1 – v0), assuming that the mass (m) remains constant while the velocity changes from v0 to v1.
IMS See - . inbreeding Mating between closely related individuals, the extreme condition being self-fertilization, which occurs in many plants and some primitive animals. A population of inbreeding individuals generally shows less variation than an *outbreeding population. Continued inbreeding among a normally outbreeding population leads to inbreeding depression (the opposite of *hybrid vigour) and an increased incidence of harmful characteristics. For example, in humans, certain mental and other defects tend to occur more often in families with a history of cousin marriages. incandescence The emission of light by a substance as a result of raising it to a high temperature. An incandescent lamp is one in which light is emitted by an electrically heated Ülament. See  . incisor A sharp Ûattened chisel-shaped *tooth in mammals that is adapted for biting food and – in rodents – for gnawing. In humans there are normally two pairs of incisors (central and lateral) in each jaw. See  . inclination 1. See . 2. The angle between the orbital plane of a planet, satellite, or comet and the plane of the earth's *ecliptic. inclusive Ütness The quality that organisms attempt (unconsciously) to maximize as the result of natural selection acting on genes that are inÛuential in controlling their behaviour and physiology. It includes the individual's own reproductive success (usually taken as the number of its offspring that survive to adulthood) and also the effects of
the individual's actions on the reproductive success of its relatives, because relatives have a higher probability of sharing some identical genes with the individual than do other members of the population. When interactions between relatives are likely to occur (which happens during the lives of many animals and plants) *kin selection will operate.
incoherent scattering See  . incompatibility 1. The condition that exists when foreign grafts or blood transfusions evoke a marked *immune response and are rejected. 2. The phenomenon in which pollen from one Ûower fails to fertilize other Ûowers on the same plant (self-incompatibility) or on other genetically similar plants. This genetically determined mechanism prevents self-fertilization (breeding between likes) and promotes cross-fertilization (breeding between individuals with different genetic compositions). See also ; ; . incomplete dominance The condition that arises when neither *allele controlling a characteristic is dominant and the aspect displayed by the organism results from the partial inÛuence of both alleles. For example, a snapdragon plant with alleles for red and for white Ûowers produces pink Ûowers. Compare . incubation 1. The process of maintaining the fertilized eggs of birds and of some reptiles and egg-laying mammals at the optimum temperature for the successful development of the embryos. A period of incubation follows the laying of the eggs and precedes their hatching. 2. The process of maintaining a *culture of bacteria or other microorganisms at the optimum temperature for growth of the culture. 3. The phase in the development of an infectious disease between initial infection and the appearance of the Ürst symptoms. incus (anvil) The middle of the three *ear ossicles of the mammalian middle ear. indeÜnite inÛorescence See  . indeÜnite integral See . indehiscent Describing a fruit or fruiting body that does not open to release its seeds or spores when ripe. Instead, release occurs
indicator when the fruit wall decays or, if eaten by an animal, is digested. Compare .
indene A colourless Ûammable hydrocarbon, C9H8; r.d. 0.996; m.p. –1.8°C; b.p. 182.6°C. Indene is an aromatic hydrocarbon with a Üve-membered ring fused to a benzene ring. It is present in coal tar and is used as a solvent and raw material for making other organic compounds. independent assortment The separation of the alleles of one gene into the reproductive cells (gametes) independently of the way in which the alleles of other genes have segregated. By this process all possible combinations of alleles should occur equally frequently in the gametes. In practice this does not happen because alleles situated on the same chromosome tend to be inherited together. However, if the allele pairs Aa and Bb are on different chromosomes, the combinations AB, Ab, aB, and ab will normally be equally likely to occur in the gametes. See ; M' . indeterminacy See  . index fossil (zone fossil) An animal *fossil of a group that existed continuously during a particular span of geological time and can therefore be used to date the rock in which it is found. Index fossils are found chieÛy in sedimentary rocks. They are an essential tool in stratigraphy for comparing the geological ages of sedimentary rock formations. Examples are *ammonites and *graptolites. indicator A substance used to show the presence of a chemical substance or ion by its colour. Acid–base indicators are compounds, such as phenolphthalein and methyl orange, that change colour reversibly, depending on whether the solution is acidic or basic. They are usually weak acids in which the un-ionized form HA has a different colour from the negative ion A–. In solution the indicator dissociates slightly HA ˆ H+ + A– In acid solution the concentration of H+ is high, and the indicator is largely undissociated HA; in alkaline solutions the equilibrium is displaced to the right and A– is formed. Useful acid–base indicators show a sharp colour change over a range of about 2 pH units. In titration, the point at which the reaction is complete is the equivalence point (i.e. the point at which equivalent
indicator species quantities of acid and base are added). The end point is the point at which the indicator just changes colour. For accuracy, the two must be the same. During a titration the pH changes sharply close to the equivalence point, and the indicator used must change colour over the same range. Other types of indicator can be used for other reactions. Starch, for example, is used in iodine titrations because of the deep blue complex it forms. Oxidation–reduction indicators are substances that show a reversible colour change between oxidized and reduced forms. See also  .
indicator species A plant or animal species that is very sensitive to a particular environmental factor, so that its presence (or absence) in an area can provide information about the levels of that factor. For example, some lichens are very sensitive to the concentration of sulphur dioxide (a major pollutant) in the atmosphere. Examination of the lichens present in an area can provide a good indication of the prevailing levels of sulphur dioxide. indigenous Describing a species that occurs naturally in a certain area, as distinct from one introduced by humans; native. indigo A blue vat dye, C16H10N2O2. It occurs as the glucoside indican in the leaves of plants of the genus Indigofera, from which it was formerly extracted. It is now made synthetically. indium Symbol In. A soft silvery element belonging to group 13 (formerly IIIB) of the periodic table; a.n. 49; r.a.m. 114.82; r.d. 7.31 (20°C); m.p. 156.6°C; b.p. 2080±2°C. It occurs in zinc blende and some iron ores and is obtained from zinc Ûue dust in total quantities of about 40 tonnes per annum. Naturally occurring indium consists of 4.23% indium–113 (stable) and 95.77% indium–115 (half-life 6 × 1014 years). There are a further Üve short-lived radioisotopes. The uses of the metal are small – some special-purpose electroplates and some special fusible alloys. Several semiconductor compounds are used, such as InAs, InP, and InSb. With only three electrons in its valency shell, indium is an electron acceptor and is used to dope pure germanium and silicon; it forms stable indium(I), indium(II), and indium(III) compounds. The element was discovered in 1863 by Ferdinand Reich (1799–1882) and Hieronymus Richter (1824–90).
indole A yellow solid, C8H7N, m.p. 52°C. Its molecules consist of a benzene ring fused to a nitrogen-containing Üve-membered ring. It occurs in some plants and in coal tar, and is produced in faeces by bacterial action. It is used in making perfumes. Indole has the nitrogen atom positioned next to the fused benzene ring. An isomer with the nitrogen two atoms away from the fused ring is called isoindole. indoleacetic acid (IAA) See . induced emission (stimulated emission) The emission of a photon by an atom in the presence of *electromagnetic radiation. The atom can become excited by the absorption of a photon of the right energy and, having become excited, the atom can emit a photon. The rate of absorption is equal to the rate of induced emission, both rates being proportional to the density of photons of the electromagnetic radiation. The relation between induced emission and *spontaneous emission is given by the *Einstein coefÜcients. The process of induced emission is essential for the operation of *lasers and *masers. See also    . induced Üssion See  . inducer See . inductance The property of an electric circuit or component that causes an e.m.f. to be generated in it as a result of a change in the current Ûowing through the circuit (selfinductance) or of a change in the current Ûowing through a neighbouring circuit with which it is magnetically linked (mutual inductance). In both cases the changing current is associated with a changing magnetic Üeld, the linkage with which in turn induces the e.m.f. In the case of self inductance, L, the e.m.f., E, generated is given by E = –L.dI/dt, where I is the instantaneous current and the minus sign indicates that the e.m.f. induced is in opposition to the change of current. In the case of mutual inductance, M, the e.m.f., E1, induced in one circuit is given by E1 = –M.dI2/dt, where I2 is the instantaneous current in the other circuit. induction 1. (in embryology) The ability of natural stimuli to cause unspecialized embryonic tissue to develop into specialized tis-
sue. 2. (in obstetrics) The initiation of childbirth by artiÜcial means; for example, by injection of the hormone *oxytocin. 3. (in physics) A change in the state of a body produced by a Üeld. See  ; .
induction coil A type of *transformer used to produce a high-voltage alternating current or pulses of high-voltage current from a low-voltage direct-current source. The induction coil is widely used in sparkignition *internal-combustion engines to produce the spark in the sparking plugs. In such an engine the battery is connected to the primary winding of the coil through a circuit-breaking device driven by the engine and the e.m.f. generated in the secondary winding of the coil is led to the sparking plugs through the distributor. The primary coil consists of relatively few turns, whereas the secondary consists of many turns of Üne wire. induction heating The heating of an electrically conducting material by *eddy currents induced by a varying electromagnetic Üeld. Induction heating may be an undesirable effect leading to power loss in transformers and other electrical devices. It is, however, useful for melting and heattreating and in forging and rolling metals, as well as for welding, brazing, and soldering. The material to be heated is inserted into a coil through which an alternating current Ûows and acts as the short-circuited secondary of a *transformer. Eddy currents induced in the material within the coil cause the temperature of the material to rise. induction motor See  . inductive effect The effect of a group or atom of a compound in pulling electrons towards itself or in pushing them away. Inductive effects can be used to explain some aspects of organic reactions. For instance, electron-withdrawing groups, such as –NO2, –CN, –CHO, –COOH, and the halogens substituted on a benzene ring, reduce the electron density on the ring and decrease its susceptibility to further (electrophilic) substitution. Electron-releasing groups, such as –OH, –NH2, –OCH3, and –CH3, have the opposite effect. indusium The kidney-shaped covering of the *sorus of certain ferns that protects the developing sporangia. It withers when the sorus ripens to expose the sporangia.
inequality industrial melanism The increase of melanic (dark) forms of an animal in areas darkened by industrial pollution. The example most often quoted is that of the peppered moth (Biston betularia), melanic forms of which markedly increased in the industrial north of England during the 19th century. Experiments have shown that the dark forms increase in polluted regions because they are less easily seen by birds against a dark background; conversely the paler forms survive better in unpolluted areas. inelastic collision A collision in which some of the kinetic energy of the colliding bodies is converted into internal energy in one body so that kinetic energy is not conserved. In collisions of macroscopic bodies some kinetic energy is turned into vibrational energy of the atoms, causing a heating effect. Collisions between molecules of a gas or liquid may also be inelastic as they cause changes in vibrational and rotational *energy levels. In nuclear physics, an inelastic collision is one in which the incoming particle causes the nucleus it strikes to become excited or to break up. Deep inelastic scattering is a method of probing the structure of subatomic particles in much the same way as Rutherford probed the inside of the atom (see R ). Such experiments were performed on protons in the late 1960s using high-energy electrons at the Stanford Linear Accelerator Centre (SLAC). As in Rutherford scattering, deep inelastic scattering of electrons by proton targets revealed that most of the incident electrons interacted very little and pass straight through, with only a small number bouncing back. This indicates that the charge in the proton is concentrated in small lumps, reminiscent of Rutherford's discovery that the positive charge in an atom is concentrated at the nucleus. However, in the case of the proton, the evidence suggested three distinct concentrations of charge and not one. inequality A relationship between two quantities in which one of the quantities is not equal to (or not necessarily equal to) the other quantity. If the quantities are a and b, two inequalities exist: a is greater than b, written a > b, and a is less than b, i.e. a < b. Similar statements can take the form: a is greater than or equal to b, written a ≥ b, and a is less than or equal to b, which is denoted a ≤ b. There are many applications of in-
inert gases equalities in physical science, an example being the Heisenberg *uncertainty principle.
inert gases See  . inertia The property of matter that causes it to resist any change in its motion. Thus, a body at rest remains at rest unless it is acted upon by an external force and a body in motion continues to move at constant speed in a straight line unless acted upon by an external force. This is a statement of Newton's Ürst law of motion. The *mass of a body is a measure of its inertia. See M' ;  .
inertial frame A *frame of reference in which bodies move in straight lines with constant speeds unless acted upon by external forces, i.e. a frame of reference in which free bodies are not accelerated. Newton's laws of motion are valid in an inertial system but not in a system that is itself accelerated with respect to such a frame. inertial mass See . inert-pair effect An effect seen especially in groups 13 and 14 of the periodic table, in which the heavier elements in the group tend to form compounds with a valency two lower than the expected group valency. It is used to account for the existence of thallium(I) compounds in group 13 and lead(II) in group 14. In forming compounds, elements in these groups promote an electron from a Ülled s-level state to an empty p-level. The energy required for this is more than compensated for by the extra energy gain in forming two more bonds. For the heavier elements, the bond strengths or lattice energies in the compounds are lower than those of the lighter elements. Consequently the energy compensation is less important and the lower valence states become favoured. infection The invasion of any living organism by disease-causing microorganisms (see ), which proceed to establish themselves, multiply, and produce various symptoms in their host. Pathogens may invade via a wound or (in animals) through the mucous membranes lining the alimentary, respiratory, and reproductive tracts, and may be transmitted by an infected individual, a *carrier, or an arthropod *vector. Symptoms in animals appear after an initial symptomless incubation period and typically consist of localized *inÛammation, often with pain and fever. Infections are combatted by the body's natural defences
(see ). Treatment with drugs (see ; ) is effective against most bacterial, fungal, and protozoan infections; some viral infections respond to *antiviral drugs. See also .
inferior Describing a structure that is positioned below or lower than another structure in the body. For example, in Ûowering plants the ovary is described as inferior when it is located below the other organs of the Ûower. Compare . inÜnite series See . inÜnitesimal Vanishingly small but not zero. InÜnitesimal changes are notionally made in the *calculus, which is sometimes called the infinitesimal calculus. inÜnity Symbol ∞. A quantity having a value that is greater than any assignable value. Minus inÜnity, –∞, is a quantity having a value that is less than any assignable value. inÛammation The defence reaction of tissue to injury, infection, or irritation by chemicals or physical agents. Activated macrophages and other cells in the affected tissue release various substances, including *histamine, *serotonin, *kinins, and *prostaglandins. These cause localized dilatation of blood vessels so that Ûuid leaks out and blood Ûow is increased. They also attract white blood cells (neutrophils and monocytes) to the site. Overall, these responses lead to swelling, redness, heat, and often pain. White blood cells, particularly *phagocytes, enter the tissue and an immune response is stimulated (see ). A gradual healing process usually follows. inÛation See  . inÛationary universe See  . inÛection A point on a curve at which the tangent changes from rotation in one direction to rotation in the opposite direction. If the curve y = f(x) has a stationary point dy/dx = 0, there is either a maximum, minimum, or inÛection at this point. If d2y/dx2 = 0, the stationary point is a point of inÛection. inÛorescence A particular arrangement of Ûowers on a single main stalk of a plant. There are many different types of inÛorescence, which are classiÜed into two main groups depending on whether the tip of the Ûower axis goes on producing new Ûower buds during growth (see  -
) or loses this ability (see  ).
information technology See IT. information theory The branch of mathematics that analyses information mathematically. Several branches of physics have been related to information theory. For example, an increase in *entropy has been expressed as a decrease in information. It has been suggested that it may be possible to express the basic laws of physics using information theory. See also L' ; Z' . infradian rhythm See . infrared astronomy The study of radiation from space in the infrared region of the spectrum (see  ). Some infrared radiation is absorbed by water and carbon dioxide molecules in the atmosphere but there are several narrow atmospheric *windows in the near-infrared (1.15–1.3 µm, 1.5–1.75 µm, 2–2.4 µm, 3.4–4.2 µm, 4.6–4.8 µm, 8–13 µm, and 16–18 µm). Longer wavelength observations must be made from balloons, rockets, or satellites. Infrared sources are either thermal, i.e. emitted by the atoms or molecules of gases or dust particles in the temperature range 100–3000 K, or electronic, i.e. emitted by high-energy electrons interacting with magnetic Üelds as in *synchrotron radiation. Detectors are either modiÜed reÛecting *telescopes or solid-state photon detectors, usually incorporating photovoltaic devices (see  ). infrared radiation (IR) Electromagnetic radiation with wavelengths longer than that of red light but shorter than radiowaves, i.e. radiation in the wavelength range 0.7 micrometre to 1 millimetre. It was discovered in 1800 by William Herschel (1738–1822) in the sun's spectrum. The natural vibrational frequencies of atoms and molecules and the rotational frequencies of some gaseous molecules fall in the infrared region of the electromagnetic spectrum. The infrared absorption spectrum of a molecule is highly characteristic of it and the spectrum can therefore be used for molecular identiÜcation. Glass is opaque to infrared radiation of wavelength greater than 2 micrometres and other materials, such as germanium, quartz, and polyethene, have to be used to make lenses and prisms. Photographic Ülm can be made sensitive to infrared up to about 1.2 µm.
infrared spectroscopy (IR spectroscopy) A technique for chemical analysis and the determination of structure. It is based on the principles that molecular vibrations occur in the infrared region of the electromagnetic spectrum and functional groups have characteristic absorption frequencies. The frequencies of most interest range from 2.5 to 16 µm; however, in IR spectroscopy it is common to use the reciprocal of the wavelength, and thus this range becomes 4000–625 cm–1. Examples of typical vibrations are centred on 2900 cm–1 for C–H stretching in alkanes, 1600 cm–1 for N–H stretching in amino groups, and 2200 cm–1 for C≡C stretching in alkynes. In an IR spectrometer there is a source of IR light, covering the whole frequency range of the instrument, which is split into two beams of equal intensity. One beam is passed through the sample and the other is used as a reference against which the Ürst is then compared. The spectrum is usually obtained as a chart showing absorption peaks, plotted against wavelength or frequency. The sample can be a gas, liquid, or solid. infrasound Soundlike waves with frequencies below the audible limit of about 20 hertz. ingestion (feeding) A method of *heterotrophic nutrition in which food is taken into an organism and subsequently digested (see ). Ingestion is the principal mechanism of animal nutrition. See also ; . inhalation See . inheritance The transmission of particular characteristics from generation to generation by means of the *genetic code, which is transferred to offspring in the gametes. See also M' . inhibition 1. (in chemistry) A reduction in the rate of a catalysed reaction by substances called inhibitors. Inhibitors may work by poisoning catalysts for the reaction or by removing free radicals in a chain reaction. Enzyme inhibition affects biochemical reactions, in which the catalysts are *enzymes. Competitive inhibition occurs when the inhibitor molecules resemble the substrate molecules and bind to the *active site of the enzyme, so preventing normal enzymatic activity. Competitive inhibition can be reversed by increasing the concentration of the substrate. In noncompetitive inhibition
inhibitory postsynaptic potential
the inhibitor binds to a part of the enzyme or *enzyme–substrate complex other than the active site, known as an allosteric site. This deforms the active site so that the enzyme cannot catalyse the reaction. Noncompetitive inhibition cannot be reversed by increasing the concentration of the substrate. The toxic effects of many substances are produced in this way. Inhibition by reaction products (feedback inhibition) is important in the control of enzyme activity. See also  . 2. (in physiology) The prevention or reduction of the activity of effectors (such as muscles) by means of certain nerve impulses. Inhibitory activity often provides a balance to stimulation of a process; for example, the impulse to stimulate contraction of a voluntary muscle may be accompanied by an inhibitory impulse to prevent contraction of its antagonist.
inhibitory postsynaptic potential (IPSP) The electric potential that is generated in a postsynaptic neuron when an inhibitory neurotransmitter (such as gamma-aminobutyric acid) is released into the synapse and causes a slight increase in the potential difference across the postsynaptic membrane. This makes the neuron less likely to transmit an impulse. Compare   . initial One of a group of cells (or, in lower plants, a single cell) that divides to produce the cells of a plant tissue or organ. The cells of the apical meristem, cambium, and cork cambium are initials. initiation codon See  . innate behaviour An inherited pattern of behaviour that appears in a similar form in all normally reared individuals of the same sex and species. See . inner Describing a chemical compound formed by reaction of one part of a molecule with another part of the same molecule. Thus, a lactam is an inner amide; a lactone is an inner ester. inner ear The structure in vertebrates, surrounded by the temporal bone of the skull, that contains the organs of balance and hearing. It consists of soft hollow sensory structures (the membranous labyrinth), containing Ûuid (endolymph), surrounded by Ûuid (perilymph), and encased in a bony cavity (the bony labyrinth). It consists of two chambers, the *sacculus and *utriculus, from
which arise the *cochlea and *semicircular canals respectively.
inner transition series See  . innervation The supply of nerve Übres to and from an organ. innominate artery A short artery that branches from the aorta to divide into the *subclavian artery (the main artery to the arm) and the right *carotid artery (which supplies blood to the head). innominate bone One of the two bones that form each half of the *pelvic girdle in adult vertebrates. This bone is formed by the fusion of the *ilium, *ischium, and *pubis. inoculation 1. See . 2. The placing of a small sample of microorganisms or any other type of cell into a *culture medium so that the cells can grow and proliferate. inoculum A small amount of material containing bacteria, viruses, or other microorganisms that is used to start a culture. inorganic chemistry The branch of chemistry concerned with compounds of elements other than carbon. Certain simple carbon compounds, such as CO, CO2, CS2, and carbonates and cyanides, are usually treated in inorganic chemistry. inositol A cyclic alcohol, C6H12O6, that is a constituent of certain cell phosphoglycerides. It is sometimes classiÜed as a member of the vitamin B complex but it can be synthesized by many animals and it is not regarded as an essential nutrient in humans. Phosphatidyl inositol, a constituent of plasma membranes, is a precursor of the intracellular messenger molecules, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol; these are produced in response to the binding of substances, e.g. serotonin, to their receptors on the cell surface. These pathways mediate such cellular events as smooth muscle contraction, adrenaline secretion, and histamine secretion. Insecta See H. insecticide See . Insectivora An order of small, mainly nocturnal, mammals that includes the hedgehogs, moles, and shrews. They have long snouts covered with stiff tactile hairs and their teeth are specialized for seizing and crushing insects and other small prey. The
insectivores have changed very little since they evolved in the Cretaceous period, 130 million years ago.
insectivore An animal that eats insects, especially a mammal of the order Insectivora (hedgehogs, shrews, etc.). insectivorous plant See  . insertion 1. (of muscles) See  . 2. (in genetics) A *point mutation in which an extra nucleotide base is added to the DNA sequence. This results in the misreading of the base sequence during the *translation stage of protein synthesis. insertion sequence See . insight learning A form of learning in which an animal responds to new situations by adapting experiences gained in other contexts. Insight learning requires an animal to solve problems by viewing a situation as a whole instead of relying wholly on trial-anderror learning. Chimpanzees are capable of insight learning. See  (Feature). insolation (from incoming solar radiation) The solar radiation that is received at the earth's surface per unit area. It is related to the *solar constant, the duration of daylight, the altitude of the sun, and the latitude of the receiving surface. It is measured in MJ m–2. inspiration (inhalation) The process by which gas is drawn into the lungs through the trachea (see  ). In mammals the rib cage is raised by contraction of the external *intercostal muscles and the muscles of the diaphragm. These actions enlarge the thorax, so that pressure in the lung cavity is reduced below atmospheric pressure, which causes an inÛux of air until the pressures are equalized. Compare . inspiratory centre See  . instantaneous value The value of any varying quantity at a speciÜed instant. instar A stage in the larval development of an insect between two moults (ecdyses). There are usually a number of larval instars before the pupal stage and metamorphosis. instinct An innate tendency to behave in a particular way, which does not depend critically on particular learning experiences for its development and therefore is seen in a
similar form in all normally reared individuals of the same sex and species. Much instinctive behaviour takes the form of Üxed action patterns. These are movements that – once started – are performed in a stereotyped way unaffected by external stimuli. For example, a frog's prey-catching tongue Ûick is performed in the same way whether or not anything is caught. Some complex instinctive behaviour, however, requires some learning by the animal before it is perfected. Birdsong, for example, consists of an innate component that is modiÜed and made more complex by the inÛuence of other birds, the habitat, etc.
insulator A substance that is a poor conductor of heat and electricity. Both properties usually occur as a consequence of a lack of mobile electrons. See  . insulin A protein hormone, secreted by the β (or B) cells of the *islets of Langerhans in the pancreas, that promotes the uptake of glucose by body cells, particularly in the liver and muscles, and thereby controls its concentration in the blood. Insulin was the Ürst protein whose amino-acid sequence was fully determined (in 1955). Underproduction of insulin results in the accumulation of large amounts of glucose in the blood and its subsequent excretion in the urine. This condition, known as diabetes mellitus, can be treated successfully by insulin injections. insulin-like growth factor (IGF) See  . integer Any one of the positive or negative whole numbers. integral calculus See . integrand See . integrated circuit A miniature electronic circuit produced within a single crystal of a *semiconductor, such as silicon. They range from simple logic circuits, little more than 1 mm square, to large-scale circuits measuring up to 8 mm square and containing a million or so transistors (active components) and resistors or capacitors (passive components). They are widely used in memory circuits, microcomputers, pocket calculators, and electronic watches on account of their low cost and bulk, reliability, and high speed. They are made by introducing impurities into speciÜc regions of the semiconductor crystal by a variety of techniques. integration 1. (in physiology) The coordi-
integument
nation within the brain of separate but related nervous processes. For example, sensory information from the inner ear and the eye are both necessary for the sense of balance. These stimuli must be integrated by the brain not only with each other but also with various motor nerves, which coordinate the muscles that control posture. 2. (in mathematics) The process of continuously summing changes in a function f(x). It is the basis of the integral *calculus and the opposite process to *differentiation. The function to be integrated is called the integrand and the result of integration on the integrand is called the integral. For example, the integration of f(x) is written ∫f(x)dx, the differential dx being added to indicate that f(x) must be integrated with respect to x. To complete the integration, a constant of integration, C, must be added where no interval over which the integration takes place is given. This is called an indefinite integral. If the interval is speciÜed, e.g. b
∫a (x)dx, no constant of integration is required and the result is a definite integral. This means that f(x) is to be integrated between the values x = +r and x = –r.
integument 1. The outermost body layer of an animal, characteristically comprising a layer of living cells – the *epidermis – together with a superÜcial protective coat, which may be a secreted hardened *cuticle, as in arthropods, or dead keratinized cells, as in vertebrates (see ). 2. The outer protective covering of a plant *ovule. It is perforated by a small pore, the *micropyle. Usually two integuments are present in angiosperms and one in gymnosperms. After fertilization the integuments form the *testa of the seed. intelligence The coordination of *memory, *learning, and reasoning in animals. Intelligence has also been deÜned as the ability of an animal to form associative links between events or objects of which it has had no previous experience (see  ). In humans intelligence is generally expressed as an intelligence quotient (IQ): the mental age of the subject (as measured by standard tests) divided by his or her real age × 100. intensity 1. The rate at which radiant energy is transferred per unit area. See  . 2. The rate at which sound energy is transferred as measured relative to
some reference value. See . 3. Magnetic intensity. See  . 4. Electric intensity. See  . 5. See  .
intensive variable A quantity in a *macroscopic system that has a well deÜned value at every point inside the system and that remains (nearly) constant when the size of the system is increased. Examples of intensive variables are the pressure, temperature, density, speciÜc heat capacity at constant volume, and viscosity. An intensive variable results when any *extensive variable is divided by an arbitrary extensive variable such as the volume. A macroscopic system can be described by one extensive variable and a set of intensive variables. interaction An effect involving a number of bodies, particles, or systems as a result of which some physical or chemical change takes place to one or more of them. See also  . interactome All the interactions that occur between the various molecules produced by an organism, in all its cells and tissues and at all stages of its life. Of central signiÜcance are the interactions of proteins – the protein interactome – because these are the molecules encoded by the organism's genes and are fundamental to all other cellular processes. Determining which proteins form complexes, or bind together in some way, enables researchers to identify the components of the complex pathways and networks that govern different aspects of cellular functioning and potentially play a role in health and disease. intercalation cell A type of secondary cell in which layered electrodes, usually made of metal oxides or graphite, store positive ions between the crystal layers of an electrode. In one type, lithium ions form an intercalation compound with a graphite electrode when the cell is charged. During discharge, the ions move through an electrolyte to the other electrode, made of manganese oxide, where they are more tightly bound. When the cell is being charged, the ions move back to their positions in the graphite. This backwards and forwards motion of the ions has led to the name rocking-chair cell for this type of system. Such cells have the advantage that only minor physical changes occur to the electrodes during the charging and discharging processes and the electrolyte is not decomposed but simply serves as a con-
ductor of ions. Consequently, such cells can be recharged many more times than, say, a lead-acid accumulator, which eventually suffers from degeneration of the electrodes. Lithium cells, based on this principle, have been used in portable electronic equipment, such as camcorders. They have also been considered for use in electric vehicles.
intercalation compound A type of compound in which atoms, ions, or molecules are trapped between layers in a crystal lattice. There is no formal chemical bonding between the host crystal and the trapped molecules (see also ). Such compounds are formed by *lamellar solids and are often nonstoichiometric; examples are graphitic oxide (graphite–oxygen) and the mineral *muscovite. intercellular (in biology) Located or occurring between cells. Compare . intercostal muscles The muscles located between the *ribs, surrounding the lungs. Comprising the superÜcial external intercostal muscles and the deep internal intercostal muscles, they play an essential role in breathing (see ; ). interference The interaction of two or more wave motions affecting the same part of a medium so that the instantaneous disturbances in the resultant wave are the vector sum of the instantaneous disturbances in the interfering waves. The phenomenon was Ürst described by Thomas Young in 1801 in light waves; it provided strong evidence for the wave theory of light. In the apparatus known as Young's slits, light is passed from a small source through a slit in a screen and the light emerging from this slit is used to illuminate two adjacent slits on a second screen. By allowing the light from these two slits to fall on a third screen, a series of parallel interference fringes is formed. Where the maximum values of the two waves from the slits coincide a bright fringe occurs (constructive interference) and where the maxima of one wave coincide with the minima of the other dark fringes are produced (destructive interference). (In Young's original experiment, two pinholes were used rather than slits). *Newton's rings are also an interference effect. Because *lasers produce *coherent radiation they are also used to produce interference effects, one application of their
intermediate bond use being *holography. See also .
interferometer An instrument designed to produce optical *interference fringes for measuring wavelengths, testing Ûat surfaces, measuring small distances, etc. See also ; F–P ; M–M . In astronomy, radio interferometers are one of the two basic types of *radio telescopes. interferon (IFN) Any of a number of proteins (see ) that increase the resistance of cells to attack by viruses. In humans, three groups of interferons have been discovered: α-interferons from white blood cells; β-interferons from connective tissue Übroblasts; and γ-interferons from T cells and *natural killer cells (NK cells). αand β-interferons induce intrinsic resistance to viral infection in all cells by triggering the expression of genes that encode antiviral proteins. Moreover, they activate NK cells, which selectively kill virus-infected cells, and promote synthesis of MHC proteins by all cell types (see ), thereby protecting uninfected cells from attack by the NK cells. The actions of γ-interferon include macrophage activation, increasing the expression of MHC molecules, and suppression of helper *T cells. It is produced by cytotoxic T cells. Interferons are produced commercially for therapeutic purposes using genetically engineered bacteria or human tissue culture. interhalogen A chemical compound formed between two *halogens. Interhalogens are highly reactive and volatile, made by direct combination of the elements. They include compounds with two atoms (ClF, IBr, etc.), four atoms (ClF3, IF3, etc.), six atoms (BrF5, IF5, etc.) and IF7 with eight atoms. interleukin Any of several *cytokines that are produced by leucocytes. Interleukin-1 (IL-1) is secreted by antigen-activated macrophages and activates *T cells. Interleukin-2 (IL-2) stimulates the proliferation of T cells, which also secrete it. Interleukin3 is a growth factor for haemopoietic cells, and interleukin-4 induces B cells to proliferate and produce antibodies. Nearly 30 interleukins are now known to exist, and some are manufactured using recombinant DNA technology, for use as therapeutic agents. intermediate bond See  .
intermediate coupling intermediate coupling See - . intermediate frequency See ;  . intermediate neutron A *neutron with kinetic energy in the range 102–105 electronvolts (1.6 × 10–17 – 1.6 × 10–14 joule). intermediate vector boson See W ; Z . intermetallic compound A compound consisting of two or more metallic elements present in deÜnite proportions in an alloy. intermolecular forces Weak forces occurring between molecules. See   W' ;  .
internal-combustion engine A *heat engine in which fuel is burned in combustion chambers within the engine rather than in a separate furnace (as with the steam engine). The Ürst working engine was the fourstroke Otto engine produced in 1876 by Nikolaus Otto (1832–91). In this type of engine a piston descends in a cylinder, drawing in a charge of fuel and air through an inlet valve; after reaching the bottom of its stroke the piston rises in the cylinder with the valves closed and compresses the charge; at or near the top of its stroke the charge is ignited by a spark and the resulting increase in pressure from the explosion forces the piston down again; on the subsequent upstroke the exhaust valve opens and the burnt gases are pushed out of the combustion chamber. The cycle is then repeated. Otto's engine used gas as a fuel; however, the invention of the carburettor and the development of the oil industry at the end of the 19th century enabled the Otto engine to become the source of power for the emerging motor car. A variation of the Otto four-stroke engine is the two-stroke engine that has no complicated valve system, the explosive charge entering and leaving the cylinder through ports in the cylinder that are covered and uncovered by the moving piston. An alternative to the Otto engine, especially for heavy vehicles where weight is not a problem, is the compression-ignition Diesel engine invented by Rudolf Diesel (1858–1913) in about 1896. In this type of engine there are no sparking plugs; instead air is compressed in the cylinder, causing its temperature to rise to about 550°C. Oil is then sprayed into the combustion chamber and ignites on contact with the hot air. While the spark-ignition petrol engine typically
works on a *compression ratio of 8 or 9 to 1, the Diesel engine has to have a compression ratio of between 15 and 25 to 1. This requires a much heavier, and therefore more expensive, engine. See also  .
internal conversion A process in which an excited atomic nucleus (see ) decays to the *ground state and the energy released is transferred by electromagnetic coupling to one of the bound electrons of that atom rather than being released as a photon. The coupling is usually with an electron in the K-, L-, or M-shell of the atom, and this conversion electron is ejected from the atom with a kinetic energy equal to the difference between the nuclear transition energy and the binding energy of the electron. The resulting ion is itself in an excited state and usually subsequently emits an Auger electron (see A ) or an X-ray photon. internal energy Symbol U. The total of the kinetic energies of the atoms and molecules of which a system consists and the potential energies associated with their mutual interactions. It does not include the kinetic and potential energies of the system as a whole nor their nuclear energies or other intra-atomic energies. The value of the absolute internal energy of a system in any particular state cannot be measured; the signiÜcant quantity is the change in internal energy, ∆U. For a closed system (i.e. one that is not being replenished from outside its boundaries) the change in internal energy is equal to the heat absorbed by the system (Q) from its surroundings, less the work done (W) by the system on its surroundings, i.e. ∆U = Q – W. See also ; ; . internal environment The conditions that prevail within the body of an organism, particularly with respect to the composition of the *tissue Ûuid. The concept of an internal environment was Ürst proposed by the French physiologist Claude Bernard (1813–78), who stated that maintenance of a constant internal environment was necessary for the survival of an organism in a varying external environment. Selective absorption of materials across plasma membranes plays a large part in controlling the internal environment of both animals and plants. Animals in addition can regulate their body Ûuids by the action of hormones and the nervous system. See .
internal resistance The resistance within a source of electric current, such as a cell or generator. It can be calculated as the difference between the e.m.f. (E) and the potential difference (V) between the terminals divided by the current being supplied (I), i.e. r = (E – V)/I, where r is the internal resistance. international candle A former unit of *luminous intensity. It has now been replaced by the *candela, to which it is approximately equal. international date line An imaginary line on the earth's surface that joins the north and south poles and approximately follows the 180° meridian through the PaciÜc Ocean. This line has been agreed internationally to mark the beginning and end of a day. A traveller moving towards the east, against the sun's apparent movement, gains 1 hour for every 15° of longitude; westward he loses time at the same rate. In crossing the dateline therefore he is deemed to compensate for this by losing or gaining (respectively) one day. The 180° meridian was chosen as the date line by the International Meridian Conference in 1884. International Nucleotide Sequence Database Collaboration (INSDC) A collaborative venture that effectively divides up the task of collecting, updating, and storing the nucleotide sequence data reported by researchers throughout the world. It comprises three databases, which exchange information on a daily basis; these are GenBank, run by the US National Institutes of Health, the DNA Data Bank of Japan (DDBJ), and the Nucleotide Sequence Database of the European Molecular Biology Laboratory (EMBL). Information from the entire collection can be accessed via any one of the partner organizations. International Practical Temperature Scale See  . International Space Station A spacebased scientiÜc research facility constructed in earth orbit at an altitude of 350 km from prefabricated modules and components. It is a joint project involving the space agencies of the United States, Russia, Japan, Canada, and the ten member nations of the European Space Agency, with additional contributions from Brazil and Italy. Assembly of the station began in 1998 and was scheduled to be completed by 2011. It has been crewed continuously since November 2000.
interpolation international system See H– M . Internet (Net) The global network that links most of the world's computer networks. It does not offer services to users, but serves primarily to interconnect other networks on which services are located. These include basic services for *electronic mail, the transfer of computer Üles, and remote log-in, and high-level services including the *World Wide Web. The Internet is informal, with a minimal level of administration by governing bodies. internode 1. (in botany) The part of a plant stem between two *nodes. 2. (in neurology) The myelinated region of a nerve Übre between two nodes of Ranvier. See  . interoceptor A *receptor that detects stimuli from the internal environment of an organism. *Chemoreceptors that detect changes in the levels of oxygen concentration in the blood are examples. Compare . interphase The period following the completion of *cell division, when the nucleus is not dividing. During this period changes in both the nucleus and the cytoplasm result in the complete development of the daughter cells. See  . interplanetary space (interplanetary medium) The space occupied by the sun and the planets, dwarf planets, asteroids, comets, *trans-Neptunian objects, and all other bodies within the *solar system. Apart from these, the interplanetary matter that saturates this region of space consists mostly of the energetic particles that make up the *solar wind. The solar wind consists primarily of protons emerging from the sun at a rate of about 109 kilograms per second. At the earth's distance from the sun the particle density is only a few particles per cm3. Apart from this very tenuous gas, there are also dust particles in interplanetary space, largely believed to originate in the belt of asteroids. Particles weighing about 1 g produce visible meteors in the earth's atmosphere; micrometeorites as small as 1 nanogram can be detected by their impact on spacecraft. interpolation An *approximation technique for Ünding the value of a function or a measurement that lies within known values. If the values f(x0), f(x1), …, f(xn) of a function f of a variable x are known in the interval
intersex [x0,xn], the value of f(x) for a value of x inside the interval [x0,xn] can be found by interpolation. One method of interpolation, called linear interpolation for x0 < x < x1, gives: f(x) ≅ f(x0) + [f(x1) – f(x0)] (x – x0)/(x1 – x0), which is derived using the assumption that between the points x0 and x1, the graph of the function f(x) can be regarded as a straight line. More complicated methods of interpolation exist, using more than two values for the function. The techniques used for interpolation are usually much better than the techniques used in *extrapolation.
intersex An organism displaying characteristics that are intermediate between those of the typical male and typical female of its species. For example, a human intersex may have testes that fail to develop, so that although he is technically a man he has the external appearance of a woman. Intersexes may be produced in various ways; for example, by malfunctioning of the sex hormones. See also . interspeciÜc competition See . interstellar molecules See . interstellar space (interstellar medium) The space between the stars. The interstellar matter that occupies this space constitutes several percent of the Galaxy's total mass and it is from this matter that new stars are formed. The matter is primarily hydrogen, in which a number of other molecules and radicals have been detected, together with small solid dust grains. On average the density of matter in interstellar space is about 106 hydrogen atoms per cubic metre, but the gas is not uniformly distributed, being clumped into interstellar clouds of various sizes and densities. interstitial See   (Feature). interstitial cell A cell that forms part of the connective tissue (the interstitium) between other tissues and structures, especially any of the cells of the *testis that lie between the seminiferous tubules and secrete androgens in response to stimulation by interstitial-cell-stimulating hormone (see  ). interstitial-cell-stimulating hormone See  . interstitial compound A compound in
which ions or atoms of a nonmetal occupy interstitial positions in a metal lattice. Such compounds often have metallic properties. Examples are found in the *carbides, *borides, and *silicides.
intervertebral disc Any of the discs of cartilage that separate the bones of the *vertebral column. The intervertebral discs allow the vertebral column a certain degree of Ûexibility and they also absorb shock. intestinal juice (succus entericus) A slightly alkaline liquid containing mucus that is secreted into the lumen of the small intestine from the cells that line the *crypts of Lieberkühn. Together with pancreatic juice, the intestinal juice provides an alkaline environment that helps in the absorption of digested food molecules entering the small intestine in chyme from the stomach. intestine The portion of the *alimentary canal posterior to the stomach. Its major functions are the Ünal digestion of food matter from the stomach, the absorption of soluble food matter, the absorption of water, and the production of *faeces. See  ;  . intracellular (in biology) Located or occurring within cells. Compare . intraspeciÜc competition See . intrinsic factor See  B . intrinsic semiconductor See . intron (intervening sequence) A nucleotide sequence in a gene that does not code for the gene product (compare ). Introns, which occur principally in eukaryotes, are transcribed into messenger *RNA but are subsequently removed from the transcript before translation (see  ). Their functional signiÜcance is still subject to debate. intrusion An upwelling of *magma or other molten rock into an existing rock. The intrusion may force its way through or follow such weaknesses as joints and bedding planes. The heat of the molten intrusion may bring about changes in the composition of the country rock it invades. There are various kinds of igneous intrusions, including *batholiths, *dykes, laccoliths, sills, and *xenoliths.
inulin A polysaccharide, made up from fructose molecules, that is stored as a food reserve in the roots or tubers of many plants, such as the dahlia. Invar A trade name for an alloy of iron (63.8%), nickel (36%), and carbon (0.2%) that has a very low *expansivity over a a restricted temperature range. It is used in watches and other instruments to reduce their sensitivity to changes in temperature. inverse Compton effect The gain in energy of low-energy photons when they are scattered by free electrons of much higher energy. As a consequence, the electrons lose energy. The effect is thought to be important in certain astrophysical processes. See also C ; GZK . inverse functions If y = f(x) and a function can be found so that x = g(y), then g(y) is said to be the inverse function of f(x). If y is a trigonometrical function of the angle x, say y = sinx, then x is the inverse trigonometrical function of y, written x = arcsiny or sin–1y. Similarly, the other trigonometrical functions form the inverse trigonometrical functions cos–1y, tan–1y, cot–1y, sec–1y, and cosec–1y. Inverse hyperbolic functions are also formed in this way, e.g. arcsinhy or sinh–1y, cosh–1y, and tanh–1y. inverse-square law A law in which the magnitude of a physical quantity is proportional to the reciprocal of the square of the distance from the source of that property. *Newton's law of gravitation and *Coulomb's law are both examples. inversion 1. (in chemistry) A chemical reaction involving a change from one optically active conÜguration to the opposite conÜguration. The Walden inversion is an example. See  . 2. (in genetics) A *chromosome mutation caused by reversal of part of a chromosome, so that the genes within that part are in inverse order. Inversion mutations usually occur during *crossing over in meiosis. 3. (in genetics) A *point mutation caused by the reversal of two or more bases in the DNA sequence within a gene. inversion layer See . inversion temperature See J– T . invertebrate Any animal that lacks a vertebral column (backbone). Invertebrates in-
clude all nonchordate animals as well as the more primitive chordates (see C).
in vitro Describing biological processes that are made to occur outside the living body, in laboratory apparatus (literally 'in glass', i.e. in a test tube). In in vitro fertilization, mature egg cells are removed from the ovary of a woman unable to conceive normally and fertilized externally; the resultant blastocyst is implanted into her uterus. Compare  . in vivo Describing biological processes as they are observed to occur in their natural environment, i.e. within living organisms. Compare  . involucre A protective structure in some Ûowering plants and bryophytes. In Ûowering plants it consists of a ring of *bracts arising beneath the Ûower cluster of those species with a *capitulum (i.e. members of the dandelion family) or an *umbel (i.e. members of the carrot family). In mosses and liverworts the involucre is a projection of tissue from the thallus that arches over the developing *archegonium. involuntary (in biology) Not under the control of the will of an individual. Involuntary responses by muscles, glands, etc., occur automatically when required; many such responses, such as gland secretion, heartbeat, and peristalsis, are controlled by the *autonomic nervous system and effected by *involuntary muscle. involuntary muscle (smooth muscle) Muscle whose activity is not under the control of the will; it is supplied by the *autonomic nervous system. Involuntary muscle comprises long spindle-shaped cells without striations. These cells occur singly, in groups, or as sheets in the skin, around hair follicles, and in the digestive tract, respiratory tract, urinogenital tract, and the circulatory system. The cells contract slowly in spontaneous rhythms or when stretched; they may show sustained contraction (tonus) for long periods without fatigue. Compare  ;  . involute See . involution 1. A decrease in the size of an organ or the body. It may be associated with functional decline, as occurs in the ageing process, or follow enlargement, as when the uterus returns to its normal size after preg-
iodic acid nancy. 2. The turning or rolling inwards of cells that occurs during the development of some vertebrate embryos.
iodic acid Any of various oxoacids of iodine, such as iodic(V) acid and iodic(VII) acid. When used without an oxidation state speciÜed, the term usually refers to iodic(V) acid (HIO3).
iodic(V) acid and also acts as an oxidizing agent.
iodine value A measure of the amount of unsaturation in a fat or vegetable oil (i.e. the number of double bonds). It is obtained by Ünding the percentage of iodine by weight absorbed by the sample in a given time under standard conditions.
iodic(V) acid A colourless or very pale yellow solid, HIO3; r.d. 4.63; decomposes at 110°C. It is soluble in water but insoluble in pure ethanol and other organic solvents. The compound is obtained by oxidizing iodine with concentrated nitric acid, hydrogen peroxide, or ozone. It is a strong acid and a powerful oxidizing agent.
iodoethane (ethyl iodide) A colourless liquid *haloalkane, C2H5I; r.d. 1.9; m.p. –108°C; b.p. 72°C. It is made by reacting ethanol with a mixture of iodine and red phosphorus.
iodic(VII) acid (periodic acid) A hygroscopic white solid, H5IO6, which decomposes at 140°C and is very soluble in water, ethanol, and ethoxyethane. Iodic(VII) acid may be prepared by electrolytic oxidation of concentrated solutions of iodic(V) acid at low temperatures. It is a weak acid but a strong oxidizing agent.
iodomethane (methyl iodide) A colourless liquid haloalkane, CH3I; r.d. 2.28; m.p. –66.45°C; b.p. 42.4°C. It can be made by reacting methanol with a mixture of iodine and red phosphorus.
iodide See . iodine Symbol I. A dark violet nonmetallic element belonging to group 17 of the periodic table (see ); a.n. 53; r.a.m. 126.9045; r.d. 4.94; m.p. 113.5°C; b.p. 184.35°C. The element is insoluble in water but soluble in ethanol and other organic solvents. When heated it gives a violet vapour that sublimes. Iodine is required as a trace element (see  ) by living organisms; in animals it is concentrated in the thyroid gland as a constituent of thyroid hormones. The element is present in sea water and was formerly extracted from seaweed. It is now obtained from oil-well brines (displacement by chlorine). There is one stable isotope, iodine–127, and fourteen radioactive isotopes. It is used in medicine as a mild antiseptic (dissolved in ethanol as tincture of iodine), and in the manufacture of iodine compounds. Chemically, it is less reactive than the other halogens and the most electropositive (metallic) halogen. It was discovered in 1812 by Bernard Courtois (1777– 1838).
iodine(V) oxide (iodine pentoxide) A white solid, I2O5; r.d. 4.799; decomposes at 300–350°C. It dissolves in water to give
iodoform See . iodoform test See  .
ion An atom or group of atoms that has either lost one or more electrons, making it positively charged (a cation), or gained one or more electrons, making it negatively charged (an anion). See also . ion channel A protein that spans a cell membrane to form a water-Ülled pore through which ions can pass in or out of the cell or cell compartment. Ion channels are found in the plasma membrane and in certain internal cell membranes. They vary in how they open and close and in their selectivity to different ions: some may be speciÜc for one particular ion, whereas others may admit two or more similar ions (e.g. K+ and Na+). The electrical and chemical environment inside cells, including the resting potential, is determined largely by the numbers, types, and activity of the cell's ion channels; they play a crucial role in the excitability of nerve and muscle cells. ion engine A type of jet-propulsion engine that may become important for propelling or controlling spacecraft. It consists of a unit producing a beam of ions, which are accelerated by an electric or electromagnetic Üeld. Reaction forces from the highspeed ions causes propulsion in much the same way as that caused by exhaust gas of a rocket. However, a separate beam of electrons or ions of opposite polarity to the propelling beam must also be ejected from the engine to enable recombination to take place behind the vehicle (to avoid the vehicle
becoming charged). Ion engines provide high *speciÜc impulse and therefore low propellant consumption. The three main components of an ion engine are the power generator, the propellant feed, and the thruster. The power generator may be a nuclear reactor or a solar-energy collector. If it is the former, a gas turbine is coupled to the reactor and the turbine drives an electric generator. A solar-energy unit provides electricity direct. The propellant chosen needs to have an ion of medium mass (low mass for high speciÜc impulse, high mass for high thrust) and a low Ürst *ionization potential. Caesium and mercury are materials currently envisaged as suitable propellants. The thruster consists of an ionizer to produce the ions, an accelerator to provide and shape the accelerating Üeld, and a neutralizer (usually an electron emitter) to neutralize the fastmoving ion beam after ejection.
ion exchange The exchange of ions of the same charge between a solution (usually aqueous) and a solid in contact with it. The process occurs widely in nature, especially in the absorption and retention of watersoluble fertilizers by soil. For example, if a potassium salt is dissolved in water and applied to soil, potassium ions are absorbed by the soil and sodium and calcium ions are released from it. The soil, in this case, is acting as an ion exchanger. Synthetic ion-exchange resins consist of various copolymers having a cross-linked three-dimensional structure to which ionic groups have been attached. An anionic resin has negative ions built into its structure and therefore exchanges positive ions. A cationic resin has positive ions built in and exchanges negative ions. Ionexchange resins, which are used in sugar reÜning to remove salts, are synthetic organic polymers containing side groups that can be ionized. In anion exchange, the side groups are ionized basic groups, such as –NH3+ to which anions X– are attached. The exchange reaction is one in which different anions in the solution displace the X– from the solid. Similarly, cation exchange occurs with resins that have ionized acidic side groups such as –COO– or –SO2O–, with positive ions M+ attached. Ion exchange also occurs with inorganic polymers such as *zeolites, in which positive ions are held at sites in the silicate lattice. These are used for water-softening, in which Ca2+ ions in solution displace Na+ ions in the
ion implantation zeolite. The zeolite can be regenerated with sodium chloride solution. Ion-exchange membranes are used as separators in electrolytic cells to remove salts from sea water (see also ) and in producing deionized water. Ion-exchange resins are also used as the stationary phase in ionexchange chromatography.
ionic bond See  . ionic crystal See . ionic product The product of the concentrations of ions present in a given solution taking the stoichiometry into account. For a sodium chloride solution the ionic product is [Na+][Cl–]; for a calcium chloride solution it is [Ca2+][Cl–]2. In pure water, there is an equilibrium with a small amount of selfionization: H2O ˆ H+ + OH– The equilibrium constant of this dissociation is given by KW = [H+][OH–] since the concentration [H2O] can be taken as constant. KW is referred to as the ionic product of water. It has the value 10–14 mol2 dm–6 at 25°C. In pure water (i.e. no added acid or added alkali) [H+] = [OH–] = 10–7 mol dm–3. See also  ; H . ionic radius A value assigned to the radius of an ion in a crystalline solid, based on the assumption that the ions are spherical with a deÜnite size. X-ray diffraction can be used to measure the internuclear distance in crystalline solids. For example, in NaF the Na – F distance is 0.231 nm, and this is assumed to be the sum of the Na+ and F– radii. By making certain assumptions about the shielding effect that the inner electrons have on the outer electrons, it is possible to assign individual values to the ionic radii – Na+ 0.096 nm; F– 0.135 nm. In general, negative ions have larger ionic radii than positive ions. The larger the negative charge, the larger the ion; the larger the positive charge, the smaller the ion. ionic strength Symbol I. A function expressing the effect of the charge of the ions in a solution, equal to the sum of the molality of each type of ion present multiplied by the square of its charge: I = Σmizi2. ion implantation The technique of im-
ionization planting ions in the lattice of a semiconductor crystal in order to modify its electronic properties. It is used as an alternative to diffusion, or in conjunction with it, in the manufacture of integrated circuits and solid-state components.
ionization The process of producing *ions. Certain molecules (see ) ionize in solution; for example, *acids ionize when dissolved in water (see also ): HCl → H+ + Cl– Electron transfer also causes ionization in certain reactions; for example, sodium and chlorine react by the transfer of a valence electron from the sodium atom to the chlorine atom to form the ions that constitute a sodium chloride crystal: Na + Cl → Na+Cl– Ions may also be formed when an atom or molecule loses one or more electrons as a result of energy gained in a collision with another particle or a quantum of radiation (see ). This may occur as a result of the impact of *ionizing radiation or of thermal ionization and the reaction takes the form A → A+ + e Alternatively, ions can be formed by electron capture, i.e. A + e → A– ionization chamber An instrument for detecting *ionizing radiation. It consists of two electrodes contained in a gas-Ülled chamber with a potential difference maintained between them. Ionizing radiation entering the chamber ionizes gas atoms, creating electrons and positive ions. The electric Üeld between the electrodes drives the electrons to the anode and the positive ions to the cathode. This current is, in suitable conditions, proportional to the intensity of the radiation. See also G . ionization energy (IE) See  . ionization gauge A vacuum gauge consisting of a three-electrode system inserted into the container in which the pressure is to be measured. Electrons from the cathode are attracted to the grid, which is positively biased. Some pass through the grid but do not reach the anode, as it is maintained at a negative potential. Some of these electrons do, however, collide with gas molecules, ionizing them and converting them to positive
ions. These ions are attracted to the anode; the resulting anode current can be used as a measure of the number of gas molecules present. Pressure as low as 10–6 pascal can be measured in this way.
ionization potential (IP) Symbol I. The minimum energy required to remove an electron from a speciÜed atom or molecule to such a distance that there is no electrostatic interaction between ion and electron. Originally deÜned as the minimum potential through which an electron would have to fall to ionize an atom, the ionization potential was measured in volts. It is now, however, deÜned as the energy to effect an ionization and is conveniently measured in electronvolts (although this is not an SI unit). The energy to remove the least strongly bound electron is the Ürst ionization potential. Second, third, and higher ionization potentials can also be measured, although there is some ambiguity in terminology. Thus, in chemistry the second ionization potential is often taken to be the minimum energy required to remove an electron from the singly charged ion; the second IP of lithium would be the energy for the process Li+ → Li2+ + e In physics, the second ionization potential is the energy required to remove an electron from the next to highest energy level in the neutral atom or molecule; e.g. Li → Li*+ + e, where Li*+ is an excited singly charged ion produced by removing an electron from the K-shell.
• Values for ionization energies of neutral atoms at the NIST website
ionizing radiation Radiation of sufÜciently high energy to cause *ionization in the medium through which it passes. It may consist of a stream of high-energy particles (e.g. electrons, protons, alpha-particles) or short-wavelength electromagnetic radiation (ultraviolet, X-rays, gamma-rays). This type of radiation can cause extensive damage to the molecular structure of a substance either as a result of the direct transfer of energy to its atoms or molecules or as a result of the secondary electrons released by ionization (see  ). In biological tissue the effect of ionizing radiation can be very serious, usually as a consequence of the ejection of an electron from a water mol-
ecule and the oxidizing or reducing effects of the resulting highly reactive species: 2H2O → e– + H2O* + H2O+ H2O* → .OH + .H H2O+ + H2O → .OH + H3O+ where the dot before a radical indicates an unpaired electron and * denotes an excited species.
ion-microprobe analysis A technique for analysing the surface composition of solids. The sample is bombarded with a narrow beam (as small as 2 µm diameter) of high-energy ions. Ions ejected from the surface by sputtering are detected by mass spectrometry. The technique allows quantitative analysis of both chemical and isotopic composition for concentrations as low as a few parts per million. ion-mobility spectrometry (IMS) A technique for detecting low concentrations of speciÜc compounds, based on the rate at which their ions migrate through an electric Üeld. The instrument operates in the gas phase at atmospheric pressure. The sample vapour enters an ionizing region, where ions can be produced by a variety of methods. In compact instruments the source is usually a small amount of radioactive material. The ions are allowed in pulses into a drift tube, where they move to a detector under the inÛuence of a homogeneous electric Üeld. The rate of movement depends on the way the ions interact with neutral molecules in the tube and this depends on the ion's size and shape. The spectrum is a plot of detector signal against time, and is characteristic of the sample being ionized. Ion-mobility spectrometers are compact, sensitive, and fastacting. They are widely used in screening for drugs and explosives at airports, border crossings, etc. often the technique is to wipe a swab over luggage and place it in the instrument. More sophisticated instruments combine IMS with gas chromatography or mass spectrometry. The technique is sometimes referred to as gas-phase electrophoresis. ionomer A thermoplastic resin with ionic bonds between the polymer chains. ionosphere See ' ;  . ionospheric wave See  .
ion pair A pair of oppositely charged ions produced as a result of a single ionization; e.g. HCl → H+ + Cl–. Sometimes a positive ion and an electron are referred to as an ion pair, as in A → A+ + e–. ion pump A type of *vacuum pump that can reduce the pressure in a container to about 1 nanopascal by passing a beam of electrons through the residual gas. The gas is ionized and the positive ions formed are attracted to a cathode within the container where they remain trapped. The pump is only useful at very low pressures, i.e. below about 1 micropascal. The pump has a limited capacity because the absorbed ions eventually saturate the surface of the cathode. A more effective pump can be made by simultaneously *sputtering a Ülm of metal, so that fresh surface is continuously produced. The device is then known as a sputter-ion pump. ion trap A device used to trap ions by electrical or magnetic Üelds (or a combination of both). There are a number of types. The Paul trap has a ring electrode with a hyperbolic section and two hyperbolic end caps. Ions are trapped in an oscillating Üeld produced by applying an oscillating voltage (about 1 MHz) between the ring electrode and the end caps. The device was invented in the 1950s by the German physicist Wolfgang Paul (1913–93). The Penning trap has a similar geometry and operation, but there is also a positive direct voltage (about 100 V) on the cap electrodes with respect to the ring electrode, and an axial magnetic Üeld (about 5 tesla) to conÜne the particles. This type of trap was developed in 1959 by the German– American physicist Hans Dehmelt (1922– ). He named it after Frans Penning, who had invented the *Penning gauge, which also uses a magnetic Üeld. Ion traps can be used for storing and investigating the properties of ions and other charged particles and can also be used in *mass spectrometry. IP See  . IP3 See . IR See  . iridium Symbol Ir. A silvery metallic *transition element (see also  ); a.n. 77; r.a.m. 192.20; r.d. 22.42; m.p. 2410°C; b.p. 4130°C. It occurs with platinum and is
iridium anomaly mainly used in alloys with platinum and osmium. The element forms a range of iridium(III) and iridium(IV) complexes. It was discovered in 1804 by Smithson Tennant (1761–1815).
iridium anomaly The occurrence of unusually high concentrations of the relatively scarce metal iridium at the boundaries of certain geological strata. Two such layers have been discovered, one at the end of the Cretaceous, 65 million years ago, and the second at the end of the Eocene, 34 million years ago. One theory to account for these suggests that on each occasion a huge iridium-containing meteorite may have collided with the earth, producing a cloud of dust that settled out to form an iridium-rich layer. The environmental consequences of such an impact, notably in causing a general warming of the earth by the *greenhouse effect, may have led to the extinction of the dinosaurs at the end of the Cretaceous and the extinction of many radiolarians at the end of the Eocene. See A . iris 1. (in anatomy) The pigmented ring of muscular tissue, lying between the cornea and the lens, in the eyes of vertebrates and some cephalopod molluscs. It has a central hole (the pupil) through which light enters the eye and it contains both circular and radial muscles. ReÛex contraction of the former occurs in bright light to reduce the diameter of the pupil; contraction of the radial muscles in dim light increases the pupil diameter and therefore the amount of light entering the eye. Colour is determined by the amount of the pigment melanin in the iris. Blue eyes result from relatively little melanin; grey and brown eyes from increasingly larger amounts. 2. (in physics) See . iron Symbol Fe. A silvery malleable and ductile metallic *transition element; a.n. 26; r.a.m. 55.847; r.d. 7.87; m.p. 1535°C; b.p. 2750°C. The main sources are the ores *haematite (Fe2O3), *magnetite (Fe3O4), limonite (FeO(OH)nH2O), ilmenite (FeTiO3), siderite (FeCO3), and pyrite (FeS2). The metal is smelted in a *blast furnace to give impure *pig iron, which is further processed to give *cast iron, *wrought iron, and various types of *steel. The pure element has three crystal forms: alpha-iron, stable below 906°C with a body-centred-cubic structure;
gamma-iron, stable between 906°C and 1403°C with a nonmagnetic face-centredcubic structure; and delta-iron, which is the body-centred-cubic form above 1403°C. Alpha-iron is ferromagnetic up to its Curie point (768°C). The element has nine isotopes (mass numbers 52–60), and is the fourth most abundant in the earth's crust. It is required as a trace element (see  ) by living organisms. Iron is quite reactive, being oxidized by moist air, displacing hydrogen from dilute acids, and combining with nonmetallic elements. It forms ionic salts and numerous complexes with the metal in the +2 or +3 oxidation states. Iron(VI) also exists in the ferrate ion FeO42–, and the element also forms complexes in which its oxidation number is zero (e.g. Fe(CO)5).
iron(II) chloride A green-yellow deliquescent compound, FeCl2; hexagonal; r.d. 3.16; m.p. 670°C. It also exists in hydrated forms: FeCl2.2H2O (green monoclinic; r.d. 2.36) and FeCl2.4H2O (blue-green monoclinic deliquescent; r.d. 1.93). Anhydrous iron(II) chloride can be made by passing a stream of dry hydrogen chloride over the heated metal; the hydrated forms can be made using dilute hydrochloric acid or by recrystallizing with water. It is converted into iron(III) chloride by the action of chlorine. iron(III) chloride A black-brown solid, FeCl3; hexagonal; r.d. 2.9; m.p. 306°C; decomposes at 315°C. It also exists as the hexahydrate FeCl3.6H2O, a brown-yellow deliquescent crystalline substance (m.p. 37°C; b.p. 280–285°C). Iron(III) chloride is prepared by passing dry chlorine over iron wire or steel wool. The reaction proceeds with incandescence when started and iron(III) chloride sublimes as almost black iridescent scales. The compound is rapidly hydrolysed in moist air. In solution it is partly hydrolysed; hydrolysis can be suppressed by the addition of hydrochloric acid. The compound dissolves in many organic solvents, forming solutions of low electrical conductivity: in ethanol, ethoxyethane, and pyridine the molecular weight corresponds to FeCl3 but is higher in other solvents corresponding to Fe2Cl6. The vapour is also dimerized. In many ways the compound resembles aluminium chloride, which it may replace in Friedel–Crafts reactions.
iron(II) oxide A black solid, FeO; cubic; r.d. 5.7; m.p. 1420°C. It can be obtained by heating iron(II) oxalate; the carbon monoxide formed produces a reducing atmosphere thus preventing oxidation to iron(III) oxide. The compound has the sodium chloride structure, indicating its ionic nature, but the crystal lattice is deÜcient in iron(II) ions and it is nonstoichiometric. Iron(II) oxide dissolves readily in dilute acids. iron(III) oxide A red-brown to black insoluble solid, Fe2O3; trigonal; r.d. 5.24; m.p. 1565°C. There is also a hydrated form, Fe2O3.xH2O, which is a red-brown powder; r.d. 2.44–3.60. (See .) Iron(III) oxide occurs naturally as *haematite and can be prepared by heating iron(III) hydroxide or iron(II) sulphate. It is readily reduced on heating in a stream of carbon monoxide or hydrogen. iron pyrites See . ironstone A sedimentary rock so-called because of its content of iron, usually in the form of the minerals haematite, limonite, or pyrite, which sometimes give the surface of the rock a typical rusty colour. It is found in beds, layers, or nodules. The iron minerals often take the form of small spherical oolites embedded in the rock. iron(II) sulphate An off-white solid, FeSO4.H2O; monoclinic; r.d. 2.970. There is also a heptahydrate form, FeSO4.7H2O; bluegreen monoclinic; r.d. 1.898; m.p. 64°C. The heptahydrate is the best known iron(II) salt and is sometimes called green vitriol or copperas. It is obtained by the action of dilute sulphuric acid on iron in a reducing atmosphere. The anhydrous compound is very hygroscopic. It decomposes at red heat to give iron(III) oxide, sulphur trioxide, and sulphur dioxide. A solution of iron(II) sulphate is gradually oxidized on exposure to air, a basic iron(III) sulphate being deposited. iron(III) sulphate A yellow hygroscopic compound, Fe2(SO4)3; rhombic; r.d. 3.097; decomposes above 480°C. It is obtained by heating an aqueous acidiÜed solution of iron(II) sulphate with hydrogen peroxide: 2FeSO4 + H2SO4 + H2O2 → Fe2(SO4)3 + 2H2O On crystallizing, the hydrate Fe2(SO4)3.9H2O is formed. The acid sulphate Fe2(SO4)3. H2SO4.8H2O is deposited from solutions containing a sufÜcient excess of sulphuric acid.
irradiance Symbol E. The *radiant Ûux per unit area reaching a surface; in SI units it is measured in watts per square metre (W m–2). Irradiance refers to electromagnetic radiation of all kinds, whereas *illuminance refers only to visible radiation. irradiation Exposure to any form of radiation; often exposure to *ionizing radiation is implied. See also  . irrational number A number that cannot be expressed as the ratio of two integers. An irrational number may be a *surd, such as √2 or √3, which can be expressed to any desired degree of accuracy but cannot be assigned an exact value. Alternatively, it may be a *transcendental number, such as π or e. Compare  . irreversibility The property of a system that precludes a change to the system from being a *reversible process. The paradox that although the equations describing the bodies in a system, such as Newton's laws of motion, Maxwell's equation, or Schrödinger's equation are invariant under *time reversal, events involving systems made up from large numbers of these bodies are not reversible. The process of scrambling an egg is an example. The resolution of this paradox requires the concept of *entropy using *statistical mechanics. Irreversibility occurs in the transition from an ordered arrangement to a disordered arrangement, which is a natural trend, since changes in a closed system occur in the direction of increasing entropy. Irreversibility also occurs in processes that violate T symmetry. According to the *CPT theorem, processes that violate CP also violate T and hence are irreversible. This has been observed in some weak interactions. irreversible reaction See  . irrigation The provision of water for crops by artiÜcial methods; for example by constructing ditches, pipe systems, and canals. Irrigation can lead to problems when the water leaches trace elements from the soil; selenium, for example, can be toxic to both local fauna and Ûora. Irrigation can also increase the salinity of the soil, if diverted rivers are used to provide the water. Evaporation of surface water leaves a crust of salt, which can drain down to deeper layers of the soil. irritability See .
IR spectroscopy IR spectroscopy See  .
2-methylbutane) is an isomer of pentane. See .
ischium The most posterior of the three bones that make up each half of the *pelvic girdle. See also ; .
isobar 1. A line on a map or chart that joins points or places that have the same atmospheric pressure. 2. A curve on a graph representing readings taken at constant pressure. 3. One of two or more nuclides that have the same number of nucleons but different *atomic numbers. Radium–88, actinium–89, and thorium–90 are isobars as each has a *nucleon number of 228.
isentropic process Any process that takes place without a change of *entropy. The quantity of heat transferred, δQ, in a reversible process is proportional to the change in entropy, δS, i.e. δQ = TδS, where T is the thermodynamic temperature. Therefore, a reversible *adiabatic process is isentropic, i.e. when δQ equals zero, δS also equals zero.
Ising model A simpliÜed model of a magnetic system consisting of an array of magnetic spins. Spins may have one of two values and interactions occur with nearest neighbours. There are also random thermal Ûuctuations depending on the temperature of the system. At low temperatures there is a net magnetization as a result of alignment of spins. At high temperature there is no net magnetization. The model was Ürst proposed by the German physicist Ernst Ising (1900–98), who studied the one-dimensional case in 1924. The two-dimensional case for a square lattice was solved exactly by Lars Onsager in 1944. Only approximate solutions have been found for three-dimensional models. The Ising model is very important in statistical mechanics and can be used to investigate other types of phase transition. ISIS/Draw A commonly used chemical drawing program for 2D and 3D structures, copyright of MDL Information Systems, Inc. The program has certain additional features including calculation of molecular weight, calculation of percentages of elements present, IUPAC name generation, and viewing in RasMol.
• A limited version of ISIS/Draw at the Elsevier MDL website (free registration required)
islets of Langerhans Small groups of cells in the pancreas that function as an endocrine gland. The alpha (or A) cells secrete the hormone *glucagon, the beta (or B) cells secrete *insulin, and the D cells secrete *somatostatin. The islets are named after their discoverer, the German anatomist and microscopist Paul Langerhans (1847–88). iso- PreÜx denoting that a compound is an isomer, e.g. isopentane (CH3CH(CH3)C2H5,
isobaric spin See  . isocline A line on a map or chart joining points or places of equal magnetic dip (see ). isocyanate See  . isocyanic acid See  . isocyanide See . isocyanide test A test for primary amines by reaction with an alcoholic solution of potassium hydroxide and trichloromethane. RNH2 + 3KOH + CHCl3 → RNC + 3KCl + 3H2O The isocyanide RNC is recognized by its unpleasant smell. This reaction of primary amines is called the carbylamine reaction. isodiaphere One of two or more nuclides in which the difference between the number of neutrons and the number of protons is the same. A nuclide and its product after losing an *alpha particle are isodiapheres. isodynamic line A line on a map or chart joining points or places at which the total strengths of the earth's magnetic Üeld are equal (see ). isoelectric point The pH of a medium at which a protein carries no net charge and therefore will not migrate in an electric Üeld. Proteins precipitate most readily at their isoelectric points; this property can be utilized to separate mixtures of proteins or amino acids. isoelectronic Describing compounds that have the same numbers of valence electrons. For example, nitrogen (N2) and carbon monoxide (CO) are isoelectronic molecules. isoenzyme See . isogamy Sexual reproduction involving the production and fusion of gametes that are similar in size and structure. It occurs in
some protoctists, e.g. certain protozoans and algae. Compare .
isogonal line A line on a map or chart joining points or places of equal magnetic declination (see ). isoindole See . isolating mechanism Any of the biological properties of organisms that prevent interbreeding (and therefore exchange of genetic material) between members of different species that inhabit the same geographical area. These mechanisms include seasonal isolation, in which the *breeding seasons of the different populations do not overlap; and behavioural isolation, in which different *courtship behaviour in the populations ensures that mating takes place only between members of the same species. Both these are examples of premating mechanisms. Postmating mechanisms include hybrid infertility and inviability. isoleptic complex A metal complex in which all the ligands are the same. isoleucine See  . isomerase Any of a class of *enzymes that catalyse the rearrangement of the atoms within a molecule, thereby converting one isomer into another. isomerism 1. (in chemistry) The existence of chemical compounds (isomers) that have the same molecular formulae but different molecular structures or different arrangements of atoms in space. In structural isomerism the molecules have different molecular structures: i.e. they may be different types of compound or they may simply differ in the position of the functional group in the molecule. Structural isomers generally have different physical and chemical properties. In stereoisomerism, the isomers have the same formula and functional groups, but differ in the arrangement of groups in space. Optical isomerism is one form of this (see  ). Another type is cis–trans isomerism (formerly geometrical isomerism), in which the isomers have different positions of groups with respect to a double bond or central atom (see illustration overleaf). Octahedral complexes can display cis–trans isomerism if they have formulae of the type MX2Y4. Octahedral complexes with formulae of the type MX3Y3 can display a different type of isomerism. If the three X lig-
ands are in a plane that includes the metal atom and the three Y ligands are in a different plane at right angles, then the structure is a mer-isomer (meridional). If the three X ligands are all on one face of the octahedron and the three Y ligands are on an opposite face, then it is a fac-isomer (facial). See illustration overleaf. See also ; E–Z . 2. (in physics) The existence of atomic nuclei that have the same atomic number and the same mass number but different energy states.
isomers See . isometric 1. (in technical drawing) Denoting a projection in which the three axes are equally inclined to the surface of the drawing and lines are drawn to scale. 2. (in crystallography) Denoting a system in which the axes are perpendicular to each other, as in cubic crystals. 3. (in physics) Denoting a line on a graph illustrating the way in which temperature and pressure are interrelated at constant volume. isomorphism The existence of two or more substances (isomorphs) that have the same crystal structure, so that they are able to form *solid solutions. isonitrile (isocyanide; carbylamine) An organic compound containing the group –NC, in which the bonding is to the nitrogen atom.
iso-octane See ;  . isopoly compound See  . isoprene A colourless liquid diene, CH2:C(CH3)CH:CH2. The systematic name is 2-methylbuta-1,3-diene. It is the structural unit in *terpenes and natural *rubber, and is used in making synthetic rubbers. isospin See  . isostasy The theoretical equilibrium that tends to exist in the earth's crust. If this equilibrium is disturbed, for example as a result of erosion or deposition, compensatory movements in the earth's crust occur: areas of deposition sink, whereas areas of erosion rise. Continental ice sheets have been an important cause of isostatic movements of the earth's crust. The growth of ice sheets and the resulting additional mass of the ice is compensated by the downward
isotactic polymer
structural isomers in which the functional group has different positions
H O
methoxymethane
structural isomers in which the functional groups are different
CH3 C
CH3 C H
trans-but-2-ene
cis-but-2-ene
cis–trans isomers in which the groups are distributed on a double bond
Pt I
CI Pt
cis–trans isomers in a square-planar complex
O C
keto form
enol form
keto–enol tautomerism
Isomerism. deÛection of the lithospheric plate and the displacement of asthenospheric material (see ) beneath it. When the ice melts, the displaced material Ûows back causing the overlying lithospheric plate to rise.
isotherm 1. A line on a map or chart joining points or places of equal temperature. 2. A curve on a graph representing readings taken at constant temperature (e.g. the relationship between the pressure and volume of a gas at constant temperature).
isotactic polymer See .
isothermal process Any process that
isotopic signature
Y trans-isomer
Y cis-isomer
fac-isomer
mer-isomer
Isomerism. takes place at constant temperature. In such a process heat is, if necessary, supplied or removed from the system at just the right rate to maintain constant temperature. Compare  .
isotone One of two or more nuclides that contain the same number of neutrons but different numbers of protons. The naturally occurring isotones, for example, strontium– 88 and yttrium–89 (both with 50 neutrons), give an indication of the stability of certain nuclear conÜgurations. isotonic Describing solutions that have the same osmotic pressure. isotope One of two or more atoms of the same element that have the same number of protons in their nucleus but different numbers of neutrons. Hydrogen (1 proton, no neutrons), deuterium (1 proton, 1 neutron), and tritium (1 proton, 2 neutrons) are isotopes of hydrogen. Most elements in nature consist of a mixture of isotopes. See  .
• Isotopic compositions at the NIST website
isotope separation The separation of the *isotopes of an element from each other on the basis of slight differences in their
physical properties. For laboratory quantities the most suitable device is often the mass spectrometer. On a larger scale the methods used include gaseous diffusion (widely used for separating isotopes of uranium in the form of the gas uranium hexaÛuoride), distillation (formerly used to produce heavy water), electrolysis (requiring cheap electrical power), thermal diffusion (formerly used to separate uranium isotopes, but now considered uneconomic), centrifuging, and laser methods (involving the excitation of one isotope and its subsequent separation by electromagnetic means).
isotopic number (neutron excess) The difference between the number of neutrons in an isotope and the number of protons. isotopic signature The relative proportions of certain isotopes in a sample of tissue, water, air or other material. In biology the proportions of stable isotopes of common elements in organic matter, especially carbon, nitrogen, and oxygen, can provide useful clues about growing conditions, lifestyle, and metabolism of the organism from which the material was derived. Differences in the relative tissue abundance of the natural stable isotopes of carbon (i.e. 12C and 13C), oxygen (16O and 18O) and nitrogen
isotopic spin (14N and 15N) reÛect their differing availability in the environment or in the diet, or some form of selective metabolism by the organism. So, for example, human hair samples with differing isotopic signatures can indicate the dietary preferences of the individuals.
isotopic spin (isospin; isobaric spin) A quantum number applied to hadrons (see  ) to distinguish between members of a set of particles that differ in their electromagnetic properties but are otherwise apparently identical. For example if electromagnetic interactions and weak interactions are ignored, the proton cannot be distinguished from the neutron in their strong interactions: isotopic spin was introduced to make a distinction between them. The use of the word 'spin' implies only an analogy to angular momentum, to which isotopic spin has a formal resemblance. isotropic Denoting a medium whose physical properties are independent of direction. Compare . isozyme (isoenzyme) One of several forms of an enzyme in an individual or population that catalyse the same reaction but differ from each other in such properties as substrate afÜnity and maximum rates of enzyme–substrate reaction (see M– M ). IT (information technology) The use of
computers and telecommunications equipment (with their associated microelectronics) to send, receive, store and manipulate data. The data may be textual, numerical, audio or video, or any combination of these. See also W W W.
iteration The process of successive approximations used as a technique for solving a mathematical problem. The technique can be used manually but is widely used by computers. IUPAC International Union of Pure and Applied Chemistry. An international nongovernmental body formed in 1919 to foster worldwide communications in chemical science, both academic and industrial. IUPAC is the international authority deÜning recommended terminology, atomic weights, isotopic abundances, and other data.
• The IUPAC home page
IUPAP International Union of Pure and Applied Physics. An international nongovernmental body formed in 1922 to stimulate and promote international cooperation in physics. IUPAP fosters the preparation and publication of tables of physical constants and promotes international agreements on the use of symbols, units, nomenclature, and standards.
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J jacinth See . Jacob–Monod hypothesis The theory postulated by the French biologists François Jacob (1920– ) and Jacques Monod (1910–76) in 1961 to explain the control of *gene expression in bacteria (see ). Jacob and Monod investigated the expression of the gene that codes for the enzyme βgalactosidase, which breaks down lactose; the operon that regulates lactose metabolism is called the *lac operon. jade A hard semiprecious stone consisting either of jadeite or nephrite. Jadeite, the most valued of the two, is a sodium aluminium pyroxene, NaAlSi2O6. It is prized for its intense translucent green colour but white, green and white, brown, and orange varieties also occur. The only important source of jadeite is in the Mogaung region of upper Burma. Nephrite is one of the amphibole group of rock-forming minerals. It occurs in a variety of colours, including green, yellow, white, and black. Important sources include Siberia, Turkistan, New Zealand, Alaska, China, and W USA. jadeite See . Jahn–Teller effect If a likely structure of a nonlinear molecule or ion would have degenerate orbitals (i.e. two molecular orbitals with the same energy levels) the actual structure of the molecule or ion is distorted so as to split the energy levels ('raise' the degeneracy). The effect is observed in inorganic complexes. For example, the ion [Cu(H2O)6]2+ is octahedral and the six ligands might be expected to occupy equidistant positions at the corners of a regular octahedron. In fact, the octahedron is distorted, with four ligands in a square and two opposite ligands further away. If the 'original' structure has a centre of symmetry, the distorted structure must also have a centre of symmetry. Jahn–Tellar splitting is the splitting of spectral lines as a result of this effect. H. A. Jahn and Edward Teller predicted this effect in 1937 using group theory. jargoon See .
jasper An impure variety of *chalcedony. It is associated with iron ores and as a result contains iron oxide impurities that give the mineral its characteristic red or reddishbrown colour. Jasper is used as a gemstone. JavaScript A scripting language designed to add features to web pages. JavaScript code is embedded in the HTML code and is run by the web browser. JavaScript is loosely based on the Java programming language and was produced by Netscape and Sun