A lecture on the preservation of health
Garnett, Thomas, 1766-1802. n 85323174
ik I >* <■ * # t A LECTURE, &c. Digitized by the Internet Archive in 2015 https://archive.org/details/b22027543 Til (DMA 3 GA RJSTJE TT, 31. 1J. ( PuNi/hf,/ MurrA •/.{** /SOO . by GuMf &’ .1 'tmnrf.. *Td A LECTURE ON THE PRESERVATION OF HEALTH. By T. GARNETT, M. D. Profeffor of Natural Philofophy and Chemiftry in the Royal Inllitution of Great Britain &c. SECOND EDITION. Such the reward of rude and fober life ; Of labour fuch. By health the peafant’s toil Is well repaid ; if exercife were pain Indeed, and temperance pain. Armjlrong. LONDON: PRINTED FOR T. CADELL, JUNIOR, AND W. DAVIES, STRAND. 1800. (R. Noble, Printer, Old Bailey.) h,stor»«^- wepio*v. To ERASMUS DARWIN , M. D. Dear Sir , THE jirjl edition of this pamphlet having been introduced to the world under the fantlion of your name , I take the liberty of prefixing it to the fecond ; and am happy in having another public opportunity of exprejfing my thanks for the high gratif- cation and inflruElion which I have received from the perufal of your medical and philo- fophical works. Iam, Dear Sir, With much efleem, Tour very obedient fervant, THO. GARNETT. Royal Infiitution, April 8th, 1 800. PREFACE. tS Most medical gentlemen will, it fuppofed, agree that the greater part of the numerous train of difeafes to which their patients are fubjett, have been brought on by improper conduct and im- prudence. That this conduct often pro- ceeds from ignorance of its bad effects, may be prefumed ; for though it cannot be denied that fome perfons are perfectly regardlefs with refpect to their health , yet FR E FACE. a yet the great mafs of mankind are too fenfible of the enjoyment and lofs of this great ef of bleffmgs, to run headlong into danger with their eyes open. It was with the hope of making the laws of life more generally known, and better underfood , and from thence de- ducing fuch rules for the preferval ion of health , as would be evident to every capacity, that the author zvas induced to deliver this leliure. It has been honoured with the attention of numerous audiences, in fome of the mofl populous towns in England, where it has gene- rally PREFACE. Hi rally been read for the benefit of chari- table infiitutions. The author flatters himfelf that be- fides the benefit produced by his humble endeavours to ferve thefe infiitutions , thofe endeavours have not totally failed in the grand object of preferving health ; and with the hope that the influence of the precepts here given, may be farther extended, he has concurred in the ideas of thofe who have advifed the publica- tion of this leElure. It is to be feared, that notwithjiand- ing all which can be done , difeafe will con- PREFACE. iv continue to be a heavy tax , which civilized Jociety mujl pay for its com- forts ; and the valetudinarian will often be tempted to envy the favage the frength and foundnefs of his conjlitution. Much however may be done towards the prevention of a number of difeafes. If this leffure ffould contribute to the attainment of fo defirable an end , it will afford the highejl gratification to the author. The firft part of the leliure is the fubfiance of an effay which was read by the author before the Royal Medical Society of Edinburgh, intended as a de- fence PREFACE, •u fence of the general -principles of the fyf- tem of Dr. Brozvn, whofe pupil he then was. It wasy according to cuJlom} tran- fcrihed into the books of the focietyy and the public have now an opportunity of judging how far Dr. Girtannery in his firjl ejfay pub life d in the Journal de Phyfique, about two years after , in zvhich he gives the theory as his own , without the leaf acknowledgment to the much injured and unfortunate author of the Elementa Medicinae> has borrowed from this ejfay. In public leflureSy novelty is not to be expected^ the principal objeli of the lebturer Vi PREFACE. Usurer being to place in a proper point of view y what has been before dif- covered. The author has therefore freely availed himfelf of the labours of others , particularly of the popular publications of Dr. Beddoes , which he takes this opportunity of acknowledg- ing. This ledlure is pub li/bed almojl ver- batim as it was delivered. On this account the experiments mentioned are not minutely defcribed, the reader being fuppofed to fee them performed. A LECTURE, f3c. The greateft blefling we enjoy is health, without it, wealth, honors, and every other coniideration, would be infipid, and even irkfome ; the prefervation of this ftate therefore, naturally concerns us all. In this lecture, I {hall not attempt to teach b * you ( 2 ) you to become your own phyficians, for when the barriers of health are once broken down, and difeafe has eftablifhed itfelf, it requires the deep- eft attention, and an accurate ac- quaintance with the extenfive fcience of medicine, to combat it ; to attain this knowledge demands the labour of years. But, a majority of the difeafes to which we are fubjedt, are the effedts of our own ignorance or imprudence, and it is often very eafy to prevent them ; mere precepts how- ever, have feldom much effedl:, unlefs the reafoning upon them be rendered evident ; on this account, I fhall firft en- ( 3 ) endeavour, in as plain and eafy a man- ner as poflible, to explain to you the laws by which life is governed ; and when we fee in what health confifts, we fhall be better enabled to take fuch methods as may preferve it. Health is the eafy and pleafant exercife of all the functions of the body and mind ; and difeafe confifts in the un- eafy and difproportioned exercife of all, or fome of the functions. When dead matter acts upon dead matter, the only effects we perceive are mechanical, or chemical ; for though there may appear to be other b 2 kinds ( 4 ) kinds of attraction, or repulfion, fuch as eleCtric and magnetic, yet thefe come under the head of mechanical attraction, as producing motion ; vve may therefore lay it down as a law, that when dead, or inanimate bodies aCt upon each other, no other than mechanical, or chemical effeCts are produced ; that is, either motion, or the decompofition, and new com- bination of their parts. If one ball ftrike another, it communicates to it a certain quantity of motion, this is called mechanical aCtion ; and if a quantity of fait, or fugar, be put into water, the particles of the fait or fugar ( 5 ) fugar will feparate from each other, and join themfelves to the particles of the water; the fait and water in thefe inftances, are faid to act on each other chemically ; and in all cafes whatever, in which inanimate, or dead bodies adt on each other, the effects produced are, motion, or che- mical attraction. / But, when dead matter adts on thofe bodies which we call living, the effects are much different ; let us take for example a very fimple inftance. — Snakes, at leaft fome fpecies of them, pafs the winter in a torpid date, which has ( 6 ) has all the appearance of death ; now heat, if applied to dead matter, will only produce motion, or chemical combination ; but if it be applied to the fnake, let us fee what will be the confequence; the reptile firft begins to move, and opens its eyes and mouth ; when the heat has been ap- plied for fome time, it crawls about in fearch of food, and performs all the functions of life. Here then, dead matter, when applied to a living body, produces living functions ; for if the heat had not been applied, the fnake would have continued fenfelefs, and apparently lifelefs. In more perfed f 7 ) perfect animals, the effedts produced by the action of dead matter on them, are more numerous, and are different f. in different living fyftems, but are in general the following — fenfe and mo- tion in almoft all animals, and in many the power of thinking, and other affections of the mind. The powers, or dead matters, which are applied, and which produce thefe fundtions, are chiefly, heat, food, and air. The proof that thefe powers do produce the living functions, is in my opinion a very convincing one, name- ly, that when their actions are fuf- pended, the living functions ceafe; take ( 8 ) take away, for inftance, heat, air, and food from animals, and they foon become dead matter ; and it is not neceflary that an animal fhould be deprived of all thefe to put a flop to the living functions ; if any one of them be taken away, the body fooner or later becomes dead matter : it is found by experience, that if a man be deprived of air, he dies in about three or four minutes ; for inftance, if he be immerfed under water ; if he be deprived of heat, or in other words, expofed to a very fevere degree of cold, he likewife foon dies ; or if he be deprived of food, his death is equally ( 9 ) equally certain, though more flow. It is fufficiently evident then, that the living functions are owing to the action of thefe external powers upon the body. What I have here faid, is not confined to animals, but the liv- ing functions of vegetables are like- wife caufed by the adtion of dead mat- ter upon them. The dead matters, which by their adtion produce thefe functions, are principally heat, moif- ture, light, and air. It clearly fol- lows therefore, from what I have faid, that living bodies muft have fome property different from dead matter, which renders them capable of being adted ( IO ) abted upon by thefe external powers, fo as to produce the living funbtions ; for if they had not, the only effects which thefe powers could produce, would be mechanical, or chemical. Though we know not exactly in what this property confifts, or in what manner it is a6ted on, yet we fee, that when bodies are poirdTed of it, they become capable of being a6ted upon by external powers, and thus the liv- ing fun&ions are produced ; we fhall therefore call this property excita- bility, and in ufing this term it is ne- celTary to mention, that I mean only to exprefs a fa6t, without the leaft in- tention ( II ) tention of pointing out the nature of that property which diftinguifhes liv- ing from dead matter, and in this we have the example of the great New- ton, who called the property which caufes bodies in certain fituations to approach each other, gravitation , without in the leaft hinting at its na- ture ; yet, though he knew not what gravitation was, he inveftigated the laws by which bodies were a6ted on by it ; in the fame manner, though we are ignorant of excitability, or the nature of that property which dif- tinguifhes living from dead matter, we can inveftigate the laws by which dead dead matter adts on living bodies through this medium. We know not what magnetic attraction is, and yet we can inveftigate its laws ; the fame holds good with regard to electricity; if we ever fhould attain a knowledge of the nature of this property, it would make no alteration in the laws which we had before difco- vered. I lhall now proceed to the invefti- gation of the laws by which the ex- citability is adted on ; but I mult firft define fome terms which it will be necelfary to ufe, to avoid circumlo- cution, ( i3 ) cution, and at the fame time to give us more diftind ideas on the fubjed. When the excitability is in fuch a flate as to be very fufceptible of the action of external powers, I fhall call , it abundant , or accumulated ; but when it is found not very capable of receiv- ing their adion, I fhall fay, it is de- ficient y or exhaujled. I would not with however, to have it thought, that by thefe terms I mean in the leaf: to' hint at the nature of excita- bility, nor that it is really one while increafed, and at another diminifhed in quantity, for the abftrad queftion is ( 14 ) is in no ffiape confidered ; we know not whether the excitability, or the vital principle, depends on a particu- lar arrangement of matter, or from whatever caufe it may originate ; by the terms here ufed, I mean only to fay, that the excitability is eafily adled on when I call it abundant, or accu- mulated ; at other times the living body is with more difficulty excited, and then I fay, the vital principle is deficient, or exhaufted. The laws by which external powers ad on living bodies, will, on a care- ful ( 1 5 ) fill examination, be found to be the following — Firft, when the powerful action of the exciting powers ceafes for fome time, the excitability accumulates, or becomes more capable of receiving their action, and is more powerfully affected by them. If we examine feparately the dif- ferent exciting powers, which ad: on the body, we fhall find abundant confirmation of this law. Let us firft confider Light ; if a perfon be kept in darknefs for fome time, and be then ( 16 ) then brought into a room in which there is only an ordinary degree of light, it will be almoft o oppreffive for him, and appear exceftively bright ; and if he have been kept for a confiderable time in a very dark place, the fenfation will be very pain- ful. In this cafe, while the retina, or optic nerve, was deprived of light, its excitability accumulated, or became more eaiily affedled by light ; for if a perfon goes out of one room, into ano- ther which has an equal degree of light, he will feel no effcd:. You may convince yourfelves of this law by a very fimple experiment — fhut your ( 17 ) \our eyes, and cover them for a minute or two with your hand, and endeavour not to think of the light, or of what your are doing ; then open them, and the day-light will for a ihort time appear brighter. If you look attentively at a window, for about two minutes, and then call your eyes upon a fhcet of white paper, the fhape of the window-frames will be perfectly vifible upon the paper ; thofe parts which exprcfs the wood- work, appearing brighter than the other parts. The parts of the optic nerve on which the image of the frame falls, are covered by the wood- c work ( 1* ) work from the adlion of the light ; the excitability of thefe portions of the nerve will therefore accumulate, and the parts of the paper which fall upon them, mult of courfe appear brighter. Exadlly the fame takes place with refped: to food, if a perfon have for fome time been deprived of food, or have taken it in fmall quantity, whe- ther it be meat or drink ; or if he have taken it of a lefs {Emulating quality, he will find, that when he returns to his ordinary mode of liv- ing, it will have more effcdt upon him ( 22 j him than before he lived abftemi- oufly. Perfons who have been fhut up in a coal-work from the falling in of the pit, and have had nothing to eat for two or three days, have been as much intoxicated by a bafon of broth, as a perfon in common circum fiances with two or three bottles of wine ; and we all know that fpirituous, or vinous liquors affed the head more in the morning, than after dinner. This circumftance was particularly evident among the poor failors who were ( 23 ) were in the boat with Captain Bligh after the mutiny. The captain was fent by government to convey fome plants of the bread-fruit tree from Otaheite, to the Weft-Indies ; foon after he left Otaheite, the crew muti- nied, and put the captain and moft of the officers, with fome of the men, on board the fhip’s boat, with a very fhort allowance of provifions, and particularly of liquors, for they had only fix quarts of rum, and fix bot- tles of wine, for nineteen people, who were driven by ftorms about the fouth-fea, expofed to wet and cold all the time, for nearly a month ; each man ( 24 ) man was allowed only a tea-fpoon full of rum a-day, but this tea-fpoon full refrelhed the poor men, benumbed as they were with cold, and faint with hunger, more than twenty times the quantity Mould have done thofe Mho Mere M7arm, and well fed ; and had it not been for the fpirit having fuch poM;er to adt upon men in their con- dition, they never could have out- lived the hardfhips they experienced. All thefe fadts, and many others which might be brought, eftablifh beyond a doubt the truth of the law I have mentioned, namely, that when the poM erful adtion of the exciting powers ceafes ( ) ceafes for fome time, the excitability accumulates, or becomes more capa- ble of receiving their actions. The lecond law is, that when the exciting powers have adted with vio- lence, or for a conliderable time, the excitability becomes exhaufted, or lefs fit to be adted on, and this we fhall be able to prove by a fimilar induction. Let us take the effedts of light upon the eye ; w hen it has adled violently for fome time upon the optic nerve, it diminifhes the excita- bility of that nerve, and renders it incapable of being afFedted by a quantity ( 26 J quantity of light that would at other times afFedt it. When you have been walking out in the fnow, if you come into your room, you will fcarcely be able to fee any thing for fome mi- nutes. Look ftedfaftly at a candle for a minute or two, and you will with difficulty difcern the letters of a book, which you were before read- ing diftindtly ; and if you happen to caft your eyes upon the fun, you will not fee any thing diftinctly for fome time afterwards. Let us next conlider the matter of heat : fuppofe water to be heated luke- ( 27 j lukewarm, if you put one hand into it, it will feel warm ; if you now put the other hand into water, heated for inftance to 120° or 130°, and keep it there fome time, we will fay, two minutes ; if then you take it out, and put it into the lukewarm water, that water will feel cold, though Bill it will feem warm to the other hand ; for, the hand which had been in the heated water, has had its excitability exhauft- ed by the application of heat. Before you go into a warm bath, the tempe- rature of the air may feem warm and agreeable to you, but after you have remained for fome time in a bath that is ( 28 ) is rather hot, when you come out, you feel the air uncommonly cool and chilling. Let us now examine the effects of fubftances taken into the ftomach ; and as the effects of fpirituous, and vinous liquors, are a little more re- markable than food, we fhall make our obfervations upon them. A perfon who is unaccuftomed to drink thefe liquors, will be intoxicated by a quantity that will produce no ef- fect upon one who has been for fome time accuftomed to take them ; and when ( 29 ) when a perfon has ufed himfelf to thefe ftimulants for fome time, the ordinary powers which in common fupport life, will not have their pro- per effedts upon him, becaufe his ex- citability has been in fome meafure exhaufted by the ftimulants. The fame holds good with refpedt to tobacco and opium ; a perfon ac- cuftomed to take opium will not be affedted by a quantity that would completely intoxicate one not ufed to it ; becaufe the excitability has been fo far exhaufted by the ufe of that drug. ( 30 ) drug, that it cannot be adted on by a fmall quantity. Thefe facts, with innumerable others, which will eafily fuggeft them- felves to you, prove the truth of our fecond proportion, namely, that when the exciting powers have adted vio- lently, or for a confiderable time, the excitability is exhaufted, or lefs fit to be adled on. This exhauftion of the excitability, may, however, be either finite, or temporary ; we fee animals, while the exciting powers continue to adl, at firfi ( 3i ) firft appear in their greateft vigour, then gradually decay, and at lafl: come into that hate, in which, from the long continued adtion of the excit- ing powers, the excitability is entirely exhaufted, and death takes place. Welikewife fee plants in the fpring, while the exciting powers have adted on them, moderately, and for a fhort time, arrayed in their verdant robes, and adorned with flowers of “ many mingling hues;” but, as the exciting powers which fupport the life of the plant, continue to be applied, and fome of them, for inflance heat, as the ( 32 ) the lummer advances become increas- ed, they firft lofe their verdure, then grow brown, and at the end of Summer ceaSe to live ; becauSc their excitability is exhaufted by the long continued adtion of the exciting powers ; and this does not happen merely in confequence of the heat of Summer decreafing, for they grow brown and die, even in a greater de- cree of heat than that which in O Spring made them grow luxuriantly. Thefe are examples of the finite, or irreparable exhauftion of the excita- bility ; but we find alfo, that it may be ( 33 ) be exhaufted for a time, and accu- mulated again. Though the eye has been fo dazzled by the fplendour of light, that it cannot fee an objecft moderately illuminated, yet, if it be fhut for fome time, the excitability of the optic nerve accumulates again, and we are again capable of feeing with an ordinary light. We find, that we are not always equally capable of performing the functions of life. When we have been engaged in any exertion, either mental or corporeal, for fome hours only, we find ourfclves fatigued, and unfit D ( 34 ) unfit to purfue our labours much longer ; if in this ftate, feveral of the exciting powers, particularly light and noife, be withdrawn; and if we are laid in a pofture which does not re- quire much mufcular exertion, we foon fall into that (late which nature intended for the accumulation of the excitability, and which we call Sleep. In this ftate, many of the exciting powers cannot adt upon us, unlefs applied with fome violence, for we i are infenfible to their moderate adfion. A moderate light, or a moderate noife, does not afiedt us, and the power of thinking, which exhaufis the excita- bility ( 35 ) bility very much, is in a great meafure fufpended. When the addon of thefe powers has been fufpended for fix or eight hours, the excitability is again capable of being aded on, and we rife frefh, and vigorous, and fit to engage in our occupations. Sleep then, is the method which nature has provided to repair the ex- haufted conftitution, and reftore the vital energy ; without its refrefhing aid, our w'orn-out habits would fcarce- ly be able to drag on a few days, or at moll a few weeks, before the vital ifpring was quite run dowrn ; how pro- d 2 perly ( 36 ) perly therefore has the great poet of nature called fleep the chief nourilher in life’s feaft. — ‘Sleep, that knits up the ravell’d fleeve of care, ‘ the death of each day’s life, fore labour’s bath, ‘ balm of hurt minds, great Nature’s fecond courfe, ‘ chief nourifher in life’s feaft.’ From the internal fenfations often excited, it is natural to conclude that the nerves of fenfe are not torpid during fleep; but that they are only precluded from the perception of ex- ternal objects, by the external organs being rendered unfit to tranfmit to them ( 37 ) them the impulfes of bodies, during the fufpenfion of the power of voli- tion ; thus, the eye-lids are clofed in deep, to prevent the impulfe of the light from acting on the optic nerve; and it is very probable that the drum of the ear is not ftretched ; it is like- wife probable that fomething fimilar happens to the external apparatus of all our organs of fenfe, which may make them unfit for their office of perception during deep. The more violently the exciting- powers have acted, the fooner is deep brought on; becaufe the excitability is ( 38 ) is fooner exhaufted, and therefore, fooner requires the means of renewing it ; and on the contrary, the more weakly the exciting powers have a6t- ed, the lefs is a perfon inclined to deep. Inftances of the firft are, ex- cefs of exercife, ftrong liquors, or ftudy ; and of the latter, ail under proportion of thefe. A perfon who has been daily accuf- tomed to much exercife, whether mental or corporeal, if he omit it, will find little or no inclination to deep ; he may however be made to deep by taking a little diffubble ftimulus ; ( 39 ) ftimulus ; for inftance, a little warm punch, or opium : thefe ad: entirely by exhaufting the excitability to that degree which is compatible with deep ; and when their ftimulant effedl is over, the perfon foon fails into that date. But though the excitability may have been fufficiently exhauded, and the adion of the external powers con- fiderably moderated, yet there 'are fome things within ourfelves, which dimulate violently, and prevent deep ; fuch as pain, third, and drong paf- fions and emotions of the mind. Thefe ( 40 ) Thefe all tend to drive away fleep, but it may be induced, by withdraw- ing the mind from thefe impreflions ; particularly from uneafy emotions, and employing it on fomething which makes a lefs imprefiion; fleep, in fuch cafes, is frequently brought on by liftening to the humming of bees*, or the murmuring of a rivulet ; by employing the mind on fubjedts which do not require much exertion, nor produce too much commotion ; fuch as counting to a thoufand, or * Hinc tibi, quae temper vicino ab limine fepes Hyblaeis apibus florem depafta falifti, Saepe levi fomnum fuadebit inire fufurro. VlRG. counting ( 41 ) counting drops of water which fall flowly. It fometimes happens, as has been well obferved by Dr. Franklin, that an uneafy heat of the fkin, from a want of perfpiration, occafioned by the heat of the bed-cloaths, will pre- vent deep ; in this cafe, he recom- mends a method, which I believe will often fucceed — namely, to get up and walk about the room till you are confiderably cooled ; when you get into bed again, the heat of the fkin will be diminifhed, and perfpiration become more free, and you will probably ( 42 ) probably fleep in a very few mi- nutes*. By indudtion we have difcovered two of the principal laws by which living bodies are governed; the fir ft is, that when the ordinary powers which fupport life have been fufpend- ed, or their adtion leflened for a time, the excitability, or vital principle ac- cumulates, or becomes more fit to re- ceive their actions ; and fecondly, when thefe powers have been adted * May not the heat, and want of perfpiration, depend on an exhaufted irritability of the fub- cutaneous veffels, which will be accumulated by the method here recommended ? upon ( 43 ) upon violently, or for a confidcrable time, the excitability is exhaufted, or becomes lefs fit to receive their ac- tions. There are therefore three ftates in which living bodies exift. — Firft, a ftate of accumulated ex- citability. Second, a ftate of exhaufted excita- bility. Third, when it is in fuch a ftate as to produce the ftrongeft and moft healthy actions, when adted upon by the external powers. From ( 44 ) . From what I have faid, it mult ap- pear, that life is a forced (fate, de- pending on the action of external powers upon the excitability ; and that, by their continued action, if they are properly regulated, the ex- citability will be gradually and in- fallibly exhaufted ; and life will be refigned into the hands of him who gave it, without a druggie, and with- out a groan. We fee then, that nature ope- rates in fupporting the living part of the creation, by laws as fimple and beautiful as thofe by which the in- animate ( 45 ) animate world is governed. In the latter we fee the order and harmony which is obferved by the planets, and their fatellites, in their revolu- tion round the great fource of heat and light. 4 All combin’d 4 and ruled unerring, by that Tingle power 4 which draws the Hone projected, to the ground.’ In the animated part of the crea- tion, we obferve thofe beautiful phe- nomena which are exhibited by an almoft infinite variety of individuals, all depending upon one fimple law, the ( 46 ) the adtion of the exciting powers on the excitability. 1 cannot exprefs my admiration of the wifdom of the creator better than in the words of Thomfon. ‘ O unprofufe magnificence divine ! ‘ O wifdom truly perfeft ! thus to call 1 from a few caufes, fuch a fcheme of things ; ‘ effe&s fo various, beautiful, and great.’ Life then, or thofe functions which we call living, are the effedts of cer- tain exciting powers, adting on the excitability, or property diftinguifh- ing ( 47 ) ing living from dead matter. When thofe effedts, namely, the fundtions, flow eafily, pleafantly, and complete- ly, from the adtion of the exciting powers, they indicate that ftate which we call Health. I have detained you a long time on this fubjedt, but it is of importance to make you acquainted with thefe laws ; for it is from a knowledge of them, that the rules for preferving health . mull be deduced; and having rendered them, as I hope, intelligible to you, I fhall proceed to point out fuch neceflary cautions for your con- dudt. ( ) dud, as are eafily deduced from them; and which experience confirms ; and I fhail follow' an arrangement in the confideration of the fubjed, which naturally prefents itfelf to us. The chief exciting powers w hich ad upon us are, air and food ; thefe I fhail refpedively confider, and afterwards make a few remarks on exercifc. The air is the main-fpring in the animal machine ; the fource of heat and adivity, without w'hich our blood w'ould foon become a black and ftag- nant mafs, and life would foon flop. It ( 49 ) It is now known, that only a part of atmofpheric air, is neceffary for refpi ration : the atmofphere near the furface of the earth, confifts of two kinds of air ; one, which is highly proper for refpiration, and combuf- tion, and in which, an animal im- merfed, will live much longer than in the fame quantity of common air ; and one, which is perfectly improper for fupporting refpiration, or com- buftion, for an inftant. The firft of thefe airs, has been called vital air, from its property of" fupporting life, and conftitutes about k one ( 5° ) one fourth of the atmofphere*. The other, from its property of deftroying life, is called azote, and forms of courfe the remaining three fourths of the atmofphere. Thefe two airs rtiay be feparated from each other by various methods. ]f a candle be inclofed in a given quantity of atmofpheric air, it will burn only for a certain time, and then be extinguished ; and from the riling of the water in the veflel in which * Oxygen gas, according to the new Nomen- clature. It C 51 ) it is inclofed, it is evident that a quantity of air has been abforbed. What has been abforbed is the vital air, and what remains, the azote, which is incapable of fupporting flame. If an animal be immerfed in 4 a given quantity of common air, it will live only a certain time ; at the end of this time, the air will be found diminilhed, about one fourth being extracted from it, and the remainder will neither fupport flame nor animal life ; this experiment might eafily be made, but it feems a piece of unne- ceflary cruelty. f. 2 By ( 52 ) By fimilar experiments to thofe I have mentioned, we get the azote pure ; here is Tome, in which a can- dle has burnt out, and in which no- thing but azote, or the impure part of the atmofphere is left*. I fhall plunge a lighted match into it, and you fee it is infhmtly extinguifhed. Some metals, and particularly man- ganefe, when expofed to the atmo- fphere, attrad the vital air from it. * The fixed air, or carbonic acid gas, form- ed during the combuftion, having been fepa- rated by agitation in contatt with lime water. with- ( S3 ) without touching the azote ; and it may be procured from thefe metals by the application of heat, in very great purity. Here is a. bottle of that kind of air, which I have expelled by heat from manganefe ; I fhall plunge a taper into it, and you will perceive that it burns with great brilliancy. An animal fhut up in it, would live about four times as long as if fhut up in an equal quantity of atmofpheric air. If I take three parts of azote, and one of vital air, I fhall form a com- pound which is fimilar to the atmo- fphcre. ( 54 ) fphere, and which is the mixture beft fuited to fupport the health of the body ; for if there were a much great- er proportion of vital air, it would adt too powerfully upon the fyftem, and bring on inflammatory difeafes ; it would likewife by its ftimulus exhauft the excitability, and bring us fooner to death ; and in the fame manner that a candle burns brighter in vital air, and would therefore be fooner exhaufted, fo would the flame of life be fooner burnt out. On the contrary, if the atmofphere contained a much lefs proportion of vital ( 55 ) vital air, it would not ftimulate the body fufficiently ; the excitability would morbidly accumulate, and difeafes of debility would occur. Combuftion, putrefaction, and the breathing of animals, are procefies which are continually diminifhing the quantity of vital air contained in the atmofphere ; and if the all-wife author of nature had not provided for its continual re-produCtion, the atmofphere would in all probability have long fince become too impure to fupport life ; but this is guarded againft in a molt beautiful manner. Water ( 56 ) Water is not a Ample element, as has been fuppofed, but is compofed of vital air, and a particular kind of air which is called inflammable ; the fame that is ufed to fill balloons. It has been found by experiment, that one hundred pounds of water, are compofed of eighty-five pounds of vital air, and fifteen of inflammable air*. Water may be decompounded by a variety of means, and its * Strittly fpeaking, water is compofed of ihe bates of thefe airs, the greatefl part of the caloric being given out on their union. com- ( 57 ) component parts feparated from each other. Vegetables effect this decompofi- tion ; they abforb water, and decom- pofe it in their glands ; and taking the inflammable air for their nourifh- ment, breathe out the vital air in a ftate of very great purity ; this may be afcertained by a very eafy experi- ment. This vital air is received by animals into their lungs, gives them their heat, and communicates a red colour to their blood ; when animals die for want < 5« > want of vital air, their blood is always found black. From what I have faid, it is evident, that in large and populous towns, where combuftion and refpiration are continually performed on a large fcale, the air mult be much lefs pure than in the country, where there are few of thefe caufes to contaminate the at- mofphere, and where vegetables are continually tending to render it more pure ; and if it was not for the winds which agitate this element, and con- ftantly occafion its change of place, the air of large towns would probably foon ( 59 ) loon become unfit for refpiration. Winds bring us the pure air of the country, and take away that from which the vital air has been in a great meafure extracted ; but Hill, from the immenfe quantity of fuel which is daily burnt, and the number of people breathing in large towns, the air very foon becomes impure. From the greater purity of the air in the country, proceeds the rofy bloom found in the rural cottage, which we in vain look for in the {late- ly palace, or the fplendid drawing room. Here then are reafons for pre- ( 6o ) preferring the country, which no one will difpute ; and whenever it can be done, fuch a fituation ought always to be chofen in preference to a large town : this cannot be better enforced than in the words of Dr. Arm- ftrong. — 4 Ye, who amid the feverifli world would wear 4 a body free of pain, of cares a mind ; 4 fly the rank city, fhun its turbid air; 4 breathe not the chaos of eternal fmoke, 4 and volatile corruption, from the dead, 4 the dying, fick’ning, and the living world 4 exhaled, to fully heaven’s tranfparent dome 4 with dim mortality. ‘While ( 61 ) ‘ While yet you breathe, away ; the rural wilds ‘ invite ; the mountains call you, and the vales ; c the woods, the (breams, andeach ambrofial breeze ‘that fans the ever undulating fky.’ But there are many whofe occupa- tions oblige them to refide in large towns ; they, therefore, fhould make frequent excurfions into the country, or to fuch fituations as will enable them to enjoy, and to breathe air of a little more purity. I fay enjoy } for who that has been for fome time {hut up in the town, without breathing w i the pure air of the country, does not feel his fpirits revived the moment he emerges from the azote of the town. Let ( 62 ) Let not therefore, if poffible, a iingle daypafs, without enjoying, if but for an hour, the pure air of the country. Doing this, only for a fhort time every day, would be much more effec- tual than fpending whole days, or even weeks in the country, and then returning into the corrupt atmofphere of the town ; for when you have for a long time breathed an impure air, the excitability becomes fo morbidly accumulated, from the want of the ftimulus of pure air, that the air of the country w ill have too great an elf eel upon you ; it will frequently, in the courfe of a day or two, bring on an in- ( 63 ) inflammatory fever, attended with ftu fling of the nofe, hoarfenefs, a great degree of heat, and drynefs of the fkin, with other fymptoms of a violent cold. Large towns are the graves of the human fpecies ; they would perifh in a few generations, if not conftantly recruited from the country. The confined, putrid air, which moll of their inhabitants breathe, their want of natural exercife, but above all their diflipation, fhorten their lives, and ruin their conftitutions. Children ( <H ) Children particularly, require a pure air ; every circumftance points out the country as the proper place for their education ; the purity of the air, the variety of ruftic fports, the plainnefs of diet, the fimplicity and innocence of manners, all con- cur to recommend it. It is a melan- choly fad:, that above half the chil- dren born in London, die before they are two years old. 