text: Carbon and Life\n\n    It is hard to overstate the importance of carbon; its unique capacity for forming multiple bonds and chains at low energies makes life as we know it possible, and justifies an entire major branch of chemistry - organic chemistry - dedicated to its compounds. In fact, most of the compounds known to science are carbon compounds, often called organic compounds because it was in the context of biochemistry that they were first studied in depth.\n\n    What makes carbon so special is that every carbon atom is eager to bond with as many as four other atoms. This makes it possible for long chains and rings to be formed out of them, together with other atoms - almost always hydrogen, often oxygen, sometimes nitrogen, sulfur or halides. The study of these is the basis of organic chemistry; the compounds carbon forms with metals are generally considered inorganic. Chains and rings are fundamental to the way carbon-based life forms - that is, all known life-forms - build themselves. Silicon is capable of forming the same sorts of bonds and structures, but opinion is divided on whether silicon-based life forms are a realistic prospect - in part because it needs higher energies to form them, and in part because whereas carbon dioxide (one of the main by-products of respiration, a process essential to all known life) is a gas and therefore easy to remove from the body, its counterpart silicon dioxide (silica) has an inconveniently high melting point, posing a serious waste disposal problem for any would-be silicon-based life form.\n\nFuel\n\n    Almost everything we use as fuel, whether in food or power stations, is also based on one kind of carbon-based chain or another; everything from natural gas through petrol and alcohol to oil to wax and plastic is composed of hydrocarbon chains of various lengths. As a general rule, the longer the chains and the more saturated they are (meaning every atom of carbon bonds to four other atoms), the less volatile and more viscous the substance will be, and the higher its evaporation temparature. This spread of evaporation temperatures makes it possible to separate the wide range of different hydrocarbons present in crude oil through a process known as fractional distillation, heating the oil to a range of temperatures in sequence and collecting the vapors released. The other big class of carbon-based fuel (used universally by plants and animals, but rarely by power stations) is the carbohydrate family - sugar molecules and the things you can make from them, like starch, cellulose and so on.\n\nElemental Carbon\n\n    Besides its millions of compounds, carbon also bonds with itself in different ways to form graphite, diamond, fullerenes and amorphous solids. Under certain conditions, it can also form white carbon - a transparent, biregringent material about which little else seems to be known.\n\n    In graphite, probably best known for its use in pencil 'lead' (for which purpose it is mixed with clay), the carbon is loosely bonded in crystalline layers which are able to slip over one another, so it is relatively soft and deposits itself readily on flat surfaces when drawn over them. It is used as a lubricant, in clay, and as an electrical conductor - one of the few solid, non-metallic conductors. Diamond, by contrast, is the hardest substance known to man; it has a dense, extremely stable crystalline structure, and conducts heat extraordinarily well but electricity hardly at all. Thanks to its hardness it has many industrial applications, while its high refractive index gives jewellery a sparkle which is hard to match.\n\n    Another form of elemental carbon, the fullerene, was not observed until 1985, although the possibility of its existence had been suggested as long ago as 1966. The most basic fullerene, (C60, the buckyball - short for buckminsterfullerene) has the form of a football - that's a soccer ball if you American, or a truncated icosahedron if you're a