Carbon

Carbon (syn. C; atomic weight 12) is one of the elementary substances most largely diffused in nature. It occurs uncombined in the mineral graphite or Black Lead (q.v.), and in the Diamond (q.v.), which is pure crystallised carbon. It is much more abundant, however, in a state of combination. United with oxygen, it occurs as Carbonic Acid, $CO\_2$ (q.v.), in the atmosphere, in natural waters, in limestone, dolomite, and iron-stone. In coal it is found combined with hydrogen and oxygen; and in plants and animals it occurs as one of the elements building up wood, starch, gum, sugar, oil, bone (gelatin), and flesh (fibrin). Indeed there is no other element which is so characteristic of plant and animal organisms, and it ranks as the only element never absent in substances obtained from the two kingdoms of organic nature. Wood charcoal, coke, lampblack, and animal charcoal are artificial varieties, more or less impure, of carbon. The specific gravity of the different forms of carbon greatly varies; that of the diamond being 3.330 to 3.550, while graphite is only about 2.300. Carbon, in its ordinary forms, is a good conductor of electricity; in the form of diamond, it is a non-conductor. Of heat, the lighter varieties of carbon, such as wood charcoal, are very bad conductors; graphite in mass has very considerable conducting powers. At ordinary temperatures, all the varieties of carbon are extremely unalterable; so much so, that it is customary to char the ends of piles of wood which are to be driven into the ground, so as by this coating of non-decaying carbon to preserve the interior wood; and with a similar object, the interior of casks and other wooden vessels intended to hold water during sea-voyages, is charred (coated with carbon), to keep the wood from passing into decay, and thereby to preserve the water sweet. Even as regards combustion there is a marked difference. Wood charcoal takes fire with the greatest readiness, bone-black (animal charcoal) less so; then follow in order of difficulty of combustion—coke, anthracite, lampblack, Black lead, and the diamond. Indeed, black lead is so non-combustible, that crucibles to withstand very high temperatures for prolonged periods without breakage or burning, are made of black lead; and the Diamond (q.v.) completely resists all ordinary modes of setting fire to it. In the property of hardness carbon ranges from the velvet-like lampblack to diamond, the hardest of gems. In 1879 it was announced that a method of producing pure crystallised carbon, or diamond, had been discovered, but, so far, the artificial crystals obtained have only been of microscopic size.

Besides the physical properties already alluded to, carbon possesses very remarkable absorbent powers; enabling it not only to decolourise syrupy liquids (see CHARCOAL, ANIMAL), but also to absorb gases. Thus freshly ignited wood charcoal will absorb ninety times its own volume of ammonia gas, and sixty-five times its volume of sulphurous anhydride gas. Owing to this property it has many important uses. When employed in the construction of respirators, so arranged that the air is drawn through a layer of charcoal, it is possible to breathe an atmosphere which otherwise would be irrespirable. Water and wine which have become tainted may be readily rendered wholesome by means of charcoal; while as a remedy for troublesome flatulence and as a tooth-powder, its value lies in the absorbent power referred to. Gardeners find that when it is mixed with potting-soil it prevents the souring of the soil, which is so injurious to the finer varieties of plants.

Most of the filters so much in use nowadays for the purification of water contain charcoal, which not only removes organic matter, but even lessens the hardness of the water. For decolourising liquids bone-black is preferable. The varieties of carbon, as mentioned above, differ very considerably in the readiness with which they combine with oxygen; but when burned they all yield Carbonic Anhydride, $CO\_2$ (see CARBONIC ACID). Carbon forms another oxide, called carbonic oxide, CO, when it is burned with only a limited supply of air. When heated with sulphuric acid carbon decomposes it, forming carbonic acid and sulphurous anhydride. Thus:

Carbon unites with metals to form Carbides (q.v.), the chief of which are those of iron. With hydrogen carbon forms a very important class of organic compounds, including such unlike substances as turpentine, oil of lemon, $C\_\{10\}H\_\{16\}$, and marsh gas, $CH\_4$. As carbon unites with nearly all the elements to form chlorides, bromides, &c., the number of carbon compounds is virtually unlimited (see AROMATIC SERIES), and those already known far surpass in numbers all other chemical compounds put together. When carbon is obtained sufficiently dense, it is found to be a good conductor of electricity, and to make an excellent electro-negative element in a galvanic pair. For this purpose graphite and the hard incrustation of carbon found in gas retorts are admirably suited; but, owing to their comparative scarcity, recourse is had to an artificial method of preparing dense carbon, devised by Professor Bunsen of Heidelberg. The carbons thus obtained for galvanic batteries, owing to their efficiency and cheapness, have given a great impetus to electrical work. The following are the more important steps in the process: A mixture of two parts of coke and one of coal in powder is placed in an iron mould and heated in a furnace. As soon as the liberation of gas has ceased, the mould is allowed to cool, and on opening it a carbon block is obtained which may be ground to the desired size. Although hard, it is still far too porous, and to remedy this it is soaked in thick gas-tar, allowed to dry, and finally heated in a fireproof crucible for some time. A second or even third time this may be repeated, care being taken to prevent the access of air to the crucible, and eventually a carbon very dense, sonorous when struck, and a good conductor of electricity, is obtained.

For a reference to the manufacture of charcoal, see separate article; and see ELECTRIC LIGHT.