CARBON is one of the elementary substances largely diffused in nature. It occurs uncombined in the mineral graphite, or black-lead (q.v.), and in the diamond (q.v.), is pure crystallized carbon. It is much more abundant, however, in a state of combination. United with oxygen, it occurs as carbonic acid (CO2) (q.v.) in the atmos phere, in natural water, in limestone, dolomite, and ironstone. In coal, it is found com bined with hydrogen and oxygen; and in plants and animals, it occurs as one of the ele ments building up wood, starch, gum, sugar, oil, bone (gelatine), and flesh (fibrine). Indeed, there is no other element which is so characteristic of plant and animal organ isms, 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 atomic weight or equivalent of C. is G (new sys. 12); the specific gravity greatly varies; that of the diamond is &MO to 3.550 (water being 1.000), and of graphite 1.800. C., 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 C., such as wood-charcoal, are very bad conductors; graphite in mass has very considerable conducting powers. At ordinary temperatures, all the varieties of C. 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 C. 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, are charred (coated with C.). to keep the wood from passing into decay, and thereby to preserve the water sweet. Its power of arresting odors and colors likewise varies much. See BONE-BLACK. In the simple property, even of combustion, there is a marked difference. Wood-char coal takes fire with the greatest readiness, bone-black 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 heats 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 prop erty of hardness, C. ranges from the velvet-like lampblack to diamond, the hardest of gems. In 1879, it was announced that a method of producing pure crystallized carbon, or diamond, had been discovered in Glasgow.
Carbon for eleetrical purposes. —When C. is obtained of sufficient density, it is found to be a good conductor of electricity, and to make an excellent electronegative element in a galvanic pair. Graphite displays these qualities to advantage, and so does the hard
incrustation of C. that is found sublimed in gas retorts. Coke and wood-charcoal are too porous to possess them to any great extent. The scarcity of graphite, and the pre carious supply of retort 0., preclude the possibility of obtaining much practical advan tage from the electrical properties of C. with these substances alone. We are indebted, however, to prof. Bunsen, of Heidelberg, for the discovery of a process whereby a C. of the requisite density can be manufactured with great ease and economy. The carbons thus obtained for galvanic batteries rival platinum in electric energy, and they have aided in no small degree, from their cheapness, in heightening the utility of galvanic electricity. The Bunsen carbons, as manufactured in Germany, are of the form of hol low cylinders, whereas those made in France and this country are solid rectangular prisms. The following are the more important details of the process. Two parts of coke, and one of baking-coal—the proportion varying to some extent with the materials —arc ground to a fine powder, and passed through a sieve. The powder so got is trans ferred to iron-plate molds of the required shape, the seams of which are merely clasped together, and listed with clay. No pressure is employed in filling them, other than that of shaking. When the molds are filled, they are placed in a furnace, and kept there till all carburetted hydrogen has escaped from them. They are then taken out, and allowed to cool before the mass within is removed, which is now found to have taken a solid form, and to be so hard that it may be turned or ground to the exact size wanted. At this stage, the carbons are destitute of electrical action, and they must consequently be rendered more dense by a subsequent process. This consists in soaking them thor oughly in thick sirup, or, better still, in gas-tar thickened by boiling, and laying them aside till dry, after which they' are packed with charcoal-dust in fire-proof crucibles, and exposed for a considerable time to a high heat. If one soaking and charring is not enough, the same may be repeated until sufficient density is obtained. Throughout the process, it is essential that all flaming matters be driven off, so as to leave only the C. in the mold; and care must be taken that no air be admitted to the mold when under the action of heat, otherwise there would be a loss of C. from combustion. The manu facture of these carbons may be carried on contemporaneously with that of gas. The sticks of C. used for the electric light are obtained by sawing up either C. made by this process or the C. of the gas retorts.