Carbon (Lat. carbo, coal; symbol, C), one of the most common and important substances in nature, occurring in a great variety of forms in the vegetable, animal, and mineral kingdoms, in the two first named being by far the most considerable element. The charcoal prepared from many substances belonging to these presents it pure; but the diamond is crystallized carbon, contaminated, when colorless, by no foreign admixture. In this form carbon possesses the most brilliant Justre, and a hardness unsurpassed, which is represented upon the mineralogical scale by the highest number, 10. (See Diamond.) Carbon is remarkable for its allotropic character, presenting itself under various forms while still in a state of purity. Besides those named, graphite may be regarded as one of its forms, the trace of other substances met with in its purest qualities being considered accidental; also gas carbon, the extremely hard substance which is deposited upon the inner surface of gas retorts; and lampblack, the soot deposited by highly combustible bodies, as they are imperfectly consumed.

When one atom of carbon is combined with two atoms of oxygen, it forms the compound carbonic anhydride, commonly called carbonic acid gas, an essential constituent of solid limestones and other carbonates, and, in a gaseous form, of the atmospheric air. Its compounds with hydrogen are called hydrocarbons; they occur in gaseous, solid, and liquid forms. The chemical equivalent of carbon is 12, established by Dumas by the diamond, when consumed in a stream of oxygen gas, combining with this in the proportion of 12 parts to 32. Carbon resists the influence of many reagents which powerfully affect other bodies; acids and alkalies at ordinary temperatures have no effect upon it in its denser forms; but charcoal is oxidized in boiling nitric acid. Neither is it affected by the strongest heat attainable in furnaces, provided it be protected from the action of air or oxygen. Pieces of charcoal are found enclosed in the cinders of the blast furnace, which have been for 24 hours or more in the intense heat of its interior, among melting ores and limestones, but protected in some way from exposure to the oxygen of the blast.

The only indications of volatilization or fusion which it can be made to exhibit, are seen by exposing it in a vacuum to the heat from a Bunsen's battery of several hundred pairs, so arranged in five or six series as to form 100 pairs of five or six times the ordinary size. It is then volatilized, and collects on the sides of the vessel in the form of a black crystalline powder. It is ultimately converted into graphite. Diamond is similarly affected. No substance, unless it be crystallized boron, is more unalterable in most conditions in which other bodies undergo a chemical change. It is taken up by some metals when these are fused in contact with it, and its presence in cast iron and steel imparts to them the qualities that distinguish them from malleable iron. In connection with the question of the volatility of carbon at high temperatures, Eisner has observed that when porcelain vessels are heated in the pottery furnace on trays made of a mixture of clay and graphite, they become brown-black throughout their mass, and covered with a light gray specular glaze. This result is attributed by him to volatilization of the carbon.

Regnault, on the contrary, ascribes it to the formation of a hydro-carbon. - The most valuable qualities of carbon in practical uses are its strong affinity for oxygen at high temperatures, and its power of resisting in some of its forms the high heat of furnaces. The former quality gives to many of its varieties their value as combustibles (see Fuel), and it also renders carbon a most powerful reducing agent of the oxides of the metals; for which purpose, as well as for generating heat by its combustion, it is employed in smelting furnaces, bloomaries, etc. Its disappearance as carbonic acid gas adds greatly to the convenience of its use for this purpose. Its refractory character admirably adapts it as a material for crucibles; and when used in the form of paste of pure charcoal powder ground very finely, and applied as a lining to earthen crucibles, it serves not merely to protect the contents from injurious contact with the outer vessel, but also furnishes to these the best reducing agent or flux. The black lead crucibles or blue pots are in part composed of graphite, which, when prepared by grinding and mixing with refractory earths, powerfully resists even the action of the blast in highly heated furnaces.

Other useful purposes served by carbon are considered in treating of the subjects in which this is the principal element, as Bone Black, Charcoal, Coal, Coke, Diamond, Fuel, Graphite, etc. - The peculiar form of carbon already referred to as being found lining gas retorts, and collected in crevices in their interior, possesses a metallic lustre, and is of mammillary structure, resulting from the aggregation of the vesicles of which it is composed. It is sometimes fibrous, resembling graphite; its specific gravity is 1.76. Its hardness exceeds that of any other form of carbon, except the diamond. It is burned with difficulty in high heat when exposed to currents of air - a property which renders it useful for the illuminating points of the voltaic light. Its origin is commonly attributed to a deposition of carbon from defiant gas, C2H4, which is generated in the distillation of bituminous coal, and is converted by parting with one atom of carbon into marsh gas or the light carburetted hydrogen, CH4, used for illumination. - Carbon is, under certain circumstances, separated from cyanides in the form of graphite.

This occurs at one stage of the soda ash manufactory according to the method of Le Blanc, and it is proposed to recover the shiny material separated in this way and apply it for the same purpose for which the natural graphite is used.