CARBON, a non-metallic element, is found in the free state as diamond, graphite and as crude forms of the former (symbol C, atomic number 6, atomic weight 12.000) ; in combination it occurs in all animal and vegetable tissues, in coal and pe troleum and (as carbonate) in many minerals such as chalk or limestone, dolomite, calcite, witherite, calamine and spathic iron ore; as carbon dioxide (q.v.) it occurs in the atmosphere. It is a solid which assumes different forms having widely different prop erties. The diamond has the highest specific gravity (3.52) of these, and its high refractive index (2.417 for sodium light) is re sponsible for its characteristic brilliance. Very small diamonds have been made artificially by causing carbon to crystallize from molten iron under very high pressures. (See H. Moissan, The Electric Furnace, 1904.) Genuine diamonds differ from "paste" imitations in being transparent to Röntgen rays. Carbonado and bort (boart) are "black diamonds" ; they are diamonds with a small percentage of impurity, and therefore valueless as gems. Graphite is an extremely soft form of carbon of much lower density (ranging from 2.0 to 2.6; the purest is about 2.25) ; it is probably of organic origin as it usually contains about 1% of hydrogen. Carbon is deposited in this form when it condenses from the vapour state in the electric furnace or in the arc. The purest artificial graphite (Acheson) contains only 0.5% of ash and is produced in a furnace of fire-brick lined with carborundum (q.v.), the space between and around the carbon electrodes being filled with petroleum coke. Graphite differs from diamond in being a conductor of electricity. The heats of combustion of diamond and graphite are both very close to 94,00o calories per 12 grams, so that it is not certain which is the stable form at ordinary temper atures, but diamonds are converted into graphite at high temper atures under ordinary pressures. Diamond and graphite crystal lize in the cubic and hexagonal systems respectively. Graphite is slowly attacked by mixtures of sulphuric acid with nitric acid, potassium chlorate or chromic acid, to give "graphitic acid" and finally mellitic acid, whereas the diamond is un affected by such treatment. Graphite (q.v.) is used in "lead" pencils, in polishes (as "black lead"), and as a lubricant for machinery.
Numerous varieties of carbon are classed as "amorphous" in contrast to the foregoing crystalline varieties; the commonest are lamp-black, gas carbon, animal charcoal, sugar charcoal and wood charcoal. They are formed by burning substances, from which they are named, in a limited supply of air. Lamp-black is thus formed from tars, resins, turpentine, etc., and is collected on blankets suspended in condensing chambers ; even after further purification by heating in closed vessels, it still contains oily im purities; it is used in printers' ink, in paints and in calico printing. Gas carbon is produced in gas manufacture (q.v.) and collects on the walls of retorts ; it is a very dense and fairly pure form of car bon and is used for the rods of arc lamps. Coke is the residue from gas retorts; specially designed retorts and processes are used for the production of the coke used in metallurgical operations. The charcoals are very porous, and consequently their specific gravity is apparently only about 0.25, but when the air is pumped out of the pores this becomes 1.4-1.9. Sugar charcoal is purified by heating in a current of hydrogen chloride, extraction with water, and further heating in a current of hydrogen until free from hy drogen chloride. Animal charcoal, from bones, horns, etc., con tains only about 1o% of carbon and about 8o% of calcium phos phate ; a purer form is obtained by calcining blood with potassium carbonate. Being very porous, it is used for decolourizing solu tions (e.g., of sugars) and for filtration of contaminated water. Wood charcoal is produced either by the wasteful process of slowly burning carefully stacked wood or by carbonizing it in retorts, in which case certain volatile products, such as acetone, wood spirit and pyroligneous acid are retained, and the last two are worked up for methyl alcohol and acetic acid respectively. Specially prepared blocks of wood charcoal are used in "blowpipe" analysis. (See CHEMISTRY : Analytical.) The absorptive powers of charcoals for gases are greatly improved by regulated heating, and are very pro nounced at low temperatures. (See CHARCOAL.) All the varieties of amorphous carbon are readily attacked by the reagents men tioned in connection with graphite.
The specific heats of all varieties of carbon are abnormally low at ordinary temperatures, but become nearly normal at about r,000° C; that of diamond is only about half that of graphite at —5o° C, but they gradually become more nearly equal at higher temperatures; graphite and wood charcoal have nearly the same specific heats at ordinary temperatures.
Carbon volatilizes at 3,600° C. In the electric arc it unites with hydrogen to give acetylene, (M. Berthelot), but small pro portions H2 of methane and ethane are also produced; at lower temperatures the conditions are more favourable for methane, and W. A. Bone and H. F. Coward obtained almost quantita tive yields of this gas from very pure carbon and hydrogen at 1,200° C. Carbon unites directly with fluorine to give the tetra fluoride, it burns in oxygen to give oxides; and when heated in sulphur vapour it gives carbon disulphide (q.v.). When heated with nitrogenous substances and alkaline carbonates, it gives cyanides (q.v.); and in the electric furnace it gives carbides (q.v.) with many elements, that with silicon being the abrasive carborun dum (q.v.). (A. D. M.)