Lithium carbide, Li2C2, is a white crystalline mass, produced by heating in the electric furnace a mixture of carbon with lithium carbonate; its formation is expressed by the equation: Li0CO3 + 4C = Li2C2 + 3CO. It is decomposed at a temperature not much higher than that at which it is formed; hence the exposure to the high temperature of the electric furnace should be only a short one. The analogous compounds of sodium and potassium do not resist such a high temperature ; they must therefore be prepared by exposing the metal for several weeks to the action of acetylene under pressure. This process yields compounds of the formula? NaHC2 and KHC2; when heated, they change, with evolution of acetylene, into the carbides Na2C2 and K2C0. Like lithium carbide, they are white crystalline substances, and with water acetylene is evolved: Na9C9 + 2H2O = 2NaOH + C2H2.
Calcium carbide, CaC2, has attained great industrial importance owing to its serving as the source of acetylene, now largely used for illuminating purposes. It was made in an impure state in 1892 by Travers by heating together a mixture of calcium chloride, carbon, and sodium ; but it is best produced by Moissan's process in the electric furnace, by heating a mixture of carbon and lime to the very high temperature (about 3000°) obtained in that manner. It forms blackish-grey, lustrous crystals, at once attacked by water: CaC2 + 2HOH = C2H2 + Ca(OH)2. Carbides of strontium and barium are made in a similar manner, and have properties analogous to those of the calcium compound.
Other carbides prepared by Moissan in a crystalline state by means of the electric furnace are: CeC2, LaC2, YC2, ThC9, which yield with water a mixture of acetylene, ethylene, methane, and hydrogen ; Al4Cg, which is decomposed by water, yielding pure methane ; Mn3C, yielding methane and hydrogen ; and U2Cg, the products from which are ethylene, methane, and hydrogen. By heating the oxide of the respective metal with calcium carbide, the carbides CrgC2, Mo2C, W2C, TiC, and SiC have also been prepared. The last of these has become known commercially under the name " carborundum." It forms extremely hard, blackish-blue, hexagonal crystals; when pure it is colourless. It is prepared on a large scale by heating together in the electric furnace a mixture of carbon (coke) and white sand. It is used for grinding and polishing metals and glass.
Steel, as is well known, differs from iron by the presence of a certain amount of carbon, which induces the iron, when cold, to persist in its allotropic state. This appears to be due to a carbide of iron mixed with the excess of iron in the steel. The compound has been found as a meteoric mass; it has been named cohenite, and hasnhe formula Fe3C. On treating steel with dilute acetic acid, the same substance remains as a black powder. Its formula is similar to that of manganese carbide, MngC.
Silicides,-Some silicides have also been prepared by aid of the electric furnace by heating elements with silicon. Among these are Fe9Si, lustrous prisms; Cr2Si, Ni9Si, Co2Si, Mn2Si, Cu9Si, and Pt2Si, with similar properties. Magnesium silicide, Mg2Si, prepared by heating a mixture of powdered silica and magnesium dust to redness, is attacked by dilute acid, evolving a mixture of hydrogen and hydrogen silicide (see p. 38).