CARBIDES (carbon + ide). Compounds of carper and the metals. The most important car l‘ides are iron carbide and calcium carbide. Mol ten iron unites directly carbon to form a series of definite chemical compounds, Fe,C, Fe,C, FeC,, etc. The presence of these com pounds, in variable quantities, in metallic iron causes important changes in certain properties of the latter: it becomes hard and brittle, and melts at a considerably lower temperature than pure iron. Even of greater practical importance than iron carbide is the carbide of calcium, now extensively employed in the manufacture of acetylene gas. When treated with water or dilute acids, most carbides are decomposed with formation of hydrocarbons. Thus, the carbide of ahnninium yields methane. or marsh-gas; the carbide of calcium yields acetylene. At present the carbides are made by the use of the electric furnace, which gives a temperature of about 4000° C. In this furnace carbon is made to act
on the oxide of the metal, when part of the car bon takes up the oxygen of the oxide, while an other part combines with the metal. The car bides of the alkali metals, sodium and potassium, are hest prepared by passing a current of pure and carefully dried acetylene gas over the melted metals, kept at a temperature of about 225° C., out of contact with the air. The name carbides is also applied to compounds of carbon with cer tain non-metallic elements, such as silicon. The carbide of silicon is a crystalline substance re markahle for its great hardness and used, under the name of carborundum, or silicate of car bon, for making whetstones, polishing cloths, etc. The carbides of non-metallic elements are not attacked by acids. See ACETYLENE; CAL