M3C  =

manganese, iron ; M2C = molybdenum ; M3C2= chromium ; MC = zirconium ; M4C3 = beryllium, aluminium ; M2C3 = uranium; MC2 = barium, calcium, strontium, lithium, thorium, &c.; M C4 = chromium . The principal methods for the preparation of carbides may be classified as follows:—(1) direct union at a high temperature, e.g. lithium, iron, chromium, tungsten, &c.; (2) by the reduction of oxides with carbon at high temperatures, e.g. calcium, barium, strontium, manganese, chromium, &c.; (3) by the reduction of carbonates with magnesium in the presence of carbon, e.g. calcium, lithium; (4) by the action of metals on acetylene or metallic derivatives of acetylene, e.g., sodium, potassium . The metallic carbides are crystalline solids, the greater number being decomposed by water into a metallic hydrate and a hydro-carbon; sometimes hydrogen is also evolved . Calcium carbide owes its industrial importance to its decomposition into acetylene; lithium carbide behaves similarly . Methane is yielded by aluminium and beryllium carbides, and, mixed with hydrogen, by manganese carbide . The important carbides are mentioned in the separate articles on the various metals . The commercial aspect of calcium carbide is treated in the article ACETYLENE .