~Properties.~ As would be expected from its place in the periodic table, silicon resembles carbon in many respects. It can be obtained in several allotropic forms, corresponding to those of carbon. The crystallized form is very hard, and is inactive toward reagents. The amorphous variety has, in general, properties more similar to charcoal. ~Compounds of silicon with hydrogen and the halogens.~ Silicon hydride (SiH_{4}) corresponds in formula to methane (CH_{4}), but its properties are more like those of phosphine (PH_{3}). It is a very inflammable gas of disagreeable odor, and, as ordinarily prepared, takes fire spontaneously on account of the presence of impurities. Silicon combines with the elements of the chlorine family to form such compounds as SiCl_{4} and SiF_{4}. Of these silicon fluoride is the most familiar and interesting. As stated in the discussion of fluorine, it is formed when hydrofluoric acid acts upon silicon dioxide or a silicate. With silica the reaction is thus expressed: SiO_{2} + 4HF = SiF_{4} + 2H_{2}O. It is a very volatile, invisible, poisonous gas. In contact with water it is partially decomposed, as shown in the equation SiF_{4} + 4H_{2}O = 4HF + Si(OH)_{4}. The hydrofluoric acid so formed combines with an additional amount of silicon fluoride, forming the complex fluosilicic acid (H_{2}SiF_{6}), thus: 2HF + SiF_{4} = H_{2}SiF_{6}. ~Silicides.~ As the name indicates, silicides are binary compounds consisting of silicon and some other element. They are very stable at high temperatures, and are usually made by heating the appropriate substances in an electric furnace. The most important one is _carborundum_, which is a silicide of carbon of the formula CSi. It is made by heating coke and sand, which is a form of silicon dioxide, in an electric furnace, the process being extensively carried on at Niagara Falls. The following equation represents the reaction SiO_{2} + 3C = CSi + 2CO. The substance so prepared consists of beautiful purplish-black crystals, which are very hard. Carborundum is used as an abrasive, that is, as a material for grinding and polishing very hard substances. Ferrosilicon is a silicide of iron alloyed with an excess of iron, which finds extensive use in the manufacture of certain kinds of steel. ~Manufacture of carborundum.~ The mixture of materials is heated in a large resistance furnace for about thirty-six hours. After the reaction is completed there is left a core of graphite G. Surrounding this core is a layer of crystallized carborundum C, about 16 in. thick. Outside this is a shell of amorphous carborundum A. The remaining materials M are unchanged and are used for a new charge. [Illustration: Fig. 73]