SILICON, a non-metallic chemical element. It is not found in the uncombined condition, but in combination with other ele ments it is, with the exception of oxygen, the most widely dis tributed and abundant of all the elements. It is found in the form of oxide (silica), either anhydrous or hydrated, as quartz, flint, sand, chalcedony, tridymite, opal, etc., but occurs chiefly in the form of silicates of aluminium, magnesium, iron, and the alkali and alkaline-earth metals, forming the chief constituent of various clays, soils and rocks. It has also been found as a constituent of various parts of plants and has been recognized in the stars. The element (symbol Si, atomic number 14, atomic weight 28-06, isotopes 28, 29, 30) exists in two forms, one amorphous, the other crystalline. The older methods used for the preparation of the amorphous form, namely the decomposition of silicon halides or silicofluorides by the alkali metals, or of silica by magnesium, give an impure product, but if a small proportion of magnesia is added in the last case, the product is of about 96% purity. The crystalline form may be prepared by heating potassium silico fluoride with aluminium, by heating silica with magnesium in the presence of zinc, or by volatilizing the amorphous variety from an electric furnace. A somewhat impure silicon (containing 90-98% of the element) is made by heating coke and sand in an electric furnace, the coke being present in insufficient quantity to form carborundum (q.v.), or by heating silica and carborundum simi larly. The crystalline solid has specific gravity about 2.4, and the amorphous about 2.3 5 ; the melting points are about 1,450° C. Amorphous silicon is a brown powder, the crystalline variety being gyey, but differing according to the method of preparation. The specific heat varies with the temperature, from 0•136 at —39° C to 0.2029 at 232° C. Silicon is attacked rapidly by fluorine at ordinary temperature, and by chlorine when heated in a current of the gas. It undergoes a slight superficial oxidation when heated in oxygen. It combines directly with many metals on heating, whilst others merely dissolve it. It decomposes am monia at a red heat, liberating hydrogen and yielding a com pound containing silicon and nitrogen. It reduces many non
metallic oxides. It is only soluble in a mixture of hydrofluoric and nitric acids or in solutions of the caustic alkalis, in the latter a reaction which has been utilized for the large-scale production of this gas for military balloons. On fusion with alkaline carbo nates and hydroxides it undergoes oxidation to silica which dis solves in excess of alkali yielding alkaline silicates.
Hydrides.—When hydrochloric acid reacts with magnesium silicide (produced by heating together two parts of magnesium and one part of silica), a spontaneously inflammable mixture of hydrogen and various hydrides of silicon is formed. By condensing the latter in liquid air and fractionating the resulting solid, A. Stock and his collaborators have obtained four "silanes" in a state of purity : (I) monosilane or silicomethane, m.p. —185° C, b.p. —42°, is a colourless gas which is very stable at the ordinary temperature and not spontaneously inflammable. (2) disilane or silicoethane, m.p. —132-5°, b.p. —15°, is stable at ordinary temperatures but decomposes below 300° and inflames in air; it reacts vigorously even with carbon tetra chloride, forming hydrogen chloride, silicon and carbon. (3) trisilane or silicopropane, m.p. —117°, b.p. 53°, is some what unstable at ordinary temperatures. (4) tetrasilane, m.p. —93-5°, b.p. 8o-9o°, is still more unstable. All the silanes reduce silver and copper salts, and they react with caustic alkalis to give a quantitative yield of hydrogen and silicate, e.g., By direct reaction with bromine, or, better, by reaction with hydrogen bromide in the presence of aluminium bromide, they give a series of compounds in which the hydrogen is replaced successively by bromine, and these in turn react with water to give oxy-compounds ; e.g., bromo-monosilane yields disiloxane; 2 = 2HBr+ (SiH3)20; and dibromosilane gives siloxane, which poly merises rapidly to (SiH20).. For further details of these and related compounds, the works of A. Stock (Berickte, 1916 et seq.) should be consulted.