While the characteristic of carbon is to form compounds in which many atoms of carbon are linked together (hydrocarbons, for example, having formulas like '. , atoms of silicon are characterised by linking by means of atoms of oxygen. This peculiarity leads to the existence of a large number of silicates, and probably, too, of a large number of silicic acids. The existence of some of these is rendered certain by a study of the oxychlorides.

Silicon tetrachloride, SiCl₄, when passed over fragments of felspar (a silicate of aluminium and calcium) heated to whiteness in a porcelain tube, exchanges chlorine for oxygen, and yields a liquid boiling at about 137°, of the formula

This liquid, passed along with oxygen through a hot glass tube, gave two other liquids, which could be separated by fractionation; the one, boiling at 153°, had the formula Si₄O₃Cl₁₀, and the other, boiling at 200°, Si₄O₄Cl_g. The vapour-densities of these liquids were determined, and led to the formulae given above. The signification of this will appear presently.

Si(OH)₄. When silica, in the form of flint, or fine sand, or powdered rock-crystal is either fused with caustic soda or potash, or heated under pressure with a solution of one or other of the alkalies, an orthosilicate is produced, possessing the formula Si(ONa)₄ or Si(OK)₄. These silicates are soluble in water, and as they resemble glass in appearance, they are usually named " soluble glass." If hydrochloric acid is added to the solution of one of them, no apparent change occurs; in reality, orthosilicic acid is produced, a compound which is hardly ionised at all, being one of the very weakest of acids.