Silica

(sil'e-kah,) or SILICIC ACID. [From L. silex, flint.] (Min. and Chem.) An oxide of silicon, which in its present state is the transparent and colorle variety of quartz known as rock-crystal. As prepared artificially in the anhydrous state, it forms a white powder, insoluble in water, and in all acids except hydrofluoric acid. It di olves, however, in alkalies, especially in heating; it requires a very high temperature to fuse it,but melts to a transparent gla before an air-gas blow-pipe, supplied with air from a double-action bellows, worked by a treadle. When heated with an alkaline carbonate it causes an evolution of carbonic acid, and melts to a perfectly transparent gla . When prepared artificially S. unites with water and forms a gelatinous hydrate which is much more soluble than anhydrous S., and by dialyzing a solution of an alkaline silicate in exce of hydrochloric acid, Graham has obtained a clear solution of S. in water. Hydrate of S. is found native as opal and also as a white chalky deposit. S. po e es the property of an acid, and, owing to its being non-volatile at a very high temperature, it displaces most of the other acids from their combinations; when united with bases these compounds are called silicates, and their chemistry is in the highest degree intricate. Many metallic silicates occur abundantly in the mineral kingdom, forming, in fact, the greater part of the earth's crust; they are mostly fusible, and are all insoluble in water with the exception of the alkaline silicates; they are all decomposed by hydrofluoric acid, but other mineral acids exert very various solvent powers upon them. Form. SiO2. [s. 826]