SILICA

(Lat.), n. f. Si2l'i2k-a3. From silex, a flint. Fr., silicone. Ger., Kiesel. Silicon dioxide, SiO2. In the crystalline form it constitutes quartz and tridymite. In the amorphous form it constitutes opal and enters into the structure of flint, agate, and chalcedony. It is found in the stalks of many cereals and of the bamboo, and in the scales of diatoms. Among animals, it occurs in the orders of sponges and Radio lari a, and has also been found in feathers. It is very hard, transparent, colorle when pure, and has a sp. gr. of from 2'3 to 26. Amorphous s. may be obtained artificially as a light white powder which may by fusion be converted into gla . It is insoluble in water and in most acids, but readily di olves in hydrofluoric acid and in alkalies. [B, 3.] [s. 483]