Silica

Silica consists of the oxide of silicon represented by the formula SiO₂. It is very plentiful upon the crust of the earth, both in a free state and combined with other oxides, and is by far the most abundant oxide. It also occurs in many grasses and bamboos, and also in birds' feathers. In minerals it occurs free :- crystaline, as quartz (q.v.), and tridymite, and in a non-crystalline form as the opal. Flint is also a form of silica, while the aqate and chalcedony are mixtures of the amorphous and crystalline varieties. Sand and sandstone, quartzite, and some other rocks, consist chiefly of silica. In combinations it acts the part of an acid, uniting with other oxides to form silicates, and, according to the guantity of silica present, rocks are known as acid, intermediate, or basic. It may be prepared artificially as a white powder, insoluble in acids, except hydrofluoric, possessing a specific gravity of 2.2. It is very infusible, but may be melted in the oxyhydrogen flame. Many hot springs and geysers contain silica, as it is soluble in alkaline hot solutions. The compound is precipitated from the water as it cools and evaporates, and by this means great deposits of silica may be found, as in the "sinter" terraces in Yellowstone Park and other localities.