SILICA

(Gr. x;'»^'f. « pebble or stone.) The earth of flints. This important substance constitutes the characteristic ingredient of a great variety of siliceous minerals; among which rock-crystal, quartz, chalcedony, and flint may be considered as silica nearly pure. It also predominates in many of the rocky ma es which constitute the crust of our globe, such as granite, the varieties of sandstone, and quartz rock. Although silica has none of the ordinary or more obvious acid properties, yet, as it combines in definite proportion with many salifiable bases, and expels carbonic acid when fused with the carbonated alkalies, it is very commonly termed silicic acid, and its various compounds have been denominated silicates. When pure and colourle rock-crystal is heated red hot, and quenched in water, it becomes opaque and friable; and if in this state it be reduced to powder, it presents one form of pure silica. If in this state (in which it is perfectly insoluble in water) it be fused with three parts of carbonate of potash, it forms a gla which is soluble in water, and from this solution (formerly called /jgwor of flints) the concentrated acids throw down the silica in the form of a gelatinous hydrate; but if the solution be diluted, and the acid gradually added^ the alkali may be [s. 1129]