SILICA and SILICON. Silica was till lately ranked among the earths proper ; but since the researches of Davy and Berzelius, it has been transferred to the chemical class of acids. It constitutes the principal portion of most of the hard stones and minerals which compose the crust of the globe ; occurring nearly pure in rock crystal, quartz, agate, chalcedony, flint, &c. Silica or silicic acid may be obtained perfectly pure, and also in the finest state of comminution, by taking the precipitate formed by passing silicat ed fluoric gas through water, filtering, washing, and igniting it, to expel the last traces of the fluoride of silicon. The powder thus obtained is so light as to be blown away with the least breath of air. Silica may be more conveniently procur ed, however, by fusing ground flint with four times its weight of a mixture, in equal parts, of dry carbonate of putassa and carbonate of soda, in a platinum or silver crucible. The alkaline carbonates should be first fused, and the flint pow der sprinkled into the liquid, as long as it dissolves with effervescence. The mass is to be then allowed to cool, dissolved in dilute muriatie acid ; the solution is to be filtered, and evaporated to dryness ; the dry crust is to be pulverized, digested for two hours with a little muriatie acid, to remove any iron and alumina that may be present, next washed with hot water, drained, dried, and ignited.
The above silicate of potasse and soda is the compound called soluble glass, which applied in solution to the surthce of wood, calico, paper, &c., renders them unsusceptible of taking fire on the con tact of an ignited body.
Silica, as thus prepared, is a white pow der, rough to the touch, gritty between the teeth, absolutely insoluble in water, acids, and most liquids. Its specific gra Pity is 2-66. It cannot be fused by the most intense heat of our furnaces, but at the flame of the oxy-hydrogen blowpipe it melts into a limpid colorless glass. By peculiar chemical methods, an aqueous solution of it may be made artificially, si milar to what nature presents nS with in many thermal springs, as in those of Reikum and of Geyser in Iceland, and of most mineral waters, in minute quantity. There is no acid except the fluoric which can directly dissolve dry or calcined sili ca. Silica is composed of '48.04 silicon, and 51.96 oxygen.