SILICA

(Or. \nXti, a 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 sa-idstnne, and quartz rock. Although silica has none of the ordinary or more obvious nciil properties, yet, as it combines indefinite proportion with m my salifiable bases,:,n ratcd to dryne , the silica remains in a state as insoluble as before. This solubility of hydrated silica, whilst when dry it is perfectly insoluble, may serve to explain the occasional occurrence of silica in mineral waters, and its de|wsiiion in various cbalcedonic Incrustations, But silica presents another very remarkable character; which is, that if we reverse the above proportions, and fuse together a mixture of one part of carbonate of potash and tl.n-,- of powdered rock-crystal or calcined flint, we then Obtain a transparent and fusible compound, which is insoluble in water, and which, in fact, is gla . l'Io and window gla , or, as it is commonly called, crown gla , are silicates of soda or pota a; and flint gla , of which our common gla utensils are made, is a similar compound, with a large addition of silicate of lead. i.lSS. in its ordinary or anhydrous state, is insoluble in all acids except the hydrofluoric, which Immediately acts i]|K>n it, and forms an extraordinary gaseous compound, the Jluoxilicic acid. Silica was long considered ns an elementary form of matter; but Sir II. Davy found that when the va|«>ur of pota ium was brought into contact with pure sIIm i bested to whitene , silicate of pota a was formed, and a dark -coloured matter separated, which was afterwards found to be the base of silica, and to which the terms silicimn and silicon have been applied. It is probably not metallic, and bears a greater nnalogy to boron than to any known principle. Silicon is a nonconductor of electricity, incombustible in air and oxygen, and not acted upon by any single acid; but it is readily soluble in a mixture of the nitric and hydrofluoric acids. It is, however, rapidly oxidized by and burns vividly at a temperature below redne , when in contact with fused soda or pota a. The exact equivalent of silica has not been very satisfactorily determined, but we are probably not very incorrect in regarding silica as a compound of equal weights of silicon and oxygen. In that case, if it be regarded as a protoxide, it is obvious that 8 will be the ('univalent of its base, and silica will consist of 8 silicon -f- 8 oxygen, and be represented by the equivalent 16. But ■ regarded as a compound of 1 atom of silicon and 3 atoms of oxygen, and Berzelius has shown that some analogies favour this view, then 24 will be its equivalent, and 4* that of silica. The chloride of si/iciicm, and its bromide, are limpid fuming liquids; the former very volatile. The tulphuret is a white earthy-looking substance. [s. 1139]