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| - 1911 Encyclopædia Britannica/Header SILICA, in chemistry, the name ordinarily given to amorphous silicon dioxide, Si02. This chemical compound is widely and most abundantly distributed in nature, both in the free state and in combination with metallic oxides. Free silica constitutes the greater part of sand and sandy rocks; when fairly pure it occurs in the large crystals which we know as [[../quartz|quartz]] (q.v.}, and which, when coloured, form the gem-stones amethyst, cairngorm, cats'-eye and jasper. [[../tridymite|Tridymite]] (q.v.) is a rarer form, crystallo- graphically different from quartz. Amorphous forms also occur: [[../chalcedony|chalcedony]] (q.v.), and its coloured modifications [[../agate|agate]], [[../carnelian|carnelian]], [[../onyx|onyx]] and [[../sard|sard]], together w
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| - 1911 Encyclopædia Britannica/Header SILICA, in chemistry, the name ordinarily given to amorphous silicon dioxide, Si02. This chemical compound is widely and most abundantly distributed in nature, both in the free state and in combination with metallic oxides. Free silica constitutes the greater part of sand and sandy rocks; when fairly pure it occurs in the large crystals which we know as [[../quartz|quartz]] (q.v.}, and which, when coloured, form the gem-stones amethyst, cairngorm, cats'-eye and jasper. [[../tridymite|Tridymite]] (q.v.) is a rarer form, crystallo- graphically different from quartz. Amorphous forms also occur: [[../chalcedony|chalcedony]] (q.v.), and its coloured modifications [[../agate|agate]], [[../carnelian|carnelian]], [[../onyx|onyx]] and [[../sard|sard]], together with [[../opal|opal]] (qq.v.) are examples. Amorphous silica can be obtained from a silicate (a compound of silica and a metallic oxide) by fusing the finely powdered mineral with sodium carbonate, decomposing the sodium silicate thus formed with hydrochloric acid, evaporating to dryness to convert the colloidal silicic acid into insoluble silica, and removing the soluble chlorides by washing with hot water. On drying, the silica is obtained as a soft white amorphous powder, insoluble in water and in all acids except hydrofluoric; it dissolves in hot solutions of the caustic alkalis and to a less extent in alkali carbonates. It melts at a high temperature, and in the electric furnace it may be distilled, the vapours condensing to a bluish-white powder. By heating a solution of sodium silicate in a glass vessel the glass is attacked (an acid silicate being formed) and silica separates at ordinary temperatures in a hydrated amorphous form, at higher temperatures but below 180° as tridymite, and above 180° as quartz.
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