SILEX, SILICIC ACID. Pure sand, rock crystal. An add, consisting of one equivalent of silicium, (22.22), a body resembling in appearance charcoal, and three equivalents of oxygen, 46.22. In the cold it is inaetive, but at a white heat it forms an exceedingly aetive aeid, combining with bases, and displacing most other acids, ex cept the phosphoric and boracic. The silicates are nearly all insoluble in pure water; glass and common earthen-ware are specimens of sili cates, but they gradually- decay in the presence of acids, and of carbonic acid and water. But the compounds of silicic acid, with two or three times its weight of carbonate of potash or soda, are soluble silicates, and have been recommended as manures for the cerealia, which always con tain a large amount of silicic acid in their stems, leaves, and husks. Most of the minerals and rocks of the earth are silicates, this acid forming from one-quarter to one-third of its entire solid mass. Soluble and other silieates are formed
by fusing together sand and the desired chemical body, usually in the state of carbonate, in a black lead crucible, at a full red heat. Potash, soda, lime, and magnesia in the soil whieh supply plants with saline matters, are often in the form of silicates; these are slowly deeomposed under the influenee of the carbonic acid of the air, or from decaying vegetable matter, whieh converts them into soluble carbonates, whereby they gain access to the plant.
siLicArEs. Salts containing silicic acid, usnally flinty and insoluble; slate, feldspar, and granite are specimens.
siuQuA, SILIQUE. A one or two-celled, many-seeded, linear fruit, dehiscent by two valves separating from a septum; the seeds are attached to two plaeentze adhering to the septum, and opposite to the lobes of the stigma. The seed pods of the mustard is an example.