SILICA, in mineralogy and chemistry, is generally found in a stony state, and, from its forming nearly the entire compo sition of flint, it has acquired the name of silica, silex, or silicious earth. This earth exists in great abundance in nature, and it constitutes the basis of some of the hardest stones of which the nucleus of the globe consists ; and, on account of its great abundance, it has been regarded as the primitive or elementary earth, the base of all the other earths.
Silica forms one of the constitut nt parts of most stony bodies ; but it exists in greatest abundance in agates, jasper flints, quartz, and rock crystal: in the lat ter it exists nearly in a state of purity. To obtain it pel.fectly pure, a quantity of quartz, or rock crystal, may be exposed to a red heat. When it is taken from the fire, and while it is yet hot, it is suddenly immersed in cold water. It is then to be reduced to powder, and if transparent rock crystal has been employed, it is then in a state of tolerable purity. To have it perfectly pure, mix one part of the pounded stone with three parts of potash, and expose them in a crucible to a heat which is sufficient for the fusion of the mixture. The mass thus obtained is so luble in water. Add a sufficient quantity of water for its solution, and drop in mu ;trifle acid as long as there is any preci pitate. Let this be repeatedly washed with water and dried. The substance thus obtained is pure silica. It is in the form of very fine white powder, which has neither taste nor smell. The parti cles are rough and harsh to the feel, as when they are rubbed between the fin gers, or touched with the tongue. The specific gravity is 2.6. Light has no sc. tion on silica, and it is one of the peculiar characters of this earth, that L. resists, unchanged, the greatest degree of heat. There is no action between silica and oxygen azote, or hydrogen ; nor is it changed by exposure to the air. It is not acted upon by carbon, phosphorus, or sulphur. It is insoluble in water; but, in a state of minute division, it absorbs a considerable portion, and forms with this liquid a transparent jelly. When it is exposed to the air, the whole of the mois ture is evaporated.
Silica is frequently found in nature in the crystallized form, and then it is dis tinguished by the name of rock crystal. It is most commonly in hexagonal prisms, terminated by hexagonal pyramids. Crys tals of silica have also been formed arti ficially. In a solution of silica in fluoric acid, which had remained at rest for two years, Bergman found crystals, some of which were cubes, and some had trun cated angles, at the bottom of the vessel. Crystals of silica have also been formed, by diluting largely with water the com bination of silica and potash, and allow ing it to remain for a long time.
Silica is only acted on by a very few of the acids. These are the phosphoric and boracic, which combine with it by fusion, and the fluoric, which dissolves silica either in the gaseous or liquid state. When silica is held in solution in water by means of an alkali, it is also dissolved by the muriatic acid. The alkalies have
a very powerful action on this earth. In the preparation of the pure earth, it was combined with potash by means of fusion. This compound is different in its nature and properties, according to the propor tions of the silica and the alkali. Two or three parts of potash, with one of silica, form a compound which is deliquescent in the air, and soluble in water. This was formerly distinguished by the name li quor silicum, or liquor of lints. It is now called silicated alkali. When this solu tion is long exposed to the air, the earth is deposited in a flaky gelatinous form. It is decomposed by acids, which combine with the alkali, and the pure earth falls to the bottom in the state of fine powder. When the solution is largely diluted with water, and if a greater quantity of the acid be added than is sufficient to saturate the alkali, the silica remains in solution. This is particularly the case when the muriatic acid is employed ; but when the silica is in greater proportion, a com pound is formed which is possessed of very different properties. The substance thus obtained is glass. This earth also enters into combination with some of the earths. If to a solution of the liquor of flints lime-water be added, a precipitate is formed, which is found to be a com pound of silica and lime. Silica also com bines with lime by means of heat, and in certain proportions a glass is formed. Silicious earth enters with difficulty into combination with magnesia ; but if equal parts of silica and magnesia be exposed to very strong heat, they melt into a white enamel. But the most important com pounds of the earths are those of silica and alumina. These earths may be com bined together, as appears from the ex periments of Guyton, in the humid way. He mixed together equal parts of alumina, dissolved by means of potash, and of silica held in solution by the same alkali. When the solutions came into contact, a brown zone was immediately formed, which spread, by agitation, through the whole mass, and communicated to it a yellowish colour. The mixture was no further changed during the space of an hour, al though it was occasionally stirred with a glass rod ; but at the end of that time the whole mass assumed the appearance of a thick, opaque, white jelly. When the si lica and alumina are mixed together, and formed into a paste with water, and ex posed to heat, they strongly cohere, and assume a considerable degree of hard ness. This compound forms the basis of all kinds of pottery and porcelain.
Barytes and strontian in some degree dissolve silica as the alkalies do. Two hundred parts of strontian, with sixty of silica, heated intensely in a crucible for an hour, produced a grey sonorous rifty mass, with only a slight caustic taste. This, being boiled in water, was partly dissolved, but could not be crystallized. Saturated with nitric acid, it gave, by evaporation, a copious jelly, which was. pure !ilex. A similar result was obtain ed when barytes was used instead of strontian.