Silicon, or SILICIUM, is one of the non-metallic elements: sym. Si; at. wt. 28.4 (O = 16); sp. gr. of crystalline form, 2.49. It may be obtained in three different forms—viz. the amorphous, the graphitoid, and the crystalline. It is amorphous silicon which is obtained by the processes in common use, the other forms being obtained from it.
Amorphous silicon presents the appearance of a dull brown powder, which adheres to the finger, is insoluble in water and in nitric and sulphuric acids, but readily soluble in hydrofluoric acid and in a hot solution of potash. It is a non-conductor of electricity, and when heated in air or oxygen its external surface burns brilliantly, and is converted into silica, which fuses from the extreme heat, and forms a coating over the unburned silicon. Graphitoid silicon is obtained by exposing the amorphous variety to an intense heat in a closed platinum crucible. This form of silicon will not take fire when heated in oxygen gas, and resists the solvent action of pure hydrofluoric acid, although it rapidly dissolves in a mixture of nitric and hydrofluoric acids; moreover, it is a conductor of electricity. Deville obtained crystallised silicon in regular double six-sided pyramids of a dark steel-gray colour.
Silicon, in a state of combination with oxygen, is the most abundant solid constituent of our globe; in less proportion, is an equally necessary ingredient of the vegetable kingdom; while in the animal kingdom it occurs in mere traces, except in a few special cases. It is never found in nature except in combination with oxygen; but, by a somewhat difficult process, it may be separated as a dark brown powder. It was first isolated by Berzelius in 1823. For our knowledge of the other modifications we are indebted to Wöhler and Deville.
Silicon forms two oxides, one of which is only known in the hydrated state, while the other is the well-known compound silica or silicic acid. Silica or silicic acid is represented by the formula , and a hydrate, , has been obtained, while other hydrates are known to exist.
Silica exists both in the crystalline and in the amorphous form. The best examples of the crystalline form are rock-crystal, quartz, chalcedony, flint, sandstone, and quartzose sand. Silica in this form has a specific gravity of about 2.9, and is only attacked with difficulty by potash or hydrofluoric acid. The amorphous form exists naturally in opal, and is obtained artificially as gelatinous silica, &c.; it differs from the former in its specific gravity, being about 2.2, and in its being rapidly dissolved by potash and by hydrofluoric acid. Pure silica (as it occurs in rock-crystal, for example) is perfectly transparent and colourless, and is sufficiently hard to scratch glass. The heat of the oxyhydrogen blowpipe is required for its fusion, when it melts into a transparent glass, capable of being drawn out into elastic threads. Perfectly pure silica in its amorphous form may be obtained by various chemical processes. If a solution of silicate of potash or soda be treated with hydrochloric acid, the silicic acid separates as a hydrate, and on evaporating this to dryness, and treating it with boiling water, silicic acid remains as an amorphous powder, which, after being washed, dried, and exposed to a red heat, may be regarded as chemically pure. The hydrated silicic acid mentioned in the above experiment is soluble in water, and (more freely) in acids and alkalies. The solubility of hydrated silicic acid in water accounts for the presence of silicic acid in mineral springs and in the geysers of Iceland, as well as for its gradual separation from these waters in the form of petrifications. That silica or silicic acid is a true acid (although a feeble one) is obvious from its uniting with bases, especially those which are capable of undergoing fusion, and forming true salts, known as silicates. These silicates occur abundantly in nature, all the forms of clay, felspar, mica, hornblende, augite, serpentine, &c. being compounds of this description.
Most of the silicates are fusible, the basic silicates fusing more readily than those which are either neutral or contain an excess of acid. Excepting the silicates of the alkalies, no silicates are soluble in water. The anhydrous, neutral, and acid silicates of the earths resist the action of all acids except the hydrofluoric.
Silica derives its name from the Latin silix, 'flint,' of which it is the essential constituent, and is largely employed in the manufacture of glass, china, and porcelain. For these purposes it is obtained in a finely comminuted state by heating flints or portions of colourless quartz to redness, and plunging them in cold water. The silica splits up into a friable mass, which may be easily ground to a fine powder. The use of silica in giving firmness and rigidity to various parts of the animal organs is exemplified in its free occurrence in the quill-part of the feathers of birds, in the shields of certain infusoria, and in the spicula occurring in sponges; while its similar use in the vegetable kingdom is seen in its more or less abundant presence in the stalks of the grasses, more particularly in the cereals and in the bamboo (where it is especially deposited about the joints, and is known as Tabascheer), in the Equisetæ, &c.
Silicon may be made to combine with several other elements besides oxygen, but, with the exception of silicofluoric acid, these compounds are of no practical value. Thus, silicon and hydrogen form a hydride of silicon, a colourless and spontaneously inflammable gas. Nitride of silicon is a bluish fibrous body, while sulphide of silicon is a white earthy powder. Silicon unites with chlorine, bromine, and probably iodine and fluorine, in two proportions corresponding to its oxygen compounds. Fluoride of silicon, , is a colourless pungent gas, liquefiable under strong pressure, and solidifying at , inflammable, and a non-supporter of combustion. It is obtained by heating powdered glass with twelve times its weight of oil of vitriol, and when a stream of this gas is transmitted through water a reaction takes place; two atoms of water and three atoms of the fluoride of silicon yielding silicofluoric acid, , which remains in solution, and silicon, which is deposited. A saturated solution of this acid forms a very sour fuming liquid, which does not directly attack glass, but if allowed to evaporate on it causes erosion from the fluoride of silicon becoming evaporised, and free hydrofluoric acid being left. A dilute solution is sometimes employed in the laboratory as a precipitant of potash, which it throws down in a transparent gelatinous form. With salts of baryta it gives a white crystalline precipitate. It combines with bases to form salts.
For soluble glass, a silicate of soda or potash, see GLASS, Vol. V. p. 245; and for silicate cotton, see SLAGS.