SILICON, or STLI'CIUM (sym. Si, eq. 14—in new system, 28—spec. gray. 2.49), is one of the non-metallic elements (see CHEMISTRY). It may be obtained in first of these, the three different forms, viz., the amorphous, the graphitoid, and the crystalline. It is the amorphous silicon, which is obtained by the processes in common use, the second and third being obtained from this first modification.
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 elec tricity, 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, as another point of difference, it is a conductor of electricity. For the description of crystallized silicon, we may refer to a treatise by Deville (in the Ann. de Chimie, 3d see. vol. 49, p. 65), who obtained it in regular double six-sided pyramids of a dark steel-gray color.
Silicon, in a state of combination with oxygen, is the most abundant solid constit uent of our globe; and, 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—which we need not here describe—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 Wohler and Deville.
• Silicon forms two oxides, one of which is known in the hydrated state, while the other is the well-known compound silica or siliic acid. Hydrated oxide of silicon is represented by the formula 2H0,3SiO, and silicic acid by SiO,. The hydrated oxide exhibits many interesting chemical properties, but is of no practical importance.
Silicic acid or silica exists both in the crystalline and in the amorphous form. The best examples of the crystalline form are rock-crystal, quartz; chalcedony, flint, sand stone, 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, etc.; 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 exam ple) is perfectly transparent and colorless, and is sufficiently hard to scratch glass. The heat of tire oxyhydrogen blowpipe is required for its fusion, when it melts into a trans parent 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 men tioned in the above experiment is soluble in water and (more freely) in acids and alka lies. 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 separa tion from these waters in the form of petrifactions. 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, feldspar, mica, horn blende, augite, serpentine, etc., being compounds of this description. Silicic acid combines with bases in various proportions. The following table, borrowed from Mil ler's Elements of Chemistry, vol. ii., shows tire combinations which are of the most usual occurrence: Examples. Formulas.