Silicon, Or Silieinm, the essential constituent of silex or flint. It is obtained in a dull brown amorphous powder by passing the vapor of chloride of silicon over heated potassium or sodium contained in a glass tube. It may also be obtained from the aqueous solution of the gaseous fluoride of silicon. Neutralized with solution of potash, this affords a silico-fluoride of potassium, which when well dried is mixed in a glass or iron tube with 3/10 or 9/10 of its weight of potassium or sodium and heated. The silicon set free partially combines with the excess of the alkali, from which it is finally removed by washing in water. When heated in air or oxygen, it'burns vividly, and with such intense heat as to fuse the external crust of silica. In its chemical properties silicon exhibits striking analogies with carbon and boron. When strongly heated in a close platinum crucible, it becomes darker and of greater specific gravity; it loses its affinity for oxygen, so that it will not ignite even if heated by the blowpipe and immersed in oxygen, and is not attacked by pure hydrofluoric acid. If aluminum be substituted for the sodium of the above experiment, silicon is obtained in a crystalline condition.

Two methods are employed to prepare crystalline silicon : 1, fuse a mixture of 5 parts pulverized glass, 10 parts cryolite, 1 part aluminum, and wash the product with hydrochloric and hydrofluoric acids; 2, fuse 15 parts silico-fluoride of sodium, 20 parts granulated zinc, 4 parts sodium, and wash with hydrochloric and nitric acids. Amorphous silicon was discovered by Berze-lius in 1824, crystalline by Deville in 1855. Crystalline silicon forms brilliant black scales having a lustre like that of specular iron ore, sometimes prismatic, at others octahedral, foliated, graphitic, with a specific gravity of 2.49. The symbol of silicon is Si; atomic weight, 28. It is a poor conductor of electricity, fuses at a temperature between that of cast iron and steel, is harder than glass, and is insoluble in all acids excepting hydrofluoric and nitric. There were at one time supposed to be three modifications of silicon, the amorphous, gra-phitoid, and crystalline, but the graphitoid is now regarded as somewhat problematical.

Silicon belongs to the class of tetrads, being equivalent in its most usual combinations to four atoms of hydrogen. - There is but one anhydrous oxide of silicon, commonly known as silicic acid or silica; its formula is Si02. Silica, or silicic anhydride, occurs in nature dimorphous: 1, in hexagonal prisms with terminated pyramids, as quartz, rock crystal, smoky quartz, amethyst, etc.; 2, in wedge-shaped crystals, with sharp angles, or hexagonal tables, or in twins (called tridymite), colorless and clear as water. The former has the specific gravity of 2.6, the latter of 2.3. Its only solvent among the acids is the hydrofluoric, by means of which it is decomposed, and a gaseous compound is obtained of its base with the acid. When passed into water this combination is broken up, and silica is reproduced in the form of little bubbles and white flocculi, which by washing and igniting become perfectly pure and snow-white silica. Pulverized silica, when mixed with an alkaline carbonate and fused, dispels the weaker carbonic acid, and itself combines with the alkali, thus exhibiting its properties as an acid. • But these are too feeble to act upon test paper.

An excess of silica in the alkaline mixture determines the production of glass, which is insoluble in water or common acids; but if no more silica be added to the melted mass after this ceases to effervesce on its introduction, the product after being cooled may be dissolved in water. When silica is separated from its alkaline combination by hydrochloric acid, it appears before evaporation as a jelly, which is a hydrate of silica, soluble in a large excess of water; but once deprived of water by heat, it can no more be dissolved. Silica of this character is met with in several mineral compounds. It constitutes the opal, in which the proportion of water varies from 3 to 10 per cent., and also great deposits of a white silicious earth made up of infusorial remains. The zeolites are hy-drated silicious compounds, which when finely pulverized and treated with hydrochloric acid swell up into the transparent jelly. - Silica is an important element in the composition of the grasses, and forms in chief part the hard external coat of the reeds. It combines with bases and forms silicates, among which are found a large proportion of the minerals.

Their variety is multiplied by the number of bases, as lime, alumina, magnesia, protoxide of iron, and several of the other metals, and by the diversity in the relative proportions of the different silicates, the substitution of one base for another. They comprise the hydrous and anhydrous silicates, the former including, besides those already named, the talcs, serpentines, and chloritcs, and the latter the augites, garnets, micas, and feldspars. They are for the most part fusible, and those melt easily which consist largely of fusible oxides. They are decomposed by vegetable acids, and gradually even by the carbonic acid gas of the atmosphere; but at high temperatures in a furnace the silica, not being volatile, takes the place of most other acids, expelling even sulphuric acid from its combinations. - Diatoma-ceous or infusorial silica, of which large deposits have been found in Nevada, New Jersey, and Virginia, is now employed in the arts for a great variety of purposes, among which are : as a polish for metals under the name of tri-poli or electric silicon; as a non-conductor in refrigerators and fire-proof safes; as an absorbent of nitro-glycerine in the manufacture of dynamite; in the manufacture of glass, enamel, pottery, and 6oluble glass.

Chloride of silicon, SiCl4, is a transparent, colorless liquid, with a pungent, acid, irritating odor. It is very volatile and fumes strongly in the air, and is prepared by the action of chlorine on a heated mixture of silica and charcoal. Fluoride of silicon, SiF4, is a colorless gas of a peculiar, pungent acid odor, which is evolved when equal parts of finely powdered fluor spar and silicious sand or powdered glass are mixed, in a capacious flask or retort, with 12 times their weight of oil of vitriol. The gas was converted into a liquid by Faraday. When a stream of gaseous fluoride of silicon is transmitted through water, it is partially decomposed and partially dissolved. Two atoms of water react on three of fluoride, and produce silico-fluoric or hydrofluosilicic acid, which is dissolved, while one third of its silicon is deposited as silica. Efforts have been made in metallurgical operations to economize the fluoride of silicon and hydrofluosilicic acid hitherto wasted, and to employ the latter in the beet-sugar refinery and for chemical uses.