SILICON SILISTEIA 39 a mixture of 5 parts pulverized glass, 10 cryolite, 1 part aluminum, and wash the roduct with hydrochloric and hydrofluoric ids; 2, fuse 15 parts silico-fluoride of sodi- a, 20 parts granulated zinc, 4 parts sodium, d wash with hydrochloric and nitric acids, morphous silicon was discovered by Berze- us in 1824, crystalline by Deville in 1855. rystalline silicon forms brilliant black scales ,ving a lustre like that of specular iron -ore, metimes prismatic, at others octahedral, foli- , graphitic, with a specific gravity of 2-49. e symbol of silicon is Si ; atomic weight, 28. 1 is a poor conductor of electricity, fuses at temperature between that of cast iron and , is harder than glass, and is insoluble in 1 acids excepting hydrofluoric and nitric, ere were at one time supposed to be three odifications of silicon, the amorphous, gra- hitoid, and crystalline, but the graphitoid is ow regarded as somewhat problematical, ilicon belongs to the class of tetrads, being uivalent in its most usual combinations to r atoms of hydrogen. There is but one hydrous oxide of silicon, commonly known silicic acid or silica; its formula is Si0 2 . Silica, or silicic anhydride, occurs in nature dimorphous : 1, in hexagonal prisms with ter- minated pyramids, as quartz, rock crystal, smo- ky quartz, amethyst, &c. ; 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 sol- vent 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 produc- tion 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 effer- vesce on its introduction, the product after be- ing cooled may be dissolved in water. When silica is separated from its alkaline combina- tion 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 con- stitutes 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 silicions compounds, which when finely Iverized 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, be- sides those already named, the talcs, serpen- tines, and chlorites, 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 grad- ually 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 sul- phuric acid from its combinations. Diatoma- ceous or infusorial silica, of which large de- posits 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 ab- sorbent of nitro-glycerine in the manufacture of dynamite ; in the manufacture of glass, en- amel, pottery, and soluble glass. Chloride of silicon, SiCl 4 , 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. Fluo- ride of silicon, SiF 4 , 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 trans- mitted through water, it is partially decom- posed 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 de- posited as silica. Efforts have been made in metallurgical operations to economize the flu- oride of silicon and hydrofluosilicic acid hith- erto wasted, and to employ the latter in the beet-sugar refinery and for chemical uses. SILISTRIA (Turk. Dristra), a fortified town of European Turkey, in Bulgaria, on the right bank of the Danube, 57 m. N. N. E. of Shumla and 230 m. N. N. W. of Constantinople ; pop. with the garrison about 20,000. The river is here more than 1,200 ft. wide, and studded with numerous islands between the town and the Wallachian shore. There are several mosques, a large Greek church and convent, capacious
