314 SILICON. 



tile liquid, of which the formula is Si C1 3 ; that of the sulphu- 

 ret of silicon Si S 3 . 



Silica or Silicic Acid, Si O 3 . This earth, which is the only 

 oxide of silicon, constitutes a number of minerals, nearly in a 

 state of purity, such as rock-crystal, quartz, flint, sandstone, the 

 amethyst, calcedony, cornelian, agate, opal, &c. The first 

 chemical examination of its properties and compounds is 

 due to Bergman. 



Preparation. Silica may be had very nearly, if not absolutely 

 pure, by heating a colourless specimen of rock crystal to redness 

 and throwing it into water, after which treatment the mineral 

 may easily be pulverized. It is obtained in a state of more 

 minute division, by transmitting the gaseous fluoride of silicon 

 (fluosilicic acid) into water ; or by the action of acids upon 

 some of the alkaline compounds of silica. Equal parts of car- 

 bonate of potash and carbonate of soda may be be fused in a 

 platinum crucible, at a temperature which is not high; and 

 pounded flint or any other siliceous mineral, thrown by little 

 and little into the fused mass, dissolves in it with an efferves- 

 cence due to the escape of carbonic acid gas. The addition of 

 the mineral may be continued so long as it determines this ef- 

 fervescence. The mass being allowed to cool, is afterwards dis- 

 solved in water acidulated with hydrochloric acid, which takes 

 up the silica as well as the alkalies; the liquor is filtered 

 and then evaporated to dryness. The silica may contain a 

 little peroxide of iron or alumina, to dissolve which the 

 saline mass, when perfectly dry, is moistened with con- 

 centrated hydrochloric acid, and after two hours the acid 

 mass is washed with hot water. The silica remains undis- 

 solved; it may be dried well and ignited. 



Properties. Silica so prepared is a white, tasteless powder, 

 which is rough to the touch, and feels gritty between the teeth. 

 It is extremely mobile when heated, and is thrown out of a 

 crucible, at a high temperature, by the slightest breath of wind. 

 It is absolutely insoluble in water, acids and most liquids. 

 Its density is 2.66. The heat of the strongest wind-furnace is 

 not sufficient to fuse silica, but it melts into a limpid colourless 

 glass in the flame of the oxihydrogen blow-pipe. Silica is found 

 frequently crystallized, its ordinary form being a six-sided prism 

 terminated by a six-sided pyramid, as in rock-crystal. Some- 



