SILICA. 121 



bases, and strongly heated, the carbonic acid is ex- 

 pelled, and the silica and alkali unite to form a glass. 

 Such a compound is called a silicate. 



263. Common glass always contains other sub- 

 stances, but the basis of all good glass is this com- 

 pound of silica and alkali — either silicate of potash 

 or soda. The proportions taken of the two ingredi- 

 ents are always such that the glass obtained shall be 

 perfectly unacted on by water; but if more alkali be 

 employed than is requisite to form a good glass, a 

 silicate will be obtained which is readily soluble in 

 water. A solution thus made is easily decomposed 

 by any acid, as the potash has comparatively but 

 a weak attraction for the silica, and hence that 

 substance is separated from its solution on the addi- 

 tion of almost any acid. 



264. Though potash is scarcely able to combine 

 with silica at a common temperature in its usual 

 states, yet when the silica is in an exceedingly fine 

 powder, the alkali is able to dissolve a small quantity ; 

 but this action is far slower and less perfect than 

 "when the two are strongly heated together. 



265. Silica is almost always an ingredient of the 

 soil, and exists there not only in its solid and insolu- 

 ble form of sand, but also in the soluble condition of 

 silicate of potash or soda ; it will easily be seen that 

 as many stones contain silica, they will, whilst gra- 

 dually decomposing and crumbling down by exposiire 

 to the air, constantly add to the soil silica in a very 

 finely divided state, and therefore well adapted to 



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