PROPERTIES OF SILICA. 83 



of new compounds, the coherence of the original body is 

 destroyed. If the minerals contain metallic sulphurets, such 

 as the pyrites in granite, these are gradually converted into 

 sulphates. 



Most kinds of rocks, such as felspar, basalt, clay-slate, por- 

 phyry, and the numerous members of the limestone formation, 

 consist of compounds of silica, with alumina, lime, potash, soda, 

 iron, and protoxide of manganese. 



Before we can properly comprehend the action of water and 

 of carbonic acid upon minerals, it is necessary to recollect the 

 properties of silica and of its compounds with alkaline bases. 



Quartz forms a very pure variety of silica, and, in this condi- 

 tion, it is quite insoluble both in cold and in hot water, is with- 

 out taste, and does not exert any action on vegetable colors. 

 The principal property of silica in this state is, that it unites with 

 alkalies, forming saline compounds, which are termed silicates. 

 Window and plate glass consist of mixtures of silicates of the 

 alkaline bases, potash, soda, and lime. In such compounds the 

 alkali is generally completely neutralized. The property of 

 neutralizing metallic oxides and alkalies belongs only to acids, 

 and it is owing to this that silica has received the name of silicic 

 acid. 



Silica is a very feeble acid, for we have already mentioned 

 that, in its crystallized form, it is destitute both of taste and of 

 solubility in water ; but t dissolves when finely pulverized and 

 boiled for a long time in alkaline leys. 



We may easily obtain compounds of silica with potash and 

 soda, by melting it either with a pure alkali, or with an alkaline 

 carbonate. By this treatment white glasses are obtained, differ- 

 ing in properties according to their amount of soluble ingre- 

 dients. When the glass contains 70 per cent, of silica and 30 

 per cent, of potash or soda, it becomes soluble in boiling water. 

 Its solution may be spread over a surface of wood or of iron, and 

 then dries into a vitreous substance, which has received the 

 name of soluble glass. When there is a smaller proportion of 

 alkali than the above quantity, or, in other words, when there is 

 a larger proportion of silica, the resulting glass diminishes in 

 solubility in a greater or less degree. 



