120 SILICA. 



mina, in not being a base — in being unable to com- 

 bine with acids ; indeed, it has rather the character of 

 an acid, for it is able to combine with the alkalies so 

 much in the manner of acids, that it is very frequently 

 termed silicic acid. Silica is the oxide of a peculiar 

 substance having many of the properties of metal, 

 and to which the name of Silicon has been given 

 (202). 



260. Silica, in its common forms, is quite insoluble 

 in water, unacted on by the air, and, under ordinary 

 circumstances, a very unchangeable substance : when, 

 however, combined with the alkalies, it is easily dis- 

 solved in water, and the compound thus formed is 

 sometimes present in very small quantity in mineral 

 waters. 



261. When silica is obtained by the decomposition 

 of any of its soluble compounds, it appears in the 

 form of a transparent jelly, which dries into a very 

 fine white powder, like flour ; when freshly precipi- 

 tated from a solution, this jelly is slightly soluble in 

 water and in dilute acids, a property, however, which 

 it loses by being thoroughly dried. 



262. The ordinary forms of silica or silicic acid, 

 such as sand or flint, are very little acted on by 

 potash ; but when finely powdered silica is mixed with 

 potash or soda, and strongly heated, they melt and 

 form a clear transparent substance, which is in fact 

 glass. Indeed, so strong is the affinity which silica 

 has for potash and soda, that, if fine sand or pounded 

 flints are mixed with the carbonate of either of those 



