SILICA. 119 



from the large quantity of this earth which they con- 

 tain; and the whole of the art of making pottery, 

 bricks, tiles, &c., is in great part dependent on this 

 property of alumina. 



256. This earth is able to combine with acids, but 

 the salts which it forms are mostly of very little im- 

 portance, and we need not inquire into their charac- 

 ters. The only one which is found native is the phos- 

 phate, but this is by no means of common occurrence; 

 it is only found in certain districts, and in very 

 limited quantity. 



257. The sulphate of alumina is largely manufac- 

 tured from certain kinds of slate or shale, which con- 

 tain alumina and sulphur. When sulphate of alumina 

 is mixed with sulphate of potash, the two salts com- 

 bine and form a double salt, the sulphate of alumina 

 and potash, or common alum. If a little potash is 

 added to a solution of alum, the sulphate of alumina 

 will be decomposed, pure alumina will be separated, 

 and sulphate of potash alone remain in solution (112). 



258. Silica, or silex, is the only other earth of 

 much importance besides the three already men- 

 tioned ; it is found abundantly, both pure and mixed 

 or combined with alumina and other substances, con- 

 stituting, in a nearly pure condition, quartz, sand, 

 flint, &c-., and, when associated with alumina, form- 

 ing clay and a numerous series of stones ; being, in 

 fact, an essential ingredient of the greater number of 

 the hard stones with which we are acquainted. 



259. Silica differs from the preceding earth, alu- 



