SILICATES. 319 



soluble substance with the gypsum and animal char- 

 coal produced in its formation. Other materials are 

 often added, increasing or diminishing, according to 

 their nature, its agricultural value. The basis of the 

 manufacture, is commonly the refuse bone black of 

 the sugar refineries employed in the process. 



809. OTHER PHOSPHATES. The phos- 



Whatis mid . 



of other phos- phate of soda is used in medicine 

 phates? and j^ tne cnem i stj to produce 



other phosphates. The phosphate of silver is 

 a beautiful yellow precipitate, obtained by pre- 

 cipitating salts of silver with phosphate of 

 soda or any other salt containing phosphoric acid. 



SILICATES 



What is said 810. The silicates form an exceedingly 

 of silicates? i ar g e c | ass o f salts. They are, for the 

 most part, insoluble, and are variously colored. 

 Mica and feldspar, two of the constituents 

 of granite, may serve as examples. As com- 

 ponents of this and other rocks, the silicates 

 make up a very considerable portion of the 

 mass of the earth. 



811. PREPARATION. Most silicates may 



How are sili- . f _ 



catespre- be artificially formed by fusing together 

 pared? quartz sand, with the proper oxide. This 



is done in the manufacture of glass, to be hereafter 

 described. Silicates may also be formed by precipita- 

 ting solutions of metals or their salts by the solution 

 of an alkaline silicate. 



