CHEMISTRY OF SOIL. 41 



rocks or soils. Metalloids also unite with metals and form 

 a class of compounds of great importance in agriculture. 

 These may be termed metalloid compounds when speaking 

 of such agriculturally. 



43. It is seen, therefore, that the elements composing 

 rocks are reduced to salts and metalloid compounds. 



But the peculiar character of tire salts formed by silicic 

 acid, will be easier understood by separating these from the 

 others, and studying them under the name of silicates. 

 These form the great bulk of the earth's crust. The distiii.- 

 guishing character of the silicic acid salts is either a crystal- 

 line appearance, with the transparency and lustre of glass, 

 united to the hardness of flint, or opacity, with the stony 

 and earthy look and common characters of mineral sub- 

 stances composing rocks. The compounds of silicic acid 

 would be hardly recognized as salts in the common and pop- 

 ular sense of that term, with which is associated the ideas of 

 softness and solubility. In the opinion of some chemists of 

 the highest authority, silicon, or silicium as they term it, is a 

 metal. Its combination with oxygen produces an acid. 

 Hence, perhaps, its salts derive their peculiarity. Silicic 

 acid is a metallic acid, which forms with bases, silicates ; 

 these are generally insoluble in water, or soluble by the aid 

 of an excess of alkali. Carbonic, sulphuric, and phosphoric 

 acids are metalloid acids, which form with bases, salts, in the 

 usual sense of the term. Nor is silicium, if a metal, the 

 only one of the class which acts as an acid, combined with 

 oxygen. Alumina acts so occasionally, so does manganese, 

 so does iron. 



44. The elements of water being included in silicates 

 and salts, all the substances which compose rocks may be 

 divided into three sections; a. silicates, b. salts, c. metalloid 

 compounds. 



