26 CHEMISTRY OF SOIL. 



The metals, iron, and manganese. These form 

 the first class, or silicates. 



42. The silicates are formed into two divisions ; 

 first, those with alkaline, and second, those with acid 

 properties, potash, soda, lime, magnesia, iron and 

 manganese, have alkaline properties ; silex and al- 

 umina, acid properties. Silex is commonly consider- 

 ed an earth, but truly it is not ; and alumina, though 

 generally acting as an alkali, sometimes acts as acid, 

 as does silex or silica. 



43. The inflammables, sulphur, phosphorus, car- 

 bon, and silicon, united with the bases of the alka- 

 line division of the silicates, form the second class, 

 or urets. 



44. The four elements (43) united to oxygen, 

 form acids. These acids, united to the alkaline di- 

 vision of silicates, form the third class, or salts. 



45. The principles (41, 42, 43, 44) may be con- 

 veniently tabulated. 



Twelve substances form all rocks, and they are 

 divided into three classes — silicates, urets, salts. 



FIRST DIVISION. 



1st. — Silicates. Acid, Silex. 



