CHEMISTRY OF SOIL. 29 



vinegar, or acetic acid, (47) a large class in which 

 are found silicates of soda, of potash, of lime, of 

 magnesia, of alumina, of iron, and of manganese. 

 This class forms the great bulk of all rocks and soil. 



49. The seven substances last mentioned (48) 

 are all metals united to oxygen. They are metallic 

 oxides. If the oxygen is removed, and replaced by 

 carbon, sulphur, phosphorus or silicon, combinations 

 are formed, called sulphurets, carburets, phosphurets, 

 siliciurets. 



50. Urets are combinations of unmetallic combus- 

 tibles, with metals in their pure, or unoxidated state. 



51. Salts are combinations of unmetallic combus- 

 tibles, with oxygen, and the metals in their rusted or 

 oxidated state. 



52. When the combustibles, carbon, &c, (43) are 

 united with oxygen, they become acids ; thus is 

 formed carbonic, sulphuric, phosphoric acids. When 

 these acids unite to the alkaline class, (42) salts are 

 formed, called carbonates, sulphates, phosphates. 



53. Hence, when a substance is named, for ex- 

 ample, sulphate of lime, a definite idea of the nature 

 of this is conveyed. It is, on the principles stated, 

 at once known to be a salt, that is a sulphate, that is, 

 sulphur and oxygen united to lime. So too phosphate 



of lime is seen to be a salt of lime. 

 3# 



