OF AGRICULTURE. 195 



The formation of carbonic, sulphuric, and phos- 

 phoric acids, has been explained. [See article 15.] 

 When these acids unite with bases, salts are formed, 

 called carbonates, sulphates, phosphates. 



Hence, when a substance is named, as for example, 

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

 compound is. conveyed. It is at once known to be a 

 salt, that is, a sulphate, that is, sulphur and oxygen 

 united with lime. So too, phosphate of lime is seen 

 to be a salt of lime, 



The thirteen substances that enter into the compo- 

 sition of rocks, [see article 14,] are subject to fixed 

 laws in their combinations, viz : 



They combine only in definite proportions, or in 

 multiples of the same. The combining number of a 

 compound, is the sum of the combining numbera of 

 the composing elements. If we assume the smallest 

 Combining weight, viz : that of hydrogen, as one, the 

 combining weight of all. other dements may be 

 represented by definite numbers, expressing the 

 proportions by weight in which they unite, not only 

 with hydrogen, but also amongst themselves. j.hus, 

 when oxygen and hydrogen unite to form water, the 

 proportions are : 



Hydrogen 



Oxygen 



Water 



It has been ascertained that the proportion in 

 which the bases of the silicates combine with oxygen, 

 are: 



Oxygen 1 eq>, or 8 with one eq., 21 of Silicatt, = 29 Silica or sand. 



Alumimen = 18 Almumina or clay. 

 Calcium = 28 Lime. 

 Magnesium = 20 Magnesia. 



Potassium = 47 Potash. 

 Sodium 31 Soda, 



