ALKALIES, URETS, SALTS, SILICATES, 181 



The most important property of acids is their constant de- 

 sire to unite with alkalies, alkaline earths and oxides; and 

 as they possess different degrees of affinity for bases, they 

 are in the soil constantly exchanging them. 



2. Alkalies, alkaline earths and metallic oxides. The al- 

 kalies are potassa, soda and ammonia. The alkaline earths 

 are lime, magnesia and alumina. The oxides are oxides of 

 iron and of manganese. These with the exception of the 

 two last, do not exist in the rocks in their pure state, but their 

 agency in the soil is of the highest practical interest to the 

 farmer. 



3. Urets. The urets are bi-elementary compounds, neither 

 acid nor alkaline. They are formed by the union of the non- 

 metallic combustibles with each other and with metals. The 

 principal are sulphuret of iron, (iron pyrites), phosphuret of 

 iron, carburet ofiron, phosphuret of lime, phosphuret of sili- 

 con, etc. 



Their distinguishing property is, a readiness to change into 

 salts through the influence of atmospherical or other agents. 



II. Secondary compounds or scdts. These are compounds 

 formed by the union of the primary compounds. The acids 

 combine with the alkalies, alkaline earths and oxides, which 

 are called, in reference to the acids, bases, and form ternary 

 or quaternary compounds. 



Salts may be conveniently classed under their respective 

 acids. 



1. Silicates. The silicates are those compounds or salts, 

 in which silicic acid combines with the bases above named. 

 Those entering into the composition of the rocks, are the 

 following : silicates of potassa, soda, lime, magnesia, alumina, 

 oxide ofiron and of manganese. Almost the entire mass of 

 rocks is composed of these silicates, and from seventy to 

 eighty per cent, of soils. These salts when neutral, are not 

 soluble in water, and therefore are not capable of being dis- 

 solved in that fluid, except in very minute quantities ; hence. 



