46 ACTION OF ELEMENTS OF SOIL. 



carbonic acid of the air, or of carbonates, upon sil- 

 icates. The potash, or alkaline part of the silicate 

 is by this means separated. The mineral no longer 

 held by the bond which had held its components, 

 falls into dust. The silica, lime, alumina, magnesia, 

 thus form the finer portions of soil. In obedience 

 to a well established fact, in chemistry, the seeming- 

 ly insoluble silica, and alumina, and magnesia, in the 

 very moment of their disunion, are each soluble in 

 water. They may then be taken up by plants, or 

 dissolved by various acids, formed in the soil, form 

 salts. 



78. The 2d mode of action, of air and moisture, 

 is upon the urets, upon the sulphurets, the phosphu- 

 rets, and siliciurets. The action of air upon all these 

 is, to oxidate, both the metallic base, and the unme- 

 tallic element. In a word, the urets, by air and 

 moisture, become salts ; the unmetallic part, becom- 

 ino* acid, and the base an oxide, which combine. 



79. The fact most important to the farmer, in 

 these changes is, that the urets are continually, in all 

 soil, becoming salts. Whenever iron pyrites, or 

 sulphuret of iron is found, and it is very universally 

 diffused, exposure to air and moisture, acidifies the 

 sulphur, it forms oil of vitriol, or sulphuric acid. 

 This immediately combines with the iron, and forms 



