ACTION OP ELEMENTS OF SOIL. 69 



76. These elements do not exist in soil, free ; they exist 

 as silicates, or salts, compounds regulated by the unbending 

 laws of affinity, and fixed as are the laws of gravitation. 

 The decompounding of these combinations, or the gradual 

 decay of rocks and soil, takes place also by similar laws. 

 Gradually acted upon by the carbonic acid of the air, the 

 agency of growing plants, the action of various salts, formed 

 by metalloids in atmospheric exposure, the silicates yield to 

 new affinities. The alkalies, freed partially or entirely from 

 the embrace of silica, dissolve, and are borne seaward ; the 

 silica itself is dissolved by the water used for drink ; the 

 insoluble alumina, still combined with a portion of potash 

 and silica, remains, forming the great mass of clays, or 

 mixed with granitic sand, forms loam. 



77. Felspar, mica, hornblende, are constantly acted upon 

 by air and moisture. This action is chemical. It is twofold. 

 1st. The action of the carbonic acid of the air, or of carbon- 

 ates, upon silicates. The potash, or alkaline part of the sili- 

 cate is by this means separated. The mineral, no longer 

 held by the bond which had united 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 seemingly insoluble silica, and alum- 

 ina, and magnesia, in the very moment of their disunion, are 

 each soluble in water. They may then ba taken up by 

 plants, or dissolved by various acids found in the soil, form 

 salts. 



78. The second mode of action, of air and moisture, is 

 upon the sulphurets, the phosphurets, and siliciurets. The 

 action of air upon all these is, to oxidate, both the metallic 

 and the unmetallic element. In a word, the metalloid com- 

 pounds, by air and moisture, become salts ; the unmetallic 

 part becoming acid, and the base an oxide, which combine. 



