250 SILICATES. 



carbonic acid and ammonia, either generated by the 

 decay of substances which the soil naturally con- 

 tains or absorbed from the air, and also yields them 

 the earthy and alkaline salts which they require. 



648. As may be supposed, the soil is very variable 

 in composition ; its nature is generally a good deal 

 dependent on the subsoil and stony matters which 

 are buried beneath the surface, many of which are 

 slowly decomposing or crumbling away, and adding 

 to the soil the substances of which they consisted. 



649. The origin of all soils appears to be the dis- 

 integration or gradual crumbling down of rocks, 

 from the action of frost, and various chemical and 

 mechanical agents ; and, therefore, all soils contain 

 innumerable little fragments of different rocks, 

 which are slowly but constantly becoming smaller, 

 as the chemical combinations of which they consist 

 are broken up and destroyed. 



650. This gradual decomposition of stony particles 

 in the soil is caused by the action of the air. Many 

 of the common rocks are compounds of several dif- 

 ferent earthy and alkaline substances, in which silica, 

 united to lime, alumina, potash, and soda, forms sili- 

 cates of those or similar bases (267). 



651. Silicates of this kind, or natural compounds 

 containing silica in combination with several earthy 

 and alkaline bases, are quite insoluble in water, and 

 are scarcely acted on, even by the strongest acids ; 

 nevertheless, they gradually decompose when ex- 

 posed to the air. Under the joint action of the moist- 



