THE SOIL. 69 



es by wliicli tlie soil was originally formed, he can 

 increase its depth, or otherwise improve it. Tliis 

 being the case, w^e will in a few words explain some 

 of the principal pulverizing agents. 



1. The action of frost. When water lodg-es in 

 the crevices of rocks, and freezes^ it expands, and 

 bursts the rock, on the same principle that causes 

 it to break a pitcher in winter. This power is verj 

 great, and by its assistance large cannon may be 

 burst. Of course, the action of frost is the same on 

 a small scale as when applied to large masses of mat- 

 ter, and, therefore, we find that when water freezes 

 in the jpores " of rocks or stones, it separates their 

 particles and causes them to crumble. The same rule 

 holds true with regard to stiif clay soils. If they are 

 ridged in autumn, and left with a rough surface ex- 

 posed to the frosts of winter, they will become much 

 lighter and finerj and can afterwards be worked with 

 less difficulty. 



2. The action of water. Many kinds of rock 

 become so soft on being soaked with water, that they 

 readily crumble. 



3. The chemical changes of the constituents of the 

 rock. Many kinds of rock are affected by exposure 

 to the atmosphere, in such a manner, that changes 

 take place in their chemical character, and cause 

 them to fall to pieces. The red kellis of New Jer- 

 sey, (a species of sandstone,) is, when first quarried, 

 a very hard stone, but on exposure to the influ- 



* The spaces between the particles. 



