14 AGRICULTURE 



hardly to be seen, a stage has been reached 

 when the operations of other weathering 

 agents become possible. Hitherto, water, 

 for instance, has been unable to penetrate 

 the mass, and, although it may have been 

 operative on the surface, its power of dis- 

 solving the constituents of the rock or stone 

 has been but limited. But directly water is 

 permitted to gain access to the interior, its 

 power as a weathering agent is greatly in- 

 creased. Water acts in various ways in the 

 formation of soil. In the first place it is an 

 almost universal solvent, so much so that there 

 are few substances that it cannot attack and 

 dissolve to a greater or less extent ; and it is 

 evident that if some substance easy of solution 

 is removed from a rock or stone, the rock 

 or stone is honeycombed and weakened in 

 the process, and its power of resistance to 

 other forces is correspondingly diminished. 

 But water also acts chemically upon rocks, 

 converting certain constituents which have 

 no water in their composition into compounds 

 in which water plays an essential part. This 

 change is called by chemists " hydration," 

 and can be illustrated by various familiar 

 examples. One can, for instance, take as an 

 illustration the slaking, or, as a chemist 

 would say, the hydration of lime. Every one 



