ENGLISH AGRICULTURE. 21 



to bear upon them to prevent the requisite molecular 

 changes. 



Atmospheric moisture has a powerful solvent effect, no 

 substance, perhaps, having a more uniformly dissolving effect 

 than pure distilled water. When we add to the effect that 

 of carbonic acid gas in solution, then we have a most potent 

 cause of disintegration of rocky material. We cannot have 

 a better reason for the crumbling down and disintegration 

 of compact soil than when its " continuity " is destroyed ; and 

 that continuity is destroyed when a certain portion of the 

 component ingredients is dissolved out. If we take, for ex- 

 ample, a granite, and dissolve out the soluble alkaline portion, 

 then the quartz element and the silicate of alumina, which are 

 integral parts of the granite, break down, and are carried away 

 probably by the action of running water. If the soluble part is 

 washed out, then we have the crumbling down of the remainder. 



We shall next notice that peculiar change brought about 

 when water assumes the solid state, which it does at the 

 freezing-point. The change is accompanied with an increase 

 of volume which no amount of force is able to check. If 

 porous stones such as sandstones become charged with water, 

 then when frost sets in there is an expansion of the water, 

 and a disruption of the particles. This is known to be a 

 cause of disintegration not only of rocks, but of buildings ; 

 and it is the cause of the obliteration of inscriptions on 

 tombstones, and shows itself in very many other ways. 



In the next place, we have the wearing action of water, 

 which must be considered in connection with the entire 

 water systems of the world. We have the wearing action 

 of water, not only as exemplified in runnels or cascades upon 

 the mountain-side, but in the wearing action of the entire 

 river system of the world ; and of all the currents and tides 

 of the ocean. These forces are in perpetual action, generation 

 after generation ; and so potent is the wearing action of water 





