20 AGRICULTURE 



one examines a heap of rough land-gathered 

 stones lying by a roadside, one will often find 

 that they are more or less yellow or red on 

 the surface. When they are broken in the 

 process of preparation as road-metal, it 

 is possible to trace a gradual transition from 

 the red, friable surface to a dark, crystalline 

 centre. The dark centre is comparatively 

 unoxidised, whereas the outer layers of the 

 stone have been in contact with the oxygen 

 of the air, which has entered into com- 

 bination with, amongst other things, the 

 iron that is present in the mass. The 

 result of such combination is the formation 

 of rust, or of some other form of iron oxide, 

 whose friability greatly assists the conver- 

 sion of the stone into soil. 



One of the most powerful weathering agents 

 is carbonic acid gas. This substance acts 

 chiefly through the increased power of solu- 

 tion that it imparts to water. Whereas 

 pure water will dissolve most things, it 

 dissolves them in many cases very slowly, 

 but when water is charged with carbonic acid 

 gas its power of dissolving all bodies, in- 

 cluding the constituents of rocks, is greatly 

 increased. Compared with oxygen or 

 nitrogen, carbonic acid gas is extremely sol- 

 uble in water, that is to say, a given volume 



