58 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Carbonic acid increases the solvent power of water. Rain water 

 contains from 5 to 10 parts (by volume) of carbon dioxide. River 

 and spring waters contain more, but most of it is in combination 

 with lime. The capillary water of soils containing much organic 

 matter, holds more carbonic acid in solution than river or spring 

 water. 



Water containing carbon dioxide is especially active in dissolv- 

 ing carbonate of lime or limestone, and removing it from the soil 



Fig. 12. Limestone cavern. 



or rock. Carbonate of lime is slightly soluble in water (20 parts 

 per million), but much more soluble in water containing carbon 

 dioxide, owing to the formation of calcium bicarbonate : 

 CaCO, + H, 2 O = Ca(HCO 3 ) 2 . 



Water saturated with carbon dioxide dissolves about 880 parts 

 per million. This solvent action has resulted in the formation 

 of large caverns in limestone regions. 



Oxygen is also dissolved in most natural waters, and acts upon 

 the ferrous or manganous compounds which occur in a great 

 number of minerals. When oxidized, these occupy a larger space 

 than before, and thus hasten the disintegration of the minerals 

 containing them. Some ferrous silicates are oxidized rapidly on 

 exposure to moist air, falling into a brown powder in a few weeks. 



