53 



Rain brings from the atmosphere into the soil two agents, 

 however, which do markedly affect the solubility of the soil min- 

 erals, namely, oxygen and carbon dioxide. The atmosphere 

 within the soil contains normally a somewhat smaller proportion 

 of oxygen than does the air above the soil. Rain in falling 

 through the air absorbs or dissolves relatively more oxygen than 

 nitrogen. Therefore when the rain water has penetrated the 

 soil to any considerable depth there should be, and probably is, 

 a liberation of dissolved oxygen into the atmosphere of the soil 

 interstices. This dissolved oxygen in becoming liberated or when 

 dissolved in the film water appears to be especially active to- 

 wards the ferrous or ferro-magnesian silicates. These minerals 

 are, moreover, as a class probably the most soluble of the rock- 

 forming silicates. Consequently oxygen brought into the soil 

 in this manner is one of the most important agencies in breaking 

 down and decomposing such minerals as the amphiboles, pyrox- 

 enes, chlorites, certain serpentines, phlogopites and biotites ; at the 

 same time there is formed ferric oxide (more or less hydrated) 

 and silica (probably as quartz) and magnesium, potassium, cal- 

 cium or sodium pass into solution, probably as bicarbonates. 

 That the concentration of the soil moisture may thus be made 

 temporarily abnormal is not impossible, though scarcely prob- 

 able. 



The soil atmosphere has normally a decidedly higher content 

 of carbon dioxide than the atmosphere above the soil. Conse- 

 quently the soil water is always more or less "charged" with 

 carbon dioxide, and the presence of the carbon dioxide decidedly 

 augments the solvent powers of the water towards a great many 

 and different kinds of rock-forming or soil minerals. 1 



What the mechanism of the reaction may be is far from clear. 



1 For references to the literature see Bull. No. 30, Bureau of Soils, 

 U. S. Dept. of Agriculture; also, The action of carbon dioxide under 

 pressure upon a few metal hydroxides at o C., by F. K. Cameron and 

 W. O. Robinson, Jour. phys. chem., 12, 561-573 (1908); The influence 

 of colloids and fine suspensions on the solubility of gases in water, 

 Part I. Solubility of carbon dioxide and nitrous oxide, by Alexander 

 Findlay and Henry Jermain Maude Creighton, Trans. Chem. Soc., 97, 

 536-561 (1910). 



