16 THE PRINCIPLES OF SOIL MANAGEMENT 



atmosphere to the sulfate by the direct union of oxygen 

 with the compound. Whether the process be an addition 

 or subtraction of material, it usually changes the stabil- 

 ity of the mineral, and perhaps the stability of the mass 

 of which the mineral is a part. The chemical action of 

 one agency often opens the way for the chemical and 

 mechanical action of other agencies, so that the decay 

 processes are hastened. This chemical breaking down 

 of minerals, and thereby of rock masses, is termed 

 decomposition. The mechanical breaking up of rocks 

 whereby only the state of division of the material is 

 changed is termed disintegration. The breaking up of 

 rocks due to expanison of heat, the freezing of water, 

 flowing of water, the grinding of glacial ice, and the 

 expansion of plant roots, are types of disintegration by 

 which the rock is simply reduced to a finer state of 

 division. The general tendency is for finer material to 

 result from decomposition than from simple disin- 

 tegration. 



6. Atmosphere. The atmosphere is composed of a 

 mixture of the gases nitrogen and oxygen, in the propor- 

 tion of four parts of the former to one part of the latter, 

 together with very minute quantities of carbon dioxide, 

 nitric oxide, ammonia, and, in even less amounts, other 

 volatile compounds, and a variable, but usually very 

 considerable amount of water-vapor, evidenced by 

 clouds, rain, snow, dew, etc. These gases, dissolved in 

 the atmospheric moisture, come in contact with rock 

 masses and change certain of its minerals into com- 

 pounds more or less soluble than they were originally. 

 The iron compounds are perhaps the most affected, 



