THE ATMOSPHERE. 9 



The action of the air in the soil is chiefly two-fold ; it causes 

 the evaporation of moisture, and the decomposition of organic 

 matter. The air which penetrates into the fissures and inter- 

 stices of the soil becomes laden with vapour and carbon 

 dioxide ; it is then forced out of the soil b}' every rise of 

 temperature, and replaced bj^ fresh air during cooling. The 

 extent of this change of air depends on the degree of porosity 

 of the soil and the daily range of temperature ; the greater 

 these are, the more rapidly will moisture and organic matter 

 (humus) disappear. The daily range of temperature is 

 seriously affected by the degree of protection which the soil 

 receives from forest vegetation ; it is greatest in fully exposed 

 soils and smallest in soils under the shelter of a crowded 

 crop of trees, especially if the foliage offers lateral as well as 

 vertical shelter against sun and air currents. In the latter 

 case the humus is generally carefully preserved, in the former 

 it disappears rapidly. 



2. Carhon Dioxide. 



About 0*0004 parts of the volume of the atmosphere 

 consist of carbon dioxide, which is received from a variety 

 of sources, as combustion or decomposition of plants, the 

 breathing of animals, volcanoes, spring water issuing from the 

 interior of the earth, combustion of coal and lignite, from 

 various minerals, as for instance calcium carbonate. Of these, 

 the first is by far the most important source of supply. 



Plants, except certain parasites and saprophytes, take the 

 carbon dioxide which they require through their leaves from 

 the atmosphere. Subsequently, when they die and are decom- 

 posed, their carbon is converted back into carbon dioxide, and 

 returned to the atmosphere ; hence plants form an important 

 link in the movement of carbon dioxide. 



3. Moisture. 

 The atmosphere is the medium through which the dry 

 land receives the greater part of the necessary moisture. 



