24 Agricultural (Ih 



ply of carbon dioxide, a constituent of the air, which is the basal 

 material for all compounds made by the growing plant. The 

 burning of wood is a chemical process in which oxygen of the air 

 unites with the chemical constituents of the wood. If the fire be 

 banked or otherwise deprived of a liberal air supply, it smoulders. 

 "When air is liberally supplied, as through the stove drafts or 

 forge bellows, combustion and the resultant heat are greatly 

 increased, as a consequence of the increased supply of oxygen. 

 The formation of humus in the soil, the fermentation of manures, 

 and many other common phenomena of the farm, are in part pro- 

 cesses of oxidation or burning on a small scale, and are dependent 

 upon proper supplies of the oxygen of the air. 



Atmosphere controls rainfall. The atmosphere contains vary- 

 ing amounts of water. Warm air has great capacity for holding 

 water and may take up large amounts from the sea and inland 

 lakes. Movements of this water-laden air control rainfall. In 

 the case of the warm, moisture-laden winds moving eatward from 

 the Pacific ocean, the water is released when the air is cooled on 

 the snow clad summits of the Rocky Mountains. As a result, a 

 large area east of the mountains, known as the Great American 

 Desert, receives little or no rainfall and farmers are forced to 

 irrigate or practice dry farming on arable land of this region. 



Atmosphere controls temperature. Dry air transmits heat 

 readily from the sun to the earth or from the earth into space. 

 For this reason the temperature falls rapidly after sunset in dry 

 winter weather. Dry air also permits rapid evaporation of water 

 from the earth's surface with consequent cooling. Moist air, on 

 the other hand, prevents rapid evaporation from the earth's sur- 

 face, absorbs heat transmitted from the sun and radiated from 

 the earth, and thereby maintains higher temperatures. 



While the phenomena of temperature, moisture content, and 

 movement of the air do not directly involve chemical processes, 

 they have fundamental significance in the supplying of water and 

 the maintaining of temperatures which regulate the chemical pro- 

 cesses of plant growth. This significance has been a prominent 



