26 



Agricultural Chemistry. 



retards transpiration. By these influences over transpiration 

 the air exercises control over plant growth. 



As has been stated, the presence of water in the air increases 

 its capacity to absorb heat and when the air is cooled it loses its 

 power to retain moisture. Water then separates from it and 

 collects upon colder objects. This is the cause of the appearance 

 of drops of water on the outer surface of an ice- water pitcher on 

 a sultry day in summer. Dew is formed in the same manner. 

 After sunset on a warm summer's day the earth cools rapidly 

 by radiation and reaches a temperature below that of the adja- 



The Average Composition of Dry Air. 



cent air. At a temperature bearing a definite relation to the 

 moisture content of the air and known as "the dew point," 

 moisture leaves the air and collects upon the surface of vegeta- 

 tion and other cool objects. In rainless regions dew becomes an 

 important source of water for crops and frequent tilling must 

 be practiced to prevent its escape by evaporation from the sur- 

 face of the soil. 



Movements of the moisture laden air distribute rainfall over 

 the land ; and some of the less prominent constituents of the air 

 are washed to the soil by rain and become factors in the supply 

 of plant food. 



