ATMOSPHERE 123 



the process of oxidation, is used to warm houses, to 

 produce steam, to run trains, to drive machinery, and 

 in many other ways. 



The readiness with which oxygen unites with most 

 other elements, makes it an important agent in the dis- 

 integration of rocks and minerals, and the decomposi- 

 tion of dead animal and plant matter. 



In spite of the fact that oxygen is being consumed 

 all the time, its amount does not appear to grow less. 

 Evidently to keep this equilibrium, it must be supplied 

 as fast as it is consumed. This is true, and the principal 

 source of this supply, as we shall see later, is green 

 plants. 



Carbon dioxide, though present only in small amounts 

 in the air, is extremely important. It is produced con- 

 stantly by the burning of fuel, by the decay of organic 

 matter, and by animal and plant respiration. From 

 these various sources it is supplied constantly to the 

 atmosphere. It has been estimated that carbon dioxide 

 is being supplied to the atmosphere at the rate of about 

 75 tons per second. 



Because carbon dioxide is formed as a result of com- 

 bustion and respiration, the amount of it in the air in 

 cities is greater than in the open country. 



Carbon dioxide is supplied very rapidly to the air, 

 yet the amount of it remains about the same. We infer, 

 therefore, that in some way this gas is being removed, 

 or taken out of the air, about as rapidly as it is formed. 

 Again we must look to the plant for the explanation. 



