222 PHYSIOGRAPHY 



Relations of gases to one another. The gases of the air are 

 mixed with one another, and each of them retains its own quali- 

 ties in the mixture. The oxygen behaves as if no nitrogen were 

 present, and the nitrogen as if there were no oxygen. 



The Functions of the Atmospheric Elements 



The various constituents of the air serve different purposes in 

 the economy of the earth. 



Nitrogen is inactive. Though it enters the lungs with oxygen in 

 breathing, it does not appear to be of direct use to animals. Both 

 animals and plants need nitrogen, but few of them are able to use 

 the nitrogen of the air directly. It must first be combined with 

 something else, into nitrogenous compounds, from which animals 

 and plants may get the nitrogen they need. 



Oxygen from the air is being consumed all the time by all ani- 

 mals. Air-breathing animals take it from the air directly, and 

 water-breathing animals take it from the water in which it is dis- 

 solved. Oxygen is consumed by plants also, especially by green 

 plants, and it is used wherever combustion (burning) or decay is 

 going on, for combustion is primarily the union of oxygen with 

 other substances, and decay is very slow combustion. When 

 oxygen enters into combination it loses its distinctive qualities. 



In spite of the fact that oxygen is being consumed all the time, 

 its amount does not appear to grow less. We infer, therefore, that 

 it is supplied to the air about as fast as it is used up. The sources 

 of supply are several. Plants break up the carbon dioxide of the 

 air into carbon and oxygen, and set some of the oxygen free. This 

 is perhaps the greatest source of supply. Oxygen also reaches the 

 atmosphere from volcanic vents, and in other ways. 



The carbon dioxide (CO 2 ) of the atmosphere, though a very 

 small constituent so far as quantity is concerned, is most impor- 

 tant. It is being produced constantly by the burning of coal, wood, 

 oil, gas, etc., and by the decay of all organic matter. It is also 

 added to the air by animal respiration, and it issues from volcanic 

 vents, often in great quantities. 



From these various sources, carbon dioxide is supplied to the 

 atmosphere rapidly. Take, for example, that supplied by burning. 



