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CHAPTER II. 

 THE ATMOSPHERE. 



ALMOST all the vital phenomena important from an agricul- 

 tural aspect occur in contact with and largely hy the action of 

 the air around. Moreover, air plays an important part in the 

 formation of soils and profoundly affects the temperature and 

 climate of the earth. It is therefore obvious that a knowledge 

 of the composition and properties of the atmosphere is abso- 

 lutely essential in order that the chemistry of the processes 

 involved in the life of animals and plants may be clearly 

 understood. 



The reader w r ill already possess some acquaintance with the 

 chemical nature of atmospheric air and with the properties of 

 its chief constituents. 



In this chapter, therefore, is given only a short summary of 

 what is known of the extent, variations in composition, and 

 functions of the constituents of air. 



With reference to the extent, it is found that air exerts an 

 average pressure of about 14-75H). per sq. in. (1033 grammes 

 per sq. cm.) at the sea-level. This pressure is due to the 

 earth's attraction for the air above and is, of course, a direct 

 measure of the weight of the atmosphere. There rests, there- 

 fore, on every square foot of surface 14-75 x 1441b. of air; or 

 upon an acre the total weight of air would be 41,300 tons. 

 Allowing for the space occupied by land above the sea-level, 

 Herschel has calculated that the mass of the atmosphere is 

 about Y^OOO f ^ na ^ f the earth. 



Although it is possible, as shown above, to estimate fairly 

 accurately the weight of the atmosphere, there is very little 

 information available from which the height to which the 

 atmosphere extends can be calculated. 



It can easily be shown that if the atmosphere were homo- 

 geneous its height would be between 5 and 6 miles ; but, as 

 is well known, the density rapidly diminishes with the height 



