CHAPTER XII 

 CONSTITUTION OF THE ATMOSPHERE 



Principal constituents. The composition of the atmosphere is 

 nearly the same at all times and at all places where it has been 

 analyzed. It is made up chiefly of two gases, (1) nitrogen, which 

 makes up nearly 79 per cent of dry air, and (2) oxygen, which makes 

 nearly 21 per cent. 



Minor constituents. Beside these two gases, the proportions 

 of which do not vary much, there are several lesser constituents. 

 The most important are (1) carbon dioxide and (2) water vapor. 

 The former makes up about -nHhnr ^Y weight of the whole atmos- 

 phere, and its amount is nearly constant from day to day and 

 from year to year. Water vapor is water in particles so small as to 

 be invisible. The total amount in the atmosphere is not known to 

 vary much, but the amount varies greatly from place to place, and 

 it varies much from time to time in the same place. Since this is 

 so, and since water frequently comes out of the atmosphere in the 

 form of rain, snow, etc., it is regarded by some as something in the 

 air, rather than as a part of the air. 



Impurities. The air always contains some gases which must 

 be looked upon as impurities, though they are not necessarily 

 harmful to life. Some such gases arise from the burning and 

 decay of organic matter, others from chemical processes used in 

 manufacturing, and still others from volcanic and other vents in 

 the earth's crust. The total amount of gas which enters the 

 atmosphere in this way is small, but in some places, as about some 

 vents, the gases are so abundant as to be injurious to life. This 

 is the case in a valley in the Yellowstone Park, where animals 

 straying into certain spots are in some cases killed by the gases. 

 The air always contains numerous solid particles called dust. 

 Though important, the dust in the air is an impurity rather than 

 a constituent. 



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