COMPOSITION OF AIR ATOMIC THEORY 73 



Sometimes it occurs in wells, cisterns, sewers, and in mines, 

 where it is called choke damp. One may drown in it as 

 truly as in water, since it fills the lungs but does not supply 

 any free oxygen for respiration. There is a very small amount 

 always present in the air, and this will be discussed later. 



76. Gases of the air. We have already found that the 

 air consists of several gases. These are merely mixed ; they 

 do not form a compound. The most striking one of those 

 that we have been studying is oxygen. It constitutes about 

 one fifth of the volume of the air. Of carbon dioxide there 

 are only about 3 parts in 10,000. 



There is yet another gas present in large quantities in the 

 air. It is called nitrogen. We shall have more to say about 

 it in connection with the food of plants and animals. Several 

 other gases have lately been discovered. Dust particles also 

 are always present, and some of these are of great importance. 

 Below is a table showing the gases which, in addition to 

 water vapor, are commonly found in the air, and the average 

 amount of each, given in percentage. The exact percentage 

 eannot be given because the amounts vary slightly from time 

 to time. 



COMPOSITION OF AIR 

 (Approximately) 



Nitrogen 78 per cent 



Oxygen 21 per cent 



Argon 1 per cent 



Carbon dioxide T ^ of 1 per cent 



77. Abundance of oxygen. Oxygen is the most abundant 

 simple substance upon the earth. In an uncombined state 

 it makes up about one fifth of the air, as noted above. It is 

 found in combination with almost every known simple sub- 

 stance, forming eight ninths (by weight) of water and about 

 one half of the rocks of the earth's crust It is estimated 

 that about 50 per cent of the earth is oxygen, and its im- 

 portance in our lives could scarcely be overestimated. 



