THE ATMOSPHERE 151 



Composition of the Air. You have learned that air is 

 made up, by weight, of four-fifths of nitrogen and one- 

 fifth of oxygen. These proportions, it should be noted, 

 refer to dry air; the figures usually given are seventy-eight 

 per cent nitrogen and twenty-one per cent oxygen. This 

 leaves, you see, one per cent which is made up of other 

 gases. Of these other gases, carbon dioxide is the most 

 important to life, but it forms, by weight, only three-ten- 

 thousandths of the whole atmosphere. Other gases also 

 exist in the atmosphere in small quantities. 



Water vapor is a gas whose presence in air is very im- 

 portant to life, but, as you have noted, the amount of this 

 gas varies so much from time to time and from place to 

 place that it is usually regarded as something in the air, 

 rather than as a part of the air itself, and it is dry air 

 only which is considered when we discuss its constitution. 



Air near the earth also always contains more or less of 

 dust, and of organic matter in the form of bacteria and 

 spores, and the presence of these in air also has im- 

 portant effects on life. Air is a medium by means of 

 which both beneficial and harmful bacteria are spread. 

 By means of it the spores of fungi and of other plants 

 are carried about. A dust-like kind of spore called 

 pollen is produced by flowers, and at certain seasons there 

 is much of this in the air. This air-transport of pollen is 

 of great importance to us in that by means of it the pollen 

 of our most important crop plants is conveyed from flower 

 to flower, a thing which is important in connection with 

 the production of seeds. 



You have seen the "motes" of dust that are visible when 

 you look through a sunbeam. You have seen them danc- 

 ing in it, and perhaps you have wondered if it could be 



