NITROGEN. THE ATMOSPHERE 293 



20,000, in each cubic centimeter. Ordinary, pure air contains 

 only 4 to 5 micro-organisms per liter. Most of these bacteria 

 come from the drying of soil and the dispersion of the resulting 

 dust. 



If dust were not present, we should soon notice its absence. 

 There would be no clouds or rain. It appears that moisture 

 will not condense to fog or rain in air which has been filtered, 

 by being drawn through a wide tube containing a long (20 inches 

 or more) plug of cotton, and has so been freed from dust. The 

 particles act as nuclei, round which the liquid grows at the expense 

 of the vapor. In the absence of dust, the condensation 

 would occur directly upon the surfaces of plants, houses, and 

 animals. Thus, in a dust- 

 less atmosphere, an open 

 shed or shelter, or an um- 

 brella, would afford no pro- 

 tection whatever against a 

 wetting. 



The formation of fog from 

 ordinary air, and its non- 

 formation in filtered air are 



easily shown in a darkened u FlG 74 



room (Fig. 74). The flask 



contains water to saturate the air. When the tube leading to 

 the water pump is opened for an instant, the saturated air in 

 the flask expands and is cooled. In such circumstances, ordinary 

 air gives a fog, brilliantly illuminated by the beam of light, while 

 filtered air (dustless) gives none. 



Air a Mixture. The air does not contain in combined con- 

 dition the various substances we have named. Each of the 

 substances in air shows precisely the same properties which it 

 exhibits when free, separate, and pure. This behavior is charac- 

 teristic of a mixture. 



