152 ELEMENTARY SCIENCE 



true that all the rest of the surrounding air is likewise 

 filled with these tiny particles. (These particles are very 

 much larger than molecules, yet their behavior as you see 

 them revealed in a bar of light suggests the behavior of 

 the molecules of a gas as you have learned of it in study- 

 ing about the kinetic theory.) The reason you can see 

 the particles of dust in a sunbeam is that they reflect the 

 light that strikes upon them; it is the dust in the air that 

 diffuses or scatters the light of the sun so that there is light 

 in shady places. Think how important this is. If it 

 were not for the dust in the air, all shady places would be 

 in darkness. The colors of the sky, of the sunset, and of 

 the sunrise are all produced by the effects of the particles 

 of dust and water in the air upon the light which strikes 

 them. Also, when water vapor condenses into rain-drops, 

 these dust particles act as points about which the water 

 first condenses. Have you ever noticed the dust specks 

 in a rain-drop even when it first strikes against a clean 

 pane of glass? 



Relations of Air to Life. The importance of nitrogen 

 to life you have already learned. The importance of oxy- 

 gen is familiar to us in connection with the process of 

 breathing. You know that in the air which we exhale 

 from our lungs the oxygen has been largely replaced by 

 carbon dioxide, oxygen being used in our bodies in connec- 

 tion with that essential life process called respiration. It 

 is similarly used by all plants * and animals as weU as 

 by ourselves. 



This raises an interesting question as to how the supply 

 of oxygen which is constantly being used is constantly 



* With the exception of certain bacteria. 



