112 GENERAL SCIENCE 



The atmosphere is washed free of various gases and of 

 suspended solid matter by the rains, and kept constantly 

 agitated and uniformly mixed by diffusion and by air cur- 

 rents. Sunlight kills many bacteria floating in it, and oxida- 

 tion disposes of much matter in the air of plant and animal 

 origin. Growing plants in sunlight are all the time with- 

 drawing carbon dioxide from the air and returning ap- 

 proximately the same volume of oxygen gas to it. Air has 

 weight just as does all other matter whether solid, liquid, or 

 gaseous. Although this weight varies under different con- 

 ditions of temperature and pressure, it is approximately one 

 and one-fourth ounces per cubic foot. 



The lower layers of the atmosphere at all times contain 

 more or less of water particles as clouds, and water in the 

 form of vapor, and varying quantities of dust and smoke 

 particles. Of the heat received from the sun it is calculated 

 that 60 per cent does not get through to the earth's surface, 

 being arrested in the atmosphere. However, that which 

 does get through to the land and water surfaces is hindered 

 even more by this same blanket of the lower atmosphere 

 from escaping into space by radiation. A frost is much less 

 likely when the sky is beclouded, or the air is filled with 

 smoke. 



About 78 per cent of the air is nitrogen gas. This nitrogen 

 is of course breathed in and out of the lungs along with the 

 oxygen. It is non-poisonous and harmless, and it does not 

 support life. It is characterized by its negative qualities, 

 and is tasteless, colorless, odorless, and chemically inert 

 (inactive). It so dilutes the oxygen of the air that the 

 oxidation in the body is kept within bounds. If the air con- 

 tained a larger per cent of oxygen, fires once started might 

 easily become uncontrollable. 



Compounds of nitrogen in the soil in soluble form are 

 indispensable in plant growth, and in this way contribute 



