THE ATMOSPHERE 113 



directly to man's supply of foodstuffs. The supply of 

 nitrogen in the atmosphere is inexhaustible. None of man's 

 efforts, however, to form food materials for plants direct 

 from atmospheric nitrogen in large quantities and at rela- 

 tively low cost have been successful to the extent desired. 

 To maintain and to increase the content of the soil in its 

 nitrogen compounds suitable for plant growth is a problem 

 of science directly concerned with the feeding of the world's 

 population. 



SUMMARY 



The atmosphere as a mixture of gases has the properties common to 

 all matter in a gaseous form. Its composition is approximately 78 per 

 cent nitrogen, about 21 per cent oxygen, i per cent argon, and 21 oo 

 per cent carbon dioxide. Water vapor and dust are present in varying 

 proportions. 



It is believed that the atmosphere may extend outward from the 

 earth's surface two hundred miles more or less, but by reason of its 

 compressibility one-half of it is within four miles of sea-level, and 

 twenty-nine thirtieths of it within fifteen miles of the earth's surface. 

 These calculations are based upon readings of barometers that have 

 been carried upward to different altitudes, and upon calculations of 

 the height of meteors when first seen. 



So completely is the atmosphere mixed by its own diffusion and by 

 winds that, aside from its content of water and dust, very little varia- 

 tion in its composition is found at different places and at different 

 times. The withdrawal of oxygen from the air by animals, and the 

 increase in its per cent of carbon dioxide due to respiration and com- 

 bustion, is counterbalanced by the action of plants in abstracting 

 carbon dioxide from the air and setting oxygen free. 



Winds are named according to the directions from which they blow. 

 It is likely that the directions of ocean currents in some cases is largely 

 shaped by the direction of prevailing winds. 



Atmospheric conditions directly affect health. A cold damp air 

 chills the body surface causing, it may be, colds and other more serious 

 ailments. Very dry air is injurious to the mucous lining of the air- 

 passages, and by its irritating effects may upset the nervous equilibrium 

 of the body. The air-passages and lungs are not only irritated by dust 

 blown about, but disease germs become widely scattered by the air. 



