The Air We Breathe 43 



air into them under pressure or by pumping out the stale air so 

 that atmospheric pressure will cause fresh air to enter to take its 

 place. Mines are ventilated by air pressure. Air pressure also is 

 used in many ways in various industries. Moreover, it is the 

 natural pressure of the air that enables birds and other flying 

 creatures, as well as kites and airplanes, to remain up in the 

 air. 



SUMMARY 



Air is one of the most common forms of matter. It occupies 

 a great space about the surface of the earth. 



Air is composed largely of oxygen and nitrogen. Only one 

 per cent of other gases is found in it. 



Air possesses both chemical and physical properties and has 

 many uses. 



The most important property of the oxygen part of the air 

 is that it will support combustion. 



An important property of nitrogen is its inactivity. In the 

 air it serves to dilute the oxygen. It is especially useful in the 

 making of protein foods by plants. 



An important property of carbon dioxide is that it will not 

 burn or sustain burning. It is useful in putting out fires. It is 

 dangerous to life if present in large quantities. 



The amount of water vapor or moisture in the air varies at 

 different times. It is necessary to both plant and animal life. 



Air has an important relation to all life. Proper methods of 

 breathing are essential to good health. 



Other important properties of air are that it will flow and 

 exert pressure. It is a real substance and has weight. 



An instrument called a barometer is used to show the changes 

 of atmospheric pressure. 



The fact that air is elastic and can be compressed makes it 

 useful in setting brakes on trains, automobiles, and buses, in pumps, 

 in tunneling under streams and in many devices in factories. 



Air pressure enables birds and other flying creatures to re- 

 main up in the air, and makes possible the flying of kites and 

 airplanes. 



