CHAPTER IV 

 THE AIR WE BREATHE 



We are living at the bottom of a great invisible ocean 

 an ocean of air. We can often feel the movements of its 

 currents as they sweep about us. At times, its wind-waves 

 are even more violent than the storm billows of the sea. 

 In this air ocean, birds dart about, just as fish swim about 

 in the water. Great airships sail through it. This air ocean 

 is valuable to us in many ways, but most important of 

 all it makes life itself possible. Without air all forms of 

 animal and vegetable life would quickly die. The earth 

 would become a barren waste, just another dead moon in 

 the heavens. 



Air is a mixture of several gases. It is one of the most com- 

 mon forms of matter. It occupies a vast amount of space, ex- 

 tending upward several miles above all parts of the earth. It is 

 commonly spoken of as the atmosphere. Air is also found in 

 water and in soil. 



Experiments to Show the Presence of Air in Water and 

 in Soil. To show that air is found dissolved in water, it is only 

 necessary to heat water gradually in a beaker or other dish. Long 

 before the water becomes hot enough to boil, small bubbles may be 

 seen coming from the bottom and breaking at the surface. These 

 bubbles are a"ir escaping from the water. If heated long enough 

 nearly all the air escapes. Perhaps you have noticed that boiled 

 water is tasteless. This is largely because of the absence of dis- 

 solved air. 



To show that air is found in soil, pour water on garden soil 

 in a pan until the soil is soaked and a thin layer of water lies on 

 top of it. Bubbles will be seen rising from the soil. They show 

 that air is escaping; so we may conclude that air is found in 

 soil. 



Composition of Air. Air is composed, or made up, largely 

 of oxygen and nitrogen, with a small amount of carbon dioxide, 

 water vapor and gases called argon, neon and krypton. It is com- 



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