THE AIR WE BREATHE. 
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burn, and when it passes through our lungs into 
the blood, it slowly ‘ burns ’ or changes the sub- 
stances that are there. And these changes are so 
necessary that, if we are unable to keep up a 
proper supply of oxygen to cause them to take 
place, we become ill. 
5. Nitrogen, like oxygen, cannot be seen, tasted, 
or smelt. It will neither burn, nor cause other 
The Effects of Oxygen and Nitrogen on a Lighted Match. 
things to burn. But it is well that the air contains 
a large proportion of nitrogen, because we could 
not live long if we breathed pure oxygen. When 
we breathe we take into our lungs only about one 
share of oxygen to four of nitrogen. 
6. All animals in breathing, all fires in burning, 
and all refuse in decaying, are continually adding 
carbonic-acid gas to the air. How is it, then, that 
there is not much more of this gas in the air than 
there was ten or a hundred years ago? 
7 . We have only to turn to the plants for an 
