AIR 



should have been used up long ago ! The car- 

 bonic acid gas is largely made up of oxygen, 

 so is the water-vapour ; every breath we take 

 changes a little oxygen into carbonic acid gas 

 and water ; in every fire that burns, coal, which 

 contains a great deal of carbon, is taking oxygen 

 out of the air, and burning it into carbonic acid 

 gas, warming us as it does so. We can see 

 this in a very simple way. If we light a candle, 

 stand it up in a saucer of water, and then put 

 over it a bottle or glass jar which touches the 

 water on all sides, so that no more air can get 

 in, the candle will go out after a short while 

 because the flame has used up all the oxygen 

 in the air inside, and cannot burn any longer 

 without a fresh supply. Somewhere, clearly 

 then, something must be dividing up the car- 

 bonic acid gas, and sending back the oxygen 

 into the air to be used again. And we may 

 also wonder what happens to this carbon, and 

 what sort of stuff it is. Now carbon exists in 

 many forms and shapes. There is a great deal 

 of it in coal, as we shall see later on. Diamonds 

 are almost pure carbon, and we sometimes hear 

 people speak of coal as " black diamonds." The 

 fat and flesh of animals and people is largely 

 made up of carbon, and, most important of all 

 for us, trees and plants and vegetables require 

 great quantities of carbon. 



We said above that the trees and other 

 plants breathed somewhat like animals, at any 



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