138 The Great World's Farm 



hand continue to mount upwards instead of falling to the 

 ground, but this is precisely what carbon dioxide does, 

 and we can but state the fact without explaining it. 

 Gases, no matter what their weight, are obliged to mix 

 one with the other. 



Put into a bottle first some heavy carbon dioxide, then 

 some oxygen, which is lighter, nitrogen, which is lighter 

 still, and lastly, hydrogen, the lightest of all, which is so 

 light that it has to be poured upwards, and though at 

 first the heaviest gas will be at the bottom, before long 

 all will be perfectly mixed, and there will be as much 

 hydrogen at the bottom as at the top. Carbon dioxide 

 moves more slowly than hydrogen, owing to its weight, 

 but move upwards it will, and that without any shaking. 



All, or part, of the carbon dioxide might, however, be 

 removed from this mixture without affecting the other 

 gases, if a piece of caustic potash were introduced; for 

 this substance has the power of attracting and absorbing 

 this particular gas. Each of the other gases might also 

 be removed by similar means, one by one substance, and 

 another by another. 



Leaves, then, act upon carbon dioxide in some such 

 way as caustic potash does. They attract it to them- 

 selves and absorb it; but by so doing, they are constantly 

 diminishing the amount of the gas in the air immediately 

 surrounding them; and as, according to the law of their 

 being, gases must mix equally one with the other, more 

 carbon dioxide flows in to supply the place of that which 

 is absorbed. Streams of the gas are therefore constantly 

 flowing towards each leaf, even when the air is still; when 

 there is wind the whole air is, of course, in motion. 



We have now to see what becomes of the carbon diox- 



