Leaves and their Work 165 



districts, its weight keeps it down for a time near the 

 ground, but gradually, in obedience to a mysterious law, 

 it rises and spreads through the air. Its weight draws 

 it down to the earth, or, more correctly speaking, the 

 earth attracts it to itself more than it attracts either 

 oxygen or nitrogen. It is heavy, because the earth 

 attracts it, just as a stone is heavier than a feather. 

 But it rises. 



We should be surprised to see a stone thrown from 

 our 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 intro- 

 duced ; 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- 



