Leaves and Their Work 137 



suffocate us. What, then, becomes of it ? Roughly 

 speaking, we may say that the carbon dioxide produced 

 during the winter of the north goes to feed the vegetation 

 of the south — the thistles, clover, and grass, for instance, 

 of the pampas, which are flourishing in all their luxuri- 

 ance while winter prevails with us. And it goes, to some 

 extent, at least, because the leaves of the southern hemi- 

 sphere draw it thither. 



The ocean of air which surrounds the world is not, it 

 must be remembered, a compound, but a mixture. If we 

 could see it we should find oxygen, nitrogen, carbon 

 dioxide, ammonia, all perfectly mixed, but perfectly 

 distinct. The combination of two gases, oxygen and 

 hydrogen, makes water — a liquid entirely different from 

 both; but there is no such combination and alteration in 

 the gases of the air. Each keeps its own character; but 

 though all are of different weights, they are so thoroughly 

 and perfectly mixed, that except under special circum- 

 stances, there is but little appreciable difference in the air 

 of different parts of the world. 



Carbon dioxide is the heaviest of these gases, and it is 

 more than twice the weight of the mixture of these gases 

 which we call the air. Where it is poured out from 

 cracks in the earth, as it is largely in some volcanic dis- 

 tricts, 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 



