1 68 THE WORLDS LUMBER ROOM. 



contains the larger proportion of land ; and as the Old World 

 is more densely populated than the New, more carbonic acid 

 must be produced in the East than in the West, in the North 

 than in the South. 



Yet the air all over the world, in the plains and on the 

 mountains, is very nearly alike, for gases possess the peculiar 

 property of intermingling or " diffusing " themselves equally 

 through one another without any reference to their com- 

 parative weights. In this they are very different from 

 liquids, such, for instance, as oil and water ; for the oil will 

 always rise to the top even if put in at the bottom of a 

 bottle. But if you put in first carbonic acid, which is the 

 heaviest gas, then oxygen, and finally hydrogen, which is the 

 lightest of all, in a little while, without any shaking, they 

 will have thoroughly mixed with one another.* 



Carbonic acid is forty-four times as heavy as hydro- 

 gen, and 1*529 heavier than air, so that but for this law 

 of " diffusion " it would rest upon the earth ; and if all 

 the carbonic acid breathed out by four million Londoners 

 were thus to settle down, they would soon be completely 

 enveloped in it and suffocated. As it is, however, except in 

 particular localities, the amount of carbonic acid present in 

 the atmosphere is nearly everywhere the same, and though 

 it forms but a small proportion of the whole, its entire weight 

 amounts to more than three billion tons. 



Though constantly receiving additions, the quantity does 

 not increase, thanks to the innumerable scavengers always at 

 work removing it. Each blade of grass in the meadow, 

 every leaf in the field, wood, or forest is busy during every 



* Hydrogen is 14^ times lighter than air. 



