164 WRITINGS OP JOSEPH HENRY. [1855- 



by plants during the day, though this cannot be the prin- 

 cipal cause; a more efficient one is probably the varying 

 quantity of moisture, which may serve as a kind of vehicle 

 for its transportation to and from the ground. There is 

 also a great difference in the amount of carbonic acid in dif- 

 ferent places, perhaps in dififerent countries, and it is pos- 

 sible that a part of the variations of fertility, the other con- 

 ditions being the same, may in some cases be referred to this 

 cause. We find, from experiment, that vegetation is favored 

 by the increase of this ingredient until, according to Saus- 

 sure, we arrive at the proportion of eight parts to one hun- 

 dred, which is eighty times more than the ordinary quan- 

 tity existing in the atmosphere. The same portion would 

 entirely extinguish the life of the red-blooded air-breathing 

 animals. It is on this fact that some geologists have founded 

 the hypothesis that the luxuriant vegetation which existed 

 on the earth during the coal period was due to an atmos- 

 phere charged with carbonic acid, and the amphibious char- 

 acter of the animals existing at that period would seem to 

 favor this supposition. 



M. Chevandier has shown that one square mile of forest 

 land produces annually 441 tons of fixed carbon in the 

 wood, ( Comptes Rendus,) and Liebig increases the quantity 

 to as much as 504 tons to the square mile. The same author 

 also shows that all other vegetable productions yield nearly 

 the same quantity of carbon to the square mile. Now a prism 

 of air extending to the upper limits of the atmosphere, and 

 having a base of one square mile, contains 4,260 tons of car- 

 bon, from which it results that the annual consumption of 

 carbon by thrifty vegetation amounts to about one-ninth of 

 all the carbon of the atmosphere which rests upon it. (Gas- 

 parin; vol. ii.) 



From this it might at first sight appear that the carbonic 

 acid of the air ought rapidly to diminish, and in a few years 

 to be entirely exhausted ; but, as we have seen, the carbon 

 thus extracted is not lost to the air, but lent as it were to the 

 organized matter of the globe ; for by the process of combus- 

 tion and decay an equal amount of the same substance is 



