ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 47 
_ oxygen in the free gaseous condition, while its carbon re- 
mains in the solid state as a constituent of the plant. Re- 
ferring to the table above, we see that twenty minutes’ 
exposure to the solar rays was sufficient in the second ex- 
periment (where the proportion of nitrogen remained 
nearly unaltered) to decompose 14 per cent of carbonic acid 
and liberate its oxygen. The total volume of air collected 
was 2.4 cubic inches, and the volume of decomposed ear- 
bonic acid was 4 of a cubic inch, that of the liberated 
oxygen being fhe. same, 
Supply of Carbonic Acid in the Atmosphere,— Although 
this body forms but j5¢50 of the weight of the atmosphere, 
yet such is the immense volume of the latter that it is cal- 
culated to contain, when taken to its entire height, no less 
than 3,400,000,000,000 tons of carbonic acid. This 
amounts to about 28 tons over every acre of the earth’s 
_ surface. 
According to Chevandier, an acre of beech-forest annu- 
ally assimilates about one ton (1950 Ibs.) of carbon, an 
amount equivalent to 34 tons of this gas. Were the whole 
- earth covered with this kind of forest, and did it depend 
solely upon the atmosphere for carbon, eight years must 
elapse before the existing supply would be exhausted, in 
case no means had been provided for restoring to the air 
what vegetation constantly removes. 
When we consider that but one-fourth of the earth’s 
surface is land, an:1 that on this the annual vegetable pro- 
duction is very far below (not one-third) the amount stat- 
ed above for thrifty forest, we.are warranted in assuming 
the atmospheric content ae carbonic acid sufficient, with. 
out renewal, fora hundred years of growth. This aoe 
ent of the atmosphere is maintained in undiminished 
quantity by the oxidation of carbon in the slow decay of 
organic matters, in the combustion of fuel, and in animal 
respiration. 
That the carbonic acid of the atmosphere may fully suf: 
