ATMOSPHERIC AIR AS TUB FOOD OF PLANTS. 47 



oxygen in the free gaseous coiulition, 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 projjortion 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 car- 

 bonic acid was ^ of a cubic inch, that of the liberated 

 oxygen being the same. 



Supply of ( arboiiic Acid iu the Atmosphere. — Although 

 tills body forms but toooo 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 lbs.) of carbon, an 

 amount equivalent to 3^ tons of this gas. AV^eie 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 

 ease 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, ami 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 of carbonic acitl sufficient, with- 

 out renewal, for a hundred years of growth. This ingredi- 

 ent of the atmosphere is maintained in undiminished 

 quantity by the oxidation of carbon in the slow decay of 

 organic matters, in tlie combustion of fuel, and in animal 

 respiration. 



That the carbonic acid of the atmosphere may fully suf- 



