94 



HUTCHINSON'S POPULAR BOTANY 



FIG. 127. THIRD YEAR OF NOR- 

 FOLK OR FOUR-COURSE ROTATION 

 OF CROPS. 



FIG. 128. FOURTH YEAR OF NOR- 

 FOLK OR FOUR-COURSE ROTATION 

 OF CROPS. 



most abundant. All the plants growing 

 upon the face of the earth absorb it in 

 large quantities. Their leaves take up the 

 carbon from the atmosphere in the form 

 of carbonic acid, and they grow and prosper. 

 Give them air purified from carbon, such as 

 we could thrive in, and they could not live ; 

 give them carbon dioxide with other matters, 

 and they nourish. Our floors, our tables, the 

 framework of the chairs on which we sit. 

 have derived all their carbon, as the trees 

 and plants derive theirs, from the atmo- 

 sphere, which carries away what is bad for 

 us * and at the same time good for them 

 what is disease to the one being health 

 to the other. " So are we made depen- 

 dent," says Faraday, " not merely upon our 

 fellow creatures, but upon our fellow existers, 

 all Nature being tied together by the laws 

 that make one part conduce to the good of 

 another." f 



Carbonic acid, or carbon dioxide as it is 

 now generally called, is present in the atmo- 

 sphere in the proportion of four parts in 

 ten thousand ; so that, in every thousand 

 cubic feet of air, we have not quite half a 

 cubic foot of carbonic acid a proportion 

 somewhat startling when we remember that 

 this is almost the sole source of supply to 

 the entire vegetable kingdom; yet so great 

 is the volume of atmosphere which sur- 

 rounds the globe that, according to careful 

 computations, at least three thousand mil- 

 lion million pounds of solid carbon must 

 be contained in it a quantity which is 

 probably far in excess of the weight of all 



* Perhaps we should be more exact in saying that 

 it is the absence of oxygen, rather than the presence of 

 CO., which vitiates the air from the animal point of 

 view. 



+ Some interesting experiments by Professor T. D. 

 Macdougal, of Minnesota, U.S.A., on the growth of 

 various plants in an atmosphere devoid of CO->, will 

 be found in the Journal of the Linnean Society 

 (Botany), vol. xxxi. 1896. 



