142 BIOLOGY OF PLANTS. 



the quantity of carbon introduced by this means, must be 

 much less. 



4. Fertile land produces carbon in the form of wood, hay, 

 grain, and other kinds of growth, the bulks of which differ 

 in a remarkable degree, but the quantity of carbon yielded by 

 equal surfaces is quite constant. On the supposition that 

 the land is equally fertile, the w^eights of forest trees, hay, 

 beet root and rye, growing on an equal surface, are as 2650, 

 2500, 20,000, 2580 ; but when these several products are de- 

 composed, the actual quantity of carbon in each is about 100 

 parts; hence, the quantity of carbon is not affected by ma- 

 nure, as forest lands, meadows and cultivated fields yield the 

 same amount. These facts show that the humus of the soil 

 is not the source of the carbon* 



Whence then is it derived? It is universally admitted, 

 that humus arises from the decay of plants. No primitive 

 humus, therefore, can have existed, for plants must have pre- 

 jeeded the humus. 



Now, whence did the first vegetables derive their carbon] 

 and in what form is the carbon contained in the atmosphere ? 

 These two questions are easily answered, when we consider, 

 that the atmosphere contains carbon in the form of carbonic 

 acid, in nearly invariable proportions. The carbonic acid, 

 amounts,, constantly, according to Saussure, to 0.000415 of 

 its volume, or about y^V^y P^^^^ ^y weight, although several 

 causes are constantly tending to increase it. The respiration 

 of animals throws off immense quantities into the atmos- 

 phere. Great quantities are also evolved from volcanic dis- 

 tricts and from certain springs. It is liberated from limestone, 

 and other. carbonates, by chemical action; and, finally, the 

 process of combustion must very much increase the amount. 

 By this latter process, and by the respiration of animals, oxy- 

 gen is consumed ; but the atmosphere always contains the 

 same proportion of oxygen, why does not the oxygen dimin- 

 ish and the carbonic acid increase 1 simply because plants 



