16 OF THE ASSIMILATION OF CARBON. 



and as the soil does not yield it, it can only be 

 extracted from the atmosphere. 



In attempting to explain the origin of carbon in 

 plants, it has never been considered that the ques- 

 tion is intimately connected with that of the origin 

 of humus. It is universally admitted that humus 

 arises from the decay of plants. No primitive 

 humus, therefore, can have existed ; for plants 

 must have preceded 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 involve the consideration of 

 two most remarkable natural phenomena, which, 

 by their reciprocal and uninterrupted influence, 

 maintain the life of the individual animals and 

 vegetables, and the continued existence of both 

 kingdoms of organic nature. 



One of these questions is connected with the in- 

 variable condition of the air with respect to oxygen. 

 One hundred volumes of air have been found, at 

 every period and in every climate, to contain 21 

 volumes of oxygen, with such small deviations, that 

 they must be ascribed to errors of observation. 



Although the absolute quantity of oxygen con- 

 tained in the atmosphere appears very great when 

 represented by numbers, yet it is not inexhaustible. 

 One man consumes by respiration 45* Hessian cubic 



* [For the proportions in English weights and measures see the table 

 at the end of the volume ] 



