68 ASSIMILATION OF HYDROGEN. 



what form the production of these constituents 

 takes place ; in this respect, the representation of 

 their formation which we have given must not 

 be received in an absolute sense, it being intended 

 only to render the nature of the process more 

 capable of apprehension ; but it must not be for- 

 gotten, that if the conversion of tartaric acid into 

 sugar, in grapes, be considered as a fact, it must 

 take place under all circumstances in the same 

 proportions. 



The vital process in plants is, with reference to 

 the point we have been considering, the very 

 reverse of the chemical processes engaged in the 

 formation of salts. Carbonic acid, zinc, and water, 

 when brought into contact, act upon one another, 

 and hydrogen is separated, while a white pulveru- 

 lent compound is formed, which contains carbonic 

 acid, zinc, and the oxygen of the water. A living 

 plant represents the zinc in this process : but the 

 process of assimilation gives rise to compounds, 

 which contain the elements of carbonic acid and 

 the hydrogen of water, whilst oxygen is separated. 



Decay has been described above as the great 

 operation of nature, by which that oxygen, which 

 was assimilated by plants during life, is again 

 returned to the atmosphere. During the progress 

 of growth, plants appropriate carbon in the form of 

 carbonic acid, and hydrogen from the decomposi- 

 tion of water, the oxygen of which is set free, 

 together with a part or all of that contained in the 



