ATTENDED WITH EVOLUTION OF OXYGEN. 39 



water, when brought into contact, act upon one another, and 

 hydrogen is SEPARATED, while a white pulverulent 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 oi' carbonic acid and the hydrogen of 

 water, whilst oxygen is separated. 



Decay has been described above as the great operation of na- 

 ture, 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 decomposition of water, 

 'he oxygen of which is set free, together with a part or all of 

 that contained in the carbonic acid. In the process of putrefac- 

 tion, a quantity of water, exactly corresponding to that of the 

 hydrogen, is again formed by extraction of oxygen from the air ; 

 while all the oxygen of the organic matter is returned to the 

 atmosphere in the i^rm of carbonic acid. Vegetable matters 

 can emit carbonic acid, during their decay, only in proportion 

 to the quantity of oxygen which they contain ; acids, therefore, 

 yield more carbonic acid than neutral compounds ; while fatty 

 acids, resin, and wax, do not putrefy ; t ley remain in the soil 

 without any apparent change. 



