AGENCY OF OXYGEN. 79 



the effect of absorbing oxygen, and giving out carbonic acid. 

 When the fruit decays, it gives out large quantities of car- 

 bonic acid. The carbon is furnished by the substance of the 

 fruit, the oxygen from the decomposition of water ; the same 

 changes which take place in the decay of woody fibre, or any 

 other vegetable body. 



From this view it appears, that the principal agency of the 

 oxygen of the air in the process of nutrition is, 



1. According to the views of Thompson and ether chemists, 

 to combine with carbon, and form carbonic acid. This change 

 takes place both in the soil, and in the living plant. In the 

 soil, mostly by the fermentation of manures, or vegetable sub- 

 stances ; and in the tree, by uniting with the carbon which 

 has been previously introduced, forming carbonic acid ; this 

 with that contained in the soil and air, and which enters the 

 vegetable organs \u solution, is conveyed to the leaves, and 

 decomposed by the influence of light ; the carbon being re- 

 tained or assimilated, and the oxygen sent out to combine 

 with fresh portions of carbon, ready again to pass through the 

 same process. 



The oxygen which is absorbed by the leaves and roots, is, 

 for the m.ost part, transpired into the atmosphere ; but a part 

 is retained, and aids still farther, by its various combinations, 

 the growth and perfection of the plant. Or, 



2. According to Liebig, the oxygen of the air combines 

 with the vegetable products, by Rpurelt/ chemical process, and 

 aids the plant in the formation of several vegetable bodies, 

 while the oxygen which plants emit is derived from water 

 and carbonic acid, which are decomposed in the process of 

 assimilation. This is the more probable theory. But what- 

 ever view we take of it, whether the oxygen is derived from 

 the air, or the water ; whether it combines in the vegetable 

 organs with carbon, or is directly assimilated, it appears to be 

 an indispensable, but subordinate agent, to the vital power, 

 forming, by its combinations, those compounds which this 



