49 



equal thereto (33.) is formed ; and if it be admit- 

 ted, that none of this gas is received into the vessels 

 of the plant (35, 6, 7.)> we must embrace the belief, 

 not only that the acid is formed out of the oxygen 

 gas originally present in the air, but that it is not 

 emitted ready formed by the plant. If the acid 

 were formed independent of this oxygen gas, the 

 whole bulk of air ought to be increased, instead of 

 being diminished, by vegetation : if it be not so 

 formed, then as no oxygen gas is received into the 

 plant, no carbonic acid can be emitted from it. 

 Plants also, in nitrogen gas, do not vegetate, and 

 cannot, therefore, by a living process, form carbonic 

 acid ; and when, in atmospheric air or in oxygen 

 gas, this acid is formed by vegetation, that gas disap- 

 pears, and the plant for a time lives : whence it fol- 

 lows, that without the presence of oxygen gas, the 

 living plant is unable to form carbonic acid, and 

 that this acid, therefore, is formed by the reciprocal 

 changes going on between this oxygen gas and the 

 plant. If we do not, indeed, admit the conversion 

 of oxygen gas into carbonic acid, there is no way 

 in which its loss can be accounted for, since there 

 is no other new product formed, and none of the 

 gas, as we maintain, enters into the vascular system, 

 and combines with the fluids of the plant. In all 

 the foregoing experiments also, the deterioration of 

 the air increased in proportion to the continuance of 

 the vegetative process, until the whole of its oxygen 

 gas completely disappeared ; and the gradual rising 

 of the mercury into the jar (32.), indicated a cor- 

 responding production of carbonic acid, until the 

 bulk of the latter nearly equalled that of the former ; 



D 



