53 



for the additional remarks which the present occa- 

 sion enables us to offer. Passing over, then, for the 

 present, the anatomical and chemical difficulties 

 which beset this opinion, let us, for a moment, ad- 

 mit the capability of the leaves to absorb or attract 

 carbonic acid ; and then examine how far such a 

 supposition is consistent with reason and with fact. 

 That the same substance, carbonic acid, should, du- 

 ring the day, be absorbed by the leaf, and decompo- 

 sed within it as salutary; and, during the night, should 

 be formed within the same leaf, and emitted from it 

 as noxious *, seems to be not only inconsistent, but 

 absurd. Where would be the advantage in the 

 carbon of the acid being retained for twelve hours 

 as food, if, for the next twelve, it must again be gi- 

 ven out as excrementitious ? Or where is there an 

 instance, in the whole circle of existence, of a living 

 agent not only first forming its own food, but feeding 

 on its own excretions? If this carbon were, during the 

 day, retained as food, whence comes that composing 

 the acid which plants, when confined in a given bulk 

 of air (31.), are constantly forming ? If oxygen gas, 

 as these chemists suppose, be during the day con- 

 stantly emitted, why does that gas gradually disap- 

 pear as the process of vegetation proceeds (32, 3.) ? 

 And why at last is none to be met with, although 

 there is present an abundance of carbonic acid, 

 out of which it is supposed to be formed ? It has 

 been proved, that during the day carbonic acid, by 

 the act of vegetation (32, 3.), is constantly forming ; 



* Thomson's Chemistry, vol. iv. p. 283. 

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