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bonic acid in sunshine, we have seen, that, when 
confined for long periods in a given quantity of air, 
they produce ir; it little or no permanent change; 
but how far these two operations thus compensate 
each other in the open air, we possess not the means 
of deciding. To us, however, it appears, that, in 
the open atmosphere, vegetables can receive carbonic 
acid only, or chiefly, by means of their absorbed 
fluids ; for the air that surrounds them, unlike that 
in closed vessels, scarcely contains any sensible 
portion of this acid, and, did it contain more, this 
gas would probably diffuse itself more readily through 
the atmosphere, than enter the substance of plants. 
In this view, therefore, although plants, in closed 
vessels, may maintain the atmosphere in a permanent 
state of purity, they may not be able to do so in the 
open air, from not receiving so abundant a supply of 
acid gas. 
,529. On the other hand, it appears, that carbonic 
acid is largely conveyed into plants, both in a state 
of simple solution, and of combination with alkaline 
matter ; that these saline compounds are decomposed 
by light ; and that the carbonic acid, at the same in- 
stant, is resolved into its constituent elements, so as 
to yield its oxygen to the air. These decompo- 
sitions we have seen to be purely chemical, and 
to depend immediately on the presence and ope- 
ration of light ; hence their extent and rapidi- 
ty will be bounded only by the supply of acid, 
and by the decomposing power which light is able to 
exert. Thus, in young and succulent plants, which 
possess abundance of saline matter, the green colour 
