104 Dr Gilby cm the Respiration of Plants. 
minishing, not only the \'ltiation of the atmosphere consequent 
on animal respiration, but is it even operative in counterba- 
lancing the exhalation of carbonic acid during the natural respi- 
ration of the plant? The extraordinary rapidity with which plants 
decompose carbonic acid in the light of the sun, can, I apprehend, 
have very little avail, as a proof of their correcting influence on 
the atmosphere. The circumstances under which the plant is 
placed, by the conditions’of the experiment, have no resemblance 
to its natural respiration. In the experiments of Ingenhousz, 
Sennebier, Saussure, &c. which have been considered as corro- 
borative of the beneficial influence of plants on the atmosphere, 
there was always present a very considerable portion of carbonic 
acid ; while, in ordinary circumstances, they are surrounded by 
an atmosphere which contains, according to Thenard, not more 
than gas. There can be no doubt, if the 
atmosphere contained a considerable portion of carbonic acid, 
that the oxygenating power of plants, when exposed to the sun, 
would have a very material influence in preventing an undue 
quantity of carbonic acid. For this effect, however, to take 
place, the difficulty is forced upon us, of supposing that the 
plant is endowed with the faculty of selecting and inhaling the 
small quantity of carbonic acid, to the exclusion of the atmo- 
spheric air. Besides the improbability of such an idea, it is 
likewise contradicted by the experiments of Ingenhousz, who 
found, that plants had the power ol‘ imbibing indifferently any 
air with which they happened to be in contact, whether oxygen, 
hydrogen, or nitrogen. Independent, too, of these objections, 
it supposes the discontinuance of what may with propriety be 
considered as the natural process of respiration. In Mr Ellis’s 
work, the distinction is very clearly maintained, between the ar- 
tificial change during sunshine, and the necessary function of the 
plant. In the one case, the oxygen of the atmosphere is consumed, 
and is replaced by an equal quantity of carbonic acid, being in 
fact precisely the process which takes place in the respiration of 
animals; and this he calls the proper and natural function of the 
plant. The reverse of this change, under exposure to light, he 
considers as the mere effect of the chemical action of the sun’s 
rays, and is by no means to be considered as a property neces- 
sary to the life of the plant. In proof of this opinion, I refer 
