230 
to judge of the conclusiveness of the methods which 
he employed. He only says, that lime or potassa, 
which destroys the thin leaves of plants which vege- 
tate in sunshine, does not exert this action upon the 
vegetation of the cactus opuntia, and other plants 
which have thick leaves ; because, says he, their 
thick parenchyme, and the little porosity of their epi- 
dermis, enable them to retain obstinately the car- 
bonic acid which they have formed *. We must 
not, however, conclude, that the life of the cactus is 
preserved, in consequence of its thus retaining car- 
bonic acid, but in consequence of its hardy constitu- 
tion, which enables it to resist the deleterious opera- 
tion of the substances to which it is exposed f. 
512. Lastly, we may observe, that, even in sun- 
shine, it is to the presence of oxygen gas (309.), 
and not to that of carbonic acid, that the vegetation 
of the plant is immediately to be ascribed ; and the 
presence of this acid, in any considerable quantity, 
is actually injurious to vegetation (30Q.), unless it be 
decomposed by the agency of solar light. But by 
its decomposition, this acid does no more than af- 
ford oxygen ; consequently, it is useful in vegeta- 
tion, not as carbonic acid, but only as it affords oxy- 
gen gas. If, therefore, this gas be supplied from 
any other source, carbonic acid is not required to af- 
ford it ; and hence vegetation goes on in atmosphe- 
ric air, though it contain not more than -roW part of 
carbonic acid, and even, as M. de Saussure has 
shewn (499.), when that small portion is removed. 
Recherches Chim. p. 8p. f Ibid. p. 6i. Note, 
