339 
fcult, or even impossible, to fix the precise limits 
where the one operation terminates, and the other 
begins. 
644. With respect to the ulterior source of the 
carbon in living vegetables, we have supposed (159.) 
that its supply, in the later periods of vegetation, is 
derived, like the other ingredients which go to the 
composition of the plant, from the soil or situation 
in which it may be placed to grow. From the fact, 
however, of carbonic acid being decomposed by 
plants which vegetate in sunshine, it has been infer- 
red by M. M. Senebier and De Saussure, that the 
carbon of this acid was retained anci assimilated so as 
to afford a constant supply of nutriment to the plant. 
But plants have been shewn to live and grow in the 
shade, where they decompose little carbonic acid, 
and can, therefore, in this manner, derive but little 
carbon j and, even in sunshine, it has been also 
shewn (305.) that they grow for long periods in at- 
mospheric air without affecting permanently either 
its purity or its volume. Consequently, if it were 
granted, that, by the decomposition of carbonic acid, 
they obtained carbon in sunshine^ they must again 
give out an equal quantity of it, when they form 
carbonic acid in the shade ; so that no excess of car- 
bon remains to be applied to the growth and aug- 
mentation of the plant. M. de Saussure, indeed, ad- 
mits that plants grow in atmospheres perfectly freed 
from carbonic acid ; but he then supposes them to 
obtain carbon by decomposing the acid which they 
had previously formed. It is, however, obvious, that 
they cannot, in this way, absolutely acquire carbon ; 
