196 
heat (348.), but is readily given out under the che- 
mical action of solar light; and, indeed, M. Vauquelin 
and others have detected it in combination with alka- 
line matter. In this state, therefore, of combination 
and concentration, it may yield to the decomposing 
power of the solar rays, although it resisted their inten- 
sest action, while retaining its elastic form. And 
thus we learn, in what manner the chemical condi- 
tion of the leaf may conspire with the agency of light 
in promoting the decomposition in question, and 
why neither the leaf alone, nor the power of light 
alone, is of itself able to effect it. 
468. But after the oxygen of the acid is expelled, 
the carbon is necessarily retained in the leaves, and 
serves, probably, some useful purpose in those or- 
gans* It has been already remarked (25.), that 
plants not only owe their green colour, but their 
odour arid combustibility, to the immediate agency of 
light ; and as the chemical rays alone are necessarily 
concerned in the formation of this green colour, we 
must, also, suppose that they essentially contribute to 
the production of those other properties, which are 
so immediately associated with it. The resinous mat- 
ter, which imparts this colour, is also the most in- 
flammable part of plants, and that, too, in which, pro- 
bably, the odour and combustibility principally re- 
side; but in what manner the chemical portion of 
light immediately acts in the production of this resin, 
no facts enable us, at present, to state. The cele- 
brated Scheele supposed light to be decomposed in 
vegetables, and to contribute to the formation of their 
combustible matter j for the generation of a green 
