92 
DECOMPOSITION OF CARBONIC ACID, ETC. 
ratory process. 5th. That this digestion is brought about in 
the sarrje way as the digestion of animals, by the decay of a 
nitrogen i zed body. 
To show that the light of the sun is the cause of the de- 
composition, having obtained a motionless spectrum by the 
aid of a heliostat, he placed in the different colored spaces 
tubes filled with water impregnated with carbonic acid gas, 
and containing some leaves of grass. The decomposition 
presently commenced, and in the course of two hours a suf- 
ficient quantity of gas was collected. On examination, it 
was found that the tubes in the yellow, the orange, and the 
green light, contained most gas ; that in the red, a much 
smaller quantity ; and those in the blue, the indigo, and the 
violet, none at all. 
But the [maximum of heat occurs in or beyond the red 
ray ; the maximum of chemical action among the more re- 
frangible colors, blue, indigo, and violet; and in these 
spaces the decomposition of the acid fails to go on. 
From this he infers that it is the light of the sun, and the 
yellow light mainly, that is the cause of the phenomenon. 
On causing leaves to decompose carbonic acid in water 
by the rays of the sun, and collecting the gas as it is evolved, 
it appears on no occasion to be pure oxygen, but a mixture 
of oxygen and nitrogen in variable proportions; from 
fifty to ninety per cent, of oxygen being found at different 
times, as is shown by explosion with hydrogen gas. But 
although there is this great variability in the proportion of 
the two gases evolved, a very simple law, which directs 
the progress of the decomposition, may be traced. On caus- 
ing leaves to decompose a known volume of carbonic acid, 
the same volume of the mixed oxygen and nitrogen makes 
its appearance. From this it is to be inferred, that plants 
during this action do not only effect the fixation of carbon, 
as is commonly supposed, but with it they absorb a certain 
amount of oxygen also. When a leaf, exposed in carbonic 
