226 Transactions of the Royal Microscopical Society. 
From this experiment we deduce that, when exposed to the 
light, or those rays of the spectrum which have the greatest effect 
in promoting the decomposition of carbonic acid, plants absorb 
carbonic acid, decompose it, and emit the greatest part of the 
oxygen, mixed, it would seem, with a certain quantity of azote. It 
is, I suppose, now-a-days quite an established fact that plants by 
this process of respiration, if I may so term it, acquire the greatest 
part of the carbonaceous matter which they contain ; for as I pre- 
viously showed in my paper on the various changes caused on the 
spectrum by different vegetable colouring matters,* if we compare 
the quantity of carbon contained in plants vegetating in darkness — 
where this process does not go on — 'with the quantity which those 
plants contain which vegetate in the usual manner, we are bound at 
least to admit that the difference is most conspicuous. Chaptal, in 
his ‘Elements of Chemistry,’ published in the year 1791, gives an 
instance of a Byssus which, when vegetating in the dark, produced 
but the -j^th part of its weight of carbonaceous matter; whereas 
the same plant, after being made to vegetate for thirty days exposed 
to the sun, gave the -j^th. Hassenfratz says, “ plants which grow 
in the dark contain much more water, and less carbon and hydrogen 
than those exposed to light,” while Sennebier speaks of similar re- 
sults having answered to his analysis, and shows that plants grow- 
ing in darkness yield less hydrogen and oil, while their resinous 
matter is to that of plants growing in the light as 2 is to 5 ■ 5, and 
their moisture as 13 is to 6. 
That leaves and plants absorb oxygen has been shown to he 
the case by many observers. It is, however, not separated from 
them by submitting them to the exhausted receiver of an air-pump ; 
by that means a little air is given off, hut it is always much less 
than the oxygen absorbed, and it is moreover of precisely the same 
character as the atmosphere in which they were confined. Neither 
does it appear that the oxygen is extracted when leaves are ex- 
posed to sufficient heat so as not to prove destructive to them. 
There is, then, every reason to assume that the oxygen absorbed 
is converted into carbonic acid within the plant, but this only 
occurs when the plant is saturated with this substance, and when 
the surrounding oxygen is partly converted into carbonic acid by 
combining with the carbonaceous matter of the plant. 
When leaves are exposed to the light, carbonic acid is decom- 
posed and oxygen thrown off, which is usually greater than that 
absorbed. But the oxygen given out in daylight, when plants 
grow in atmospheres destitute of carbonic acid, is always propor- 
tional to that inspired during the night, being always greatest 
when the plant has absorbed the largest quantity of oxygen. 
As with carbonic acid, so with oxygen ; plants differ very much 
* ‘Royal Microscopical Transactions,' vol. xvii. p. 225. 
