respiration and digestion: that the first is analogous to the same
process in animals; and that by it oxygen is absorbed from, and carbonic
acid returned to the atmosphere, though to a limited degree: and that
digestion consists in _the decomposition of carbonic acid by the green
tissues of the leaves under the stimulus of the light, the fixation of
solid carbon, and the evolution of pure oxygen_. The theory of distinct
respiration has been somewhat doubted by the highest botanical authority
of this country; but the theory of digestion is indisputable. And it is
no less certain that all forms of vegetation give to the air much more
free oxygen than they take from it, and much less carbonic acid, as
their carbonaceous composition shows. If fresh leaves are placed in
a bell-glass containing air charged with seven or eight per cent. of
carbonic acid, and exposed to the light of the sun, it will be found
that a large proportion of the carbonic acid will have disappeared, and
will be replaced by pure oxygen. But this change will not be effected in
the dark, nor by any degree of artificial light. Under water the oxygen
evolved from healthy vegetation can be readily collected as it rises, as
has been repeatedly proved.

Why carbonic acid is, to a limited degree, given off by the plant in the
night, is merely because the vital process, or the fixation of carbon
and evolution of oxygen, ceases when the light is withdrawn. The plant
is only in a passive state. Ordinary chemical forces resume their sway,
and the oxygen of the air combines with the newly deposited carbon to
reproduce a little carbonic acid. But this must be placed to the account
of decomposing, not of growing vegetation; for by so much as plants
grow, they decompose carbonic acid and give its oxygen to the air, or,
in other words, purify the air.

It has been found by experiment, that every six pounds of carbon in