ange 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and evolution of oxygen, ceases when the light is withdrawn.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
existing plants has withdrawn twenty-two pounds of carbonic acid gas
from the atmosphere, and replaced it with sixteen pounds of oxygen gas,
occupying the same bulk. And when we consider the amount of carbon that
is contained in the tissues of living, and of extinct vegetation also,
in the form of peat and coal, we may have some idea of the vast body of
oxygen which the vegetable kingdom has added to the atmosphere.

And it is also to be considered, that this is the only means we know of
whereby free oxygen is given to supply the quantity constantly consumed
in respiration, combustion, and other vast and endless oxygen-using
processes. It follows, therefore, that animals are dependent upon plants
for their pure oxygen, as well as for their food. But the vegetable
kingdom might exist independently of the animal; since plants may derive
enough carbon from the soil, enriched by the decaying members of their
own race.

There is, however, one exception to the law that plants increase the
amount of oxygen in the air. During flowering and fruiting, the stores
of carbon laid up in the plant are used to support the process, and,
combining with the oxygen of the air, both carbonic acid and heat are
given off. This has been frequently proved. In large tropical plants,
where an immense number of blossoms are crowded together, the
temperature has risen twenty to fifty degrees above that of the
surrounding air.

As most of the aquatic plants are cryptogamous, or producing by spores,
and not by flowers, it seems probable that the evolution of carbonic
acid and heat is much