rom
the water itself, in order to obtain its hydrogen, and from carbonic
acid, in order to obtain its carbon."[A]

[Footnote A:_The Book of the Aquarium_, by Sidney Hibbert.]

Thus the water, if the due proportion of its animal and vegetable
tenants be observed, need never be changed. This is the true Aquarium,
which aims to imitate the balance of Nature. By this balance the whole
organic world is kept living and healthy. For animals are dependent upon
the vegetable kingdom not only for all their food, but also for
the purification of the air, which they all breathe, either in the
atmosphere or in the water. The divine simplicity of this stupendous
scheme may well challenge our admiration. Each living thing, animal or
plant, uses what the other rejects, and gives back to the air what the
other needs. The balance must be perfect, or all life would expire, and
vanish from the earth.

This is the balance which we imitate in the Aquarium. It is the whole
law of life, the whole scheme of Nature, the whole equilibrium of our
organic world, inclosed in a bottle.

For the rapid evolution of oxygen by plants the action of sunlight is
required. That evolution becomes very feeble, or ceases entirely, in the
darkness of the night. Some authorities assert even that carbonic acid
is given off during the latter period. So, too, they claim that there
are two distinct processes carried on by the leaves of plants,--namely,
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 ch