respiration. Any one who will observe the plants in an aquarium, when
the sun shines through the tank, will see the leaves studded with bright
beads, some of them sending up continuous streams of minute bubbles.
These beads and bubbles are pure oxygen, which the plants distil from
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,