Thus the real Aquarium is a water-garden and a menagerie combined,--and
aims to show life beneath the waters, both animal and vegetable, in
all the domestic security of its native home, and in all the beauty,
harmony, and nice adaptation of Nature herself. It is no sudden
discovery, but the growth of a long and patient research by naturalists.

"What happens, when we put half a dozen gold-fish into a globe? The
fishes gulp in water and expel it at the gills. As it passes through the
gills, whatever free oxygen the water contains is absorbed, and carbonic
acid given off in its place; and in course of time, the free oxygen of
the water is exhausted, the water becomes stale, and at last poisonous,
from excess of carbonic acid. If the water is not changed, the fishes
come to the surface and gulp atmospheric air. But though they naturally
breathe air (oxygen) as we do, yet they are formed to extract it from
the water; and when compelled to take air from the surface, the gills,
or lungs, soon get inflamed, and death at last puts an end to their
sufferings.

"Now, if a fish-globe be not overcrowded with fishes, we have only
to throw in a goodly handful of some water-weed,--such as the
_Callitriche_, for instance,--and a new set of chemical operations
commences at once, and it becomes unnecessary to change the water. The
reason of this is easily explained. Plants absorb oxygen as animals
do; but they also absorb carbonic acid, and from the carbonic add thus
absorbed they remove the pure carbon, and convert it into vegetable
tissue, giving out the free oxygen either to the water or the air, as
the case may be. Hence, in a vessel containing water-plants in a state
of healthy growth, the plants exhale more oxygen than they absorb, and
thus replace that which the fishes require for maintaining healthy