less in degree in them, and therefore less in the
water than in the air. We may, therefore, venture to lay it down as a
general principle, that plants evolve free oxygen in water, when in
the sunlight, and remove the carbonic acid added to the water by the
respiration of the animals.

But since this is a digestive or nutritive process, it follows that
aquatic plants may derive much or all of their food from the water
itself, or the carbon in it, in the same manner as the so-called
air-plant, which grows without soil, does from the air. It is true, at
any rate, that, in the fresh-water aquarium, the river and brook plants
need no soil but pebbles; and that the marine plants have no proper
root, but are attached by a sort of sucker or foot-stalk to stones and
masses of rock. It is very easy to see, then, how the aquarium may
be made entirely self-supporting; and that, excepting for the larger
carnivorous fish, who exhaust in a longer or shorter period the minute
creatures on which they live, no external food is required.

A very simple experiment will prove the theory and practicability of the
aquarium. In a glass jar of moderate size was placed a piece of _Ulva
latissima_, or Sea-Lettuce, a broad-leaved, green, aquatic plant, and a
small fish. The mouth was closed by a ground glass stopper. The jar was
exposed to the light daily; the water was never changed; nor was the
glass stopper removed, excepting to feed the fish, once or twice a week,
with small fragments of meat. At the end of eight months both remained
flourishing: the fish was lively and active; and the plant had more than
half filled the bottle with fresh green leaves.

Any vessel that will hold water can, of course, be readily converted
into an aquarium. But as we desire a clear view of the contents at all
times, glass is the best material. And since glass globes refract the
light irregularly and magnify and distort whatever is within them, we
shall find an advantage in having the sides of the aquarium parallel and
the form rectangular. As the weight of the aquarium, when filled with
water, is enormous,--far more than we should at first imagine,--it
follows that it must be capable of resisting pressure both from above
and from within. The floor and stand, the frame and joints must be
strong and compact, and the walls of plate or thick crown glass. The
bottom should be of slate; and if it is designed to attach arches of
rock-work inside to the ends, they, to