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 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,