And it is also to be considered, that this is the only means we know of whereby free oxygen is given to supply the quantity constantly consumed in respiration, combustion, and other vast and endless oxygen-using processes. It follows, therefore, that animals are dependent upon plants for their pure oxygen, as well as for their food. But the vegetable kingdom might exist independently of the animal; since plants may derive enough carbon from the soil, enriched by the decaying members of their own race. 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.