When a globe is filled with fresh water, and with mollusks, crustaceans, or fishes, it is observed, after a few days, that the water loses its transparency and purity, and becomes slightly corrupt. It necessarily follows that the water must be changed from time to time. Changing the water, however, causes much suffering, and even death to the animals. Besides, the new water does not always present the same composition, the same aeration, or the same temperature with that which is replaced. To obviate this defect, and taking a leaf out of Nature's book, M. Moulins proposed to put into the vase a certain number of aquatic plants floating or submerged--duckweed, for example--which would act upon the water in a direction inverse to that of the animals inhabiting it. It is known that vegetables assimilate carbon, while decomposing the carbonic acid produced by the respiration of animals, thus disengaging the oxygen indispensable to animal life. In this simple manner was the necessary change of water obviated. The same happy idea has been successfully applied to salt water, and aquariums for salt-water plants and animals have been proposed on a great scale. That of the Zoological Gardens of Paris, in the Bois de Boulogne, inaugurated in 1861, is perhaps the largest in the world. It is a solid stone building of fifty yards in length by about twelve broad, presenting a range of forty reservoirs of Angers slate, running north and south. The reservoirs are nearly cubical, presenting in front the strong glass of Saint Gobain, which permits of the interior being seen. They are lighted from above; but the light is weak, greenish, uniform, and consequently mysterious and gloomy, giving a pretty exact imitation of the submarine light some fathoms down. Each reservoir contains about two hundred gallons of water. It is furnished with rocks disposed a little in the form of an amphitheatre, and in a picturesque manner. Upon the rocks various species of marine vegetables are planted. The bottom is of shingle, gravel, and sand, in order to give certain animals a sufficiently natural retreat.