The successful treatment of aquatic plants and animals, in the confined space of a glass Aquarium, depends entirely upon the discovery that there exists in Nature a self-adjusting balance between the supply of oxygen created in water, with the quantity consumed by aquatic animals. And it became equally necessary to know the means by which that supply was continually generated. Without the knowledge of these facts, and the principles by which they are regulated, it would have been impossible to establish such a marine Aquarium as that we may now any day examine in the Regent’s Park; where, in a few glass tanks of very moderate size, we may see examples of some of the most curious forms of animal and vegetable life peculiar to the depths of the ocean—forms so singular, that their first exhibition created a sense of wonder little less intense than that which must have been caused, long years ago, by the first public display of the mountain form of the elephant to the people of cold northern countries; and much more so than the recent introduction of the giraffe or hippopotamus, although they have never been seen in Europe since the days of the Romans. Those principles, the knowledge of which was requisite to enable us thus to view the wonders of the ocean in their living state in an Aquarium, were not mastered at once, or by one man, or in one generation. The nature of certain relations between animal and vegetable life, upon which they are founded, was first advanced by Priestley, towards the close of the last century, who proved that plants give forth the oxygen necessary to animal life. The learned Ingenhauss, a native of Breda, but who principally resided in England, defined this principle still more clearly, in a work the title of which pretty fully explains the entire nature of his discovery. It was published in French, at Leyden, in 1778, and in London, in English, in 1779. The French edition is before me, the title of which I translate, “Experiments upon Plants, which prove their important influence in the purification of the atmospheric air when they are exposed to the rays of the sun, and the contrary results which ensue when they are placed in the shade, or during the night.” The action of the sun’s rays in disengaging the oxygen generated in plants is thus clearly announced, and the knowledge of this principle is one of those which have mainly conduced, as I have said, to the successful establishment of Aquaria. In the course of his essay Ingenhauss states, still more directly, that plants “immersed in water,” when exposed to the action of light, emit an air which he announces as oxygen gas; and this idea is the key-stone of the Aquarium.