6o THE OCEAN WORLD. 



beings it is intended to contain, serves the same purpose for the 

 inhabitants of the deep which the aviary does for the birds of the 

 air — cages of glass being used in place of cages of iron wire or -wicker- 

 work, and water taking, in a measure, the place of atmospheric au\ 



When the globe is filled with fresh water, and molluscs, crus- 

 taceans, or fishes are placed in it, 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, too frequently 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. des 

 Moulins proposed to put into the vase a certain number of aquatic 

 plants, floating or submerged — duckweed, for example — which would 

 oxygenate the water, and so keep it fit for the animals inhabiting it. 

 It is known that plants assimilate carbon, while decomposing the 

 carbonic acid produced by the respiration of animals, thus dis- 

 engaging the oxygen indispensable to animal life. In this simple 

 manner was the too constant change of the 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, such as that of the Zoological Gardens of Paris, belong- 

 ing to the French Acclimatisation Society, in the Bois de Boulogne, 

 inaugurated in 1861. It is a solid stone building of fifty yards in 

 length by about twelve broad, presenting a range of forty tanks made 

 of Angers slate, running north and south. The tanks are nearly 

 cubical, and are lined in front with the strong glass of Saint Gobain. 

 They are lighted from above ; but the light is weak, greenish, uniform, 

 and consequently mysterious and gloomy, giving, however, a pretty 

 exact imitation of the submarine light some fathoms down. Each 

 tank contains about 200 gallons of water, and is furnished with rocks 

 disposed a little in the form of an amphitheatre, and arranged in 

 a picturesque manner. Upon the rocks various species of marine 

 algae are planted. The bottom is of shingle, gravel, and sand, in 

 order to give certain animals a sufficiently natural habitat. 



Ten of these tanks are intended for marine animals. The water 

 employed is never changed, but it is kept in continual agitation by 

 a circulation, which is produced by a current of water led from the 

 great pipe which feeds the Bois de Boulogne. This water, being 

 subjected to a strong pressure, compresses a certain portion of air, 

 which, being permitted to act on a portion of the sea water contained 



