60 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 air. 
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 
aération, 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 
alge 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 
