Page:Popular Science Monthly Volume 50.djvu/30

18 inmates in providing that the water of each pool shall be clean and well aërated. To attain the former end, the water is constantly drained from the aquaria and replaced by fresh and filtered water; and to insure proper aëration, the incoming stream is usually passed into the tank in such a way that it draws downward with it in its current a cloud of air-bubbles—these to subdivide finely and to be in part absorbed. In the sea-water basins the "reservoir system" has been found most effective in securing the healthfulness of the water, and is at present in general use. It has certainly an advantage from the standpoint of economy, since by its means a given bulk of water may be used and re-used for months and even years, with better results, indeed, than if a fresh supply of sea water had been employed, for the latter, it is claimed, introduces a constant stream of impurities which can not be removed by filtration. The reservoir system is certainly an easy one to understand. In the basement or cellar of the aquarium building is situated a concreted cistern, whose capacity is ten to fifty times as great as that of the sum of the sea-water tanks throughout the building. In this cool, dark, and uniformly temperatured cistern the water seems to have the power, even in the course of a few days, to purify and "rest," its sediment settling and its air-drinking power becoming restored. It is into this cistern, accordingly, that the water drained from all parts of the building is returned after it has been roughly filtered; and it is directly from this cistern again that the water is pumped upward as the resupply. By this plan of circulation it is usually arranged that the water of each tank may become changed several times during the day.

From this review of the general subject we may next pass to the examination of the various aquariums of Europe.

—First in importance, as has already been noted, stands the aquarium at Naples, highest in rank, also, as a station of marine biological research. Its situation and surroundings are eminently attractive; it stands in a public garden on the side of the gulf, amid fashionable driveways, surrounded by bright-colored lawns and a wealth of century plants and cactus; in front are the outlines of distant Capri and the blue waters of the gulf; in full view is Vesuvius. The building itself is like a huge white palace, conspicuous from nearly every higher part of the city. Its main wing, shown in the foreground in the adjoining picture, is the older, dating from 1875, when the station was founded by Prof. Anton Dohrn; the wing immediately behind it is the newly built physiological laboratory. The aquarium occupies the basement of the main structure, and is open to the public daily, although to the rest of the building, including the laboratories, library, and rooms of investigators, strangers are not generally