The aquarium is now not only an interesting plaything and a handsome house ornament, but an important means of studying the habits of those plants and animals that live in water, and of watching the effect of the different species upon each other and upon the purity of the element in which they live. It therefore deserves the careful attention of those who are interested in these subjects, and consequently demands more than a passing notice at our hands.

The term aquarium was formerly applied to any tank or small pond used for growing aquatic plants; and in this sense it is used by Loudon. But since the principles which regulate the balance of organic nature have been studied in connection with this subject, the name has been restricted to those tanks or vessels in which a self-supporting system of plants and animals has been placed. The principles which control the successful management of an aquarium are very beautiful, and not difficult to understand.

Animals which live constantly under water breathe just as truly as do those animals that live on land, the difference in the methods of breathing of the two kinds being that while land animals take in the air directly into their breathing apparatus, the water animals depend for their supply of oxygen upon the air that is dissolved in the water that they inhabit. The proportion of air which is held in solution by water is considerable, being greater in cold weather and under increased pressure. It is a curious fact that the oxygen dissolves in water more freely than does nitrogen; consequently the air which is supplied to fishes through the medium of water is always richer in oxygen than is the air that is breathed by land animals. But under any circumstances the oxygen contained in a few gallons of water is soon exhausted by a comparatively small number of fish, and its place is occupied by carbonic acid, - a gas which is entirely unfit for supporting life. The carbonic acid, into which the breathing of animals converts oxygen, may be removed, and its place supplied by the life-sustaining gas in two ways: (1) by mechanically agitating the water and exposing it freely to the air, and (2) by the action of plants. The first method has been frequently employed in aquaria in public museums, - a pair of bellows or some such device being employed to force air in at the bottom of the tank, and in this way agitate it and "aerate" it. This, however, is a crude and unscientific makeshift. The action of plants is far more efficient and more interesting. Every plant, when its leaves are exposed to light, absorbs the gas that is exhaled by animals (carbonic acid), decomposes it, appropriates the carbon to itself, and sets the oxygen free. The plant, in its action on the air, is thus directly antagonistic to the animal: it undoes what the animal does, and the two forms of life thus constitute a balance which maintains the air in its purity, and the waters of rivers, lakes, and oceans, in their life-giving qualities. This is the principle which is made use of in the management of a properly kept aquarium: plants are introduced in numbers and quantities sufficient to decompose the noxious gases given off by the animals, and the latter, in their turn, supply carbon to the plants.

If no death, no decay, and no obnoxious growths ever occurred in the tanks, they would keep healthy and clear for an indefinite time, provided they were once properly balanced in the way we have described. But since minute animals will die and remain unseen, and plants will drop their dead leaves, death in some form or another is present all the time, and this tends to disturb the pleasant condition of things. Therefore, in addition to ordinary plants and fish, it is necessary to introduce certain scavengers who will devour any dead vegetable or animal matter, and thus put a stop to its evil influence. Snails and tadpoles are the great scavengers of the aquarium, as indeed they are in nature, for a well-kept aquarium is merely a natural lake on a very small scale. The dissolved portion of dead plants and animals, as well as of their excreta, - whether the latter be solid faecal matter or the excretions which are undoubtedly given off by the external surfaces of all animals, fish as well as others - are taken up by the roots of the plants and rapidly removed from the water; and so nicely may all these interdependent actions be adjusted that an aquarium has been covered with a tightly closed glass plate and the plants and animals kept in good health for months.

A careful study of these general laws will enable any one to manage an aquarium successfully; and there are few more beautiful objects in a room than a well-kept aquarium, with the water clear and the plants and animals in good health. But without a knowledge of these principles and a careful attention to them, the owner of an aquarium will be constantly groping in the dark and committing all sorts of blunders and mistakes.

Aquaria are of two classes, - fresh water and marine, - according as the water is salt or fresh. Dwellers on the seashore, who have facilities for procuring stock and water, find the marine aquaria by far the most beautiful and interesting; and even far inland this form is a favorite with experts, as the water, plants, and animals are easily sent by rail; or, if desirable, an artificial sea-water may be used, which will answer every purpose.

When sea-water can not be procured for the marine aquarium, a substitute for it may be made as follows: Mix with 970,000 grains of rain-water 27,000 grains of chloride of sodium, 3,000 of chloride of magnesium, 750 of chloride of potassium, 29 of bromide of magnesium, 2,300 of sulphate of magnesia, 1400 of sulphate of lime, 35 of carbonate of lime, and 5 of iodide of sodium. These, all being finely powdered and mixed first, are to be stirred into the water, through which a stream of air may be caused to pass from the bottom until the whole is dissolved. On no account is the water to be boiled, or even to be heated. Into this water, when clear, the rocks and seaweed may be introduced. As soon as the latter are in a flourishing state the animals may follow. Care must be taken not to have too many of these, and to remove immediately any dead ones. The loss that takes place from evaporation is to be made up by adding clear rain-water.

In such aquaria the beautiful anemones and other inhabitants of the ocean may be kept in perfect health for years. We would, however, advise our readers to commence with the fresh-water aquarium, as being the most easily procured, the most readily stocked, and as requiring the simplest management; and the following directions are intended to apply chiefly to that form.