One great point in favour of an aquarium, and one by no means generally understood is, that having once filled the tank with salt water, it will last for months, and even years, if proper care be taken, without requiring one particle of sea-water to be again added; for as the water evaporates, the salt falls to the bottom, and the deficiency may be supplied with _fresh_ water from the cistern or filter. In order to ascertain when the sea-water is of the proper density, you require to have a 'gravity bubble,' which can be had for sixpence. This may always be kept in the tank. When 'all's well' it sinks to the bottom, and when anything comes amiss it rises to the surface, but falls again quickly upon the introduction of the fresh water.[20] [20] _Vide_ author's "Sea-side and Aquarium." A more simple plan is, to mark on the glass the height of the fluid when the tank is first filled, then as the water sinks, raise it again to its original level by means of fresh water. Many persons decline starting an aquarium on account of the great difficulty of procuring a proper supply of sea-water. This objection, of course, can be offered only by those who happen to reside inland; but even these need not now be discouraged, for an ingenious plan has lately been devised for sending the commodity in question through the post! Mr. Bolton, chemist, Holborn Bars, London, supplies, not sea-water, but 'marine salts for the instantaneous production of sea-water,' About six ounces is sufficient to make a gallon, by the application of _fresh_ water. The saline material here alluded to, is not an artificial chemical compound, but is produced by the simple process of evaporating sea-water itself. Those individuals so fortunate as to possess a marine villa, or any other more humble residence at or near the sea-coast, have no occasion to resort to the scheme above-mentioned for filling their tanks, a pure supply of sea-water being attainable with scarcely any trouble whatever. A stone jar should be kept for this purpose only, and care taken that the vessel is perfectly free from any smell, as that of spirits, dirty corks, or the like, as any such impurity would quickly spoil the water.