trouble it may cost, in the lessons it affords of the workings of Almighty Wisdom, in those regions of life and wonder to which it introduces us.

The Philosophy of the Aquarium must be clearly understood by those who purpose to cultivate it. It is a self-supporting, self-renovating collection, in which the various influences of animal and vegetable life balance each other, and maintain within the vessel a correspondence of action which preserves the whole. A mere globe of fish is not an aquarium in the sense here indicated; because, to preserve the fish for any length of time, the water must be frequently changed; and even then the excess of light to which they are exposed, and the confinement in a small space, in which they quickly exhaust the vital properties of the water, are circumstances at variance with their nature, and sooner or later prove fatal to their lives.

In an aquarium, the water is not changed at all, or at least only at long intervals, as we shall explain hereafter; and besides the enclosure of fishes in a vessel of water, growing plants of a suitable kind, always form a feature of the collection. Formed on this plan, an aquarium is an imitation of Nature on a small scale. The tank is a lake containing aquatic plants and animals, and these maintain each other in the water in the same way as terrestrial plants and animals contribute mutually to each other's support in the preservation of the purity of the air.

What happens when we put half-a-dozen gold fish into a globe? The fishes gulp in water and expel it at the gills. As it passes through the gills, whatever free oxygen the water contains is absorbed, and carbonic acid given in its place; and in course of time the free oxygen of the water is exhausted, the water becomes stale, and at last poisonous, from excess of carbonic acid. If the water is not changed the fishes come to the surface and gulp atmospheric air. But, though they naturally breathe air as we do, yet they are formed to extract it from the water; and when compelled to take air from the surface, the gills, or lungs, soon get inflamed, and death at last puts an end to their sufferings.

Now if a gold-fish globe be not over-crowded with fishes we have only to throw in a goodly handful of some water weed—such as the Callitriche, for instance—and a new set of chemical operations commences at once, and it becomes unnecessary to change the water. The reason of this is easily explained. Plants absorb oxygen as animals do; but they also absorb carbonic acid, and from the carbonic acid thus absorbed, they remove the pure carbon, and convert it into vegetable tissue, giving out the free oxygen either to the water or the air, as the case may be. Hence, in a vessel containing water plants in a state of healthy growth, the plants exhale more oxygen than they absorb, and thus replace that which the fishes require for maintaining healthy respiration. Any one who will observe the healthy plants in an aquarium, when the sun shines through the tank, will see the leaves studded with bright beads, some of them sending up continuous streams of minute bubbles. These beads and bubbles are pure oxygen, which the plants distil from the water itself, in order to obtain its hydrogen, and from carbonic acid, in order to obtain its carbon.

There is one more feature, which no writer on the aquarium has yet noticed, namely—when a tank is properly stocked, the water soon gets crowded with infusorial animalculæ, which swarm among the plants, and on the sides of the glass in countless thousands, visible only by the aid of the microscope. These are in accordance with a natural law; the presence of vegetable matter in water always induces them. But observe their value: they contribute to the sustenance of the smaller fishes, by supplying them with food; and, strangely enough, the researches of modern chemists have proved that these minute creatures respire in much the same way as plants. While all other animals absorb oxygen, and perish if the supply of that gas is withdrawn, these minute organisms absorb carbonic acid, and give out oxygen in abundance. This has been proved by Professor Liebig, who collected several jars of oxygen from tanks containing infusoria only. Every one who has had experience in the management of tanks must have noticed that the water in a tank which has been established some months will sustain a much greater amount of animal life than one of the same dimensions, but recently stocked. The presence of infusoria in immense numbers is one of the reasons for this.

So far I have endeavoured to explain the theory of the aquarium, in the merest outline. Still, brief as this chapter must be, I must here impress upon the mind of the beginner, that unless the leading features of the theory are borne in mind, success can never be achieved in the establishment of water collections of any kind.

If a tank requires frequent cleansing, or frequent changing of water, if the fishes come to the surface for air, or perish through the presence in the water of offensive matter—in fact, if the whole affair has not a distinctly self-supporting character, such as will preserve its purity, and strength, and beauty, without alteration of any kind—it must be concluded that it has been either unskilfully stocked or injudiciously managed.

It is my object to explain briefly, but clearly, the whole rationale of aquarium management, whether the tank be adopted as a mere ornament—than which there is nothing more beautiful—or as a museum of instruction and a school of study—than which there is nothing more suggestive, nothing that can afford finer lessons of the subtlety of the forces, or the refinement of the instincts, that give life and loveliness to the "world of waters."

CHAPTER II.

PROPER KINDS OF VESSELS.

Rectangular Tanks.—Any vessel that will hold water may be quickly converted into an aquarium; but as we desire to have at all times a clear view of the contents of the vessel, glass takes pre-eminence among the materials for tanks. For elegance and general utility, a properly built vessel of rectangular outline, having at least two sides of glass, is found by most aquarians to be the best. Of course, no rule can be laid down as to the dimensions or forms of tanks—those details will best be determined by the means and tastes of the persons requiring them—but a few general remarks may prove useful.

VALLISNERIA SPIRALIS, ANACHARIS ALSINASTRUM—GOLD CARP, ROACH, AND MINNOW.

The tanks in use at the Regent's Park Gardens were constructed by Messrs. Saunders and Woolcot, of 54, Doughty Street, London, and that firm has since set apart a portion of the premises in Doughty Street, to meet the new and increasing demand for vessels for domestic aquaria, and have brought the manufacture to a perfection which leaves little to desire.

For the adornment of a dwelling room or a conservatory, an oblong tank, measuring three feet by one foot four inches, and one foot six inches deep, would be very suitable, and would be supplied by Messrs. Saunders and Woolcot for £5, though vessels of smaller dimensions are sent out by them at from £2 to £3. In my work on "Rustic Adornments," I have given several designs for rectangular tanks, but must here beg my reader to remain content with a simple explanatory outline. Messrs. Treggon and Co., of 57, Gracechurch Street, and 22, Jewin Street, London, are also manufacturers of tanks for aquaria. I