One of the first principles, in constructing a tank for an aquarium, is to give the water the greatest possible exposure to the air. The simple rectangular form is the best. This is generally constructed of iron and glass; the iron should be japanned, and the glass be French plate, to insure brilliancy and strength. The breadth and height of the tank should be about one half of the length. Cheap tanks can be made of wood and glass, the frame and bottom being of wood, and the sides of glass. In order to make the joints watertight, care must be taken to get a proper aquarium putty or cement. The following is a good recipe: Put an eggcupful oil and 4 ozs. tar to 1 lb. resin; melt over a gentle fire. Test it to see if it has the proper consistency when cooled; if it has not, heat longer or add more resin and tar. Pour the cement into the angles in a heated state, but not boiling hot, as it would crack the glass. The cement will be firm in a few minutes. Then tip the aquarium in a different position, and treat a second angle likewise, and so on. The cement does not poison the water. It is not advisable to make the aquarium of great depth; about eight inches of water is sufficient. In regard to the light, great care must be taken. Too much often causes blindness, and is a common source of disease. The light fish receive in rivers comes from above, and an aquarium should be constructed so as to form no exception to this rule. All cross-lights should be carefully avoided, at least if the light is very strong. Never place the aquarium in front of a window so that the light passes through it; for, when viewing an aquarium, the source of light should come from behind us. Not enough light is as injurious as too much, and causes decay of the vegetation. Having constructed a watertight aquarium, the bottom is strewn over with clean sand to the depth of 1 to 3 inches; on this a little gravel is spread; then a few stones or rock-work. Heavy large rocks should be avoided; they displace a large amount of water, and in crease the danger of breaking the glass sides. Pumice-stone, well washed, is the best kind, being light and with a rough surface suitable for the rooting of plants, etc.; and if fancy forms are desired (bridge-work, etc.), the pumice-stone can be cut quite easily to the desired shapes. The plants are rooted in the sand and the vessel left at rest for a week for the plants to vegetate. The following plants will be found useful: Utricularia in flata, utricularia vulgaris, myriophyllum spicatum , anarcharis Canadensis, and hottonia inflata.

In obtaining plants, procure all the roots and see that they are well rooted. If fungus should form, add snails {planorbis trivolcis); they will completely destroy it. After the plants are well started, add the shells and amphibious animals. The following shells will be found desirable: Planorbis trivolvis, physa heterostrapha, unio complanatus. Many shells are not needed. Snails act the part of scavengers; and where the different elements of an aquarium are rightly balanced, two or more snails will be found sufficient.

If amphibious animals are introduced, the rock-work must extend above the surface of the water, or a float of some kind must be substituted. It is impossible for them to live under water all the time, and they would die without some such arrangement.

The turtles claim first rank. The enys punctata, or spotted water-turtle, and the chrysemys picta, or painted water-turtle, will be found to be the best for the aquarium, and should be procured when very young, as they are very destructive when old. The tritons (triton tigrinus, triton niger), the red salamander, the cray-fish (astacus Bartoni), are all suitable, and present a very odd and yet a very natural look to the aquarium.

In selecting the fishes, a few only thrive in confinement. Among these, and the first, is the gold-fish. He can live for months without introduced food, and is, without comparison, the most hardy, standing remarkable changes in the temperature; and he is the most gaudy and attractive. A large number of the fishes prey upon each other, and will only do for the aquarium when in the young state. Among these may be mentioned pomotis vulgaris, or sun-fish, esoxreticulatus, or common pickerel, and perca florescens, or yellow perch. The leuciscus pygmaeus, or rock-fish, is a great addition, and is found very plentifully in our streams. The pimelodus atrarius, or common black catfish, is another worthy of a place. So also is the hydrargia diaphana, or transparent minnow. But few fish can live in an aquarium, and the needless crowding together, so often seen, is very hurtful to health, and causes sound, strong fish in a short time to become weak and poor. The great difficulty in keeping an aquarium is to secure enough oxygen for the fish. To a slight degree, it is the duty of the plants to supply this; but if too much vegetation be present, decomposition takes place and ruin follows. It has been demonstrated that only a small amount is necessary to absorb the carbonic acid given off by the fish and amphibians; consequently, if the water be daily aerated with a syringe, it will absorb an abundant supply of oxygen for the animal life, and the trouble arising from the decay of much vegetable matter will bo lessened or altogether avoided.