Any vessel that will hold water can, of course, be readily converted into an aquarium. But as we desire a clear view of the contents at all times, glass is the best material. And since glass globes refract the light irregularly and magnify and distort whatever is within them, we shall find an advantage in having the sides of the aquarium parallel and the form rectangular. As the weight of the aquarium, when filled with water, is enormous,--far more than we should at first imagine,--it follows that it must be capable of resisting pressure both from above and from within. The floor and stand, the frame and joints must be strong and compact, and the walls of plate or thick crown glass. The bottom should be of slate; and if it is designed to attach arches of rock-work inside to the ends, they, too, must be of slate, as cement will not stick to glass. The frame should be iron, zinc, or well-turned wood; the joints closed with white-lead putty; the front and back of glass. There is one objection to having the side which faces the light of transparent glass, and that is that it transmits too much glare of sunlight for the health of the animals. In Nature's aquarium the light enters only from above; and the fish and delicate creatures have always, even then, the shady fronds of aquatic plants or the shelter of the rocks,--as well as the power of seeking greater depths of water, where the light is less,--to protect themselves from too intense a sunshine. It is, therefore, sometimes advisable to have the window side of the aquarium made of glass stained of a green color. It is desirable that all aquarial tanks should have a movable glass cover to protect them from dust, impure gases, and smoke. When we speak of an aquarium, we mean a vessel holding from eight to thirty gallons of water. Mr. Gosse describes his larger tank as being two feet long by eighteen inches wide and eighteen inches deep, and holding some twenty gallons. Smaller and very pretty tanks may be made fifteen inches long by twelve inches wide and twelve deep. Great varieties in form and elegance may be adapted to various situations. There are two kinds of aquaria, the fresh- and the salt-water: the one fitted for the plants and animals of ponds and rivers; the other for the less known tenants of the sea. They are best described as the River and the Marine Aquarium, and they differ somewhat from each other. We shall speak first of the fresh-water aquarium.