Aquarium, a tank of water for live plants and animals. An aquarium is not expensive nor hard to keep up. It may be a large glass receptacle or a wooden box with glass ends. Put in an inch of sand, a few stones for shelter, some snails, some water drawn with a dipping tube from a weedy pond bottom for tiny animals, a minnow or two, and a tadpole. Root some water plants in the sand to purify the water, and let a chip float on the surface. Keep it in the light, but not in the sun, and you have an aquarium to experiment with. Country schoolteachers sometimes convert a washtub into an aquarium with great success. Some high schools and nearly all colleges and universities possess aquaria in connection with their botany and zoology departments. Entire rooms, or even an entire building, may be used in connection with natural history museums or biological stations. One of the largest and most beautiful salt-water aquaria in the world is at Naples, Italy. It occupies the ground floor of a building more than 100 feet long. Glass tanks, reaching from the floor to the ceiling, are built into the walls. Salt water is being forced constantly into these tanks from the sea. Each tank is numbered, only animals representing a certain marine type are placed in each; e. g., in one, only spiny skinned animals are seen; in another worms only; in still another bony fishes only. Each tank is designed to furnish the conditions or environment to which the animals are accustomed in their free state in the sea. Therefore one sees grottoes, rocks, variously colored sand, sticks of wood, debris, green, red, and brown plants, empty shells, etc., as the furniture of the tanks. Some of the animals are themselves very highly colored, red, green, yellow, blue; some are all of one color; others are spotted, striped, or speckled. The observer sees them moving about him in much the same way as if he had himself been lowered into the sea, and the plants and animals left in their accustomed places.