Aqua'rium, a vessel constructed wholly or partly of glass, and containing salt or fresh water, in which are kept living specimens of marine or fresh-water animals, along with aquatic plants. In principle the aquarium depends on the relations of animal and vegetable life; animals consuming oxygen and exhaling carbonic acid, plants reversing the process by absorbing carbonic acid and giving out oxygen. The aquarium must consequently be stocked both with plants and animals, and for the welfare of both, something like a proper proportion should exist between them. The simplest form of aquarium is that of a glass vase; but aquariums on a larger scale consist of a tank or a number of tanks with plate-glass sides and stone floors, and contain sand and gravel, rocks and seaweed. By improved arrangements, light is admitted from above, passing through the water in the tanks and illuminating their contents. Aquariums on a large scale have been constructed in connection with public parks or gardens, and the name is also given to places of public entertainment in which large aquariums are exhibited. The largest aquarium in the world is at Castle Garden, N. Y. It contains 150 tanks for smaller fish, and a number of gigantic tanks for sharks and other large and dangerous fish. The aquarium of the American Fish Commission at Washington is also important, since it contains many specimens of our best food fishes. Among foreign aquariums, those at the Naples Marine Station, Paris, Hamburg, Saint Petersburg and Brighton, Eng., are the most important.