Aquarium, a tank or receptacle in which aquatic animals and plants are kept as nearly as possible under natural conditions, for scientific purposes. In 1790 Sir John Dalyell formed a collection of living marine animals which he kept in tanks and glass jars, changing the water once, and sometimes twice a day. But such tanks were not aquaria. The first to apprehend the true principle on which an aquarium should be maintained was Dr. Ward (the inventor of the Wardian case) who endeavoured to reproduce in his tanks the actual conditions of life in a pond. He introduced plants to absorb the carbon dioxide given off by the animals, and to aerate the water. Gosse followed, and his book on the subject, Kingsley's Glaucus, and the writings of the Rev. J. G. Wood did much to make aquaria popular. In 1852 the Zoological Society of London erected a house for marine aquaria - the first official recognition of their scientific value. They are distinguished as marine, freshwater, and microscopic, according to the forms of life kept in them. For the first two the tanks may be of almost any shape; the worst is the glass globe, in which one often sees unfortunate gold fish imprisoned, without a spray of weed to shelter them from the glare of the sun. The best is an oblong tank, of which the width should be greater than the depth, to expose as large a surface as possible to the action of the atmosphere. Microscopic aquaria for the cultivation of minute organisms may be maintained in any small glass vessel. Some observers use zoophyte-troughs; and infusoria are generally bred in test-tubes containing water in which hay, straw, etc., is infused. The beginner may easily gain from books sufficient information to start with; he will soon acquire experience and find friends ready and even eager to help him. It will, however, greatly enhance his pleasure if he has some definite object in view, say the working out of the life-history of some animal or plant, and in this way he may make some solid contribution to the sum of scientific knowledge. Aquaria are part of the equipment of every zoological station (q.v.); the name aquarium is often used to denote a place of entertainment in which the scientific meaning of the word is quite secondary or altogether lost sight of.