76 THE BOOK OF NATURE STUDY 



goldfish often suffer from the disadvantage of being relatively 

 narrow at the top as compared with the base and the depth, 

 whereas the ideal vessel should be wide at the top and shallow, 

 so as to expose the water as fully as possible to the air. An 

 earthenware milkpan has many advantages, and, like a pie-dish, 

 is easy to obtain and keep clean. Both admit light from the 

 top only, which is the natural condition, while the disadvantage 

 of a glass aquarium is that it admits light all round, to an extent 

 which is directly injurious to some forms of life. If a glass 

 receptacle be used the direct rays of sunlight should never be 

 allowed to pass through it, as the resultant rise of temperature 

 of the water will at once destroy many forms of life. Indeed, 

 for an aquarium containing animals as well as plants a north 

 window should always be chosen. 



If, however, something more ambitious than the homely 

 receptacles named above is desired, an oblong or square tank 

 may be bought or made. This may be made with all four sides 

 glass and a wooden or metal base, or with only one side of glass 

 and the others wood or metal. Slate has also been recommended, 

 as well as potter's clay. In all cases a good rule is that the 

 depth should not exceed a foot, both because the difficulties of 

 aeration increase with depth, and because with a greater depth 

 it is difficult to reach the bottom, which will be necessary for 

 the removal of dead organisms, surplus food, and so on. Before 

 beginning, the aquarium should be thoroughly cleansed ; an 

 odour of pitch from caulking, or the presence of a cement which 

 has not completely set, may be fatal. After the cleansing pro- 

 cess, the vessel should be filled with pure water, and left for a 

 day or two, to ensure first that there is no leakage, and second 

 that there are no injurious soluble substances in the interstices. 

 In the simple forms of aquarium these precautions are unneces- 

 sary, and all that is necessary is to put the desired animals into 

 the vessel with water and a handful of floating weed, or in the 

 saltwater aquarium a piece of stone with seaweed attached. 

 The tank will require more careful treatment, which will differ 

 according as it is intended for freshwater or marine forms. In 

 the former case the plants will be least chiefly flowering plants 

 whose roots may require to be planted in sand, while in the latter 



