276 THE AQUARIUM AND WATER PLANTS. 



The weight of these tanks, when filled with water, is 

 very great, therefore strength is a matter for consideration. 

 The joints must be strong to resist internal pressure, and 

 the table firm enough to bear the weight. The slate bot- 

 tom and ends must have suitable grooves to receive the 

 glass, which should be heavy plate or very thick crown. 



White lead putty may be used; there are, however, 

 many kinds of water -proof cement equally good. 



The most elegant outline of straight lines and right 

 angles will be attained by giving the tank the form of 

 the double cube, in which the length is exactly double 

 that of the width, the width and depth being equal. 



A window is the proper place for the- aquarium. An 

 abundance of light promotes the growth of the plants, 

 and secures that succession of tiny globules which the 

 vegetation sends up from the bottom, and which in the 

 sunlight rise rapidly to the surface. This is one of 

 the prettiest features of an aquarium. These bubbles, 

 which ascend from some plants in continuous streams, are 

 globules of oxygen, and are produced by the plants which 

 decompose the water into its elements, retaining the 

 hydrogen for their own growth, and liberating the oxygen. 



When there is no sufficient exposure to the sun, none of 



