A SUITABLE ASPECT. 259 



smell which is perceived on lifting the cover, espe- 

 cially if there he many sea-weeds in the Tank. 



Aspect. — The free access of light to the plants is 

 indispensable, and therefore that situation is the best 

 where the sun's rays fall most freely on their leaves. 

 It is beautiful to see the thousands of tiny globules 

 forming on every plant, and even all over the stones, 

 where the infant vegetation is beginning to grow ; — to 

 see these globules presently rising in rapid succession 

 to the surface all over the vessel, and to see this 

 process going on uninterruptedly as long as the rays 

 of the sun are uninterrupted. 



Now these globules consist of pure oxygen, elimi- 

 nated by the vegetation under the stimulus of light ; 

 and as this is the vivifying principle of animal life, 

 the importance of the process will be readily acknow- 

 ledged. The difference between the profusion of 

 oxygen-bubbles produced on a sunny day, and the 

 paucity of those seen in a dark, cloudy day, or in a 

 northern aspect, is very marked. 



Yet there is one caution required. In summer the 

 heat of the solar rays is very great, as well as their 

 light ; and if the vessel be small, and the volume of 

 water very limited, it will become tepid in the mid- 

 day sun, and the animals will be killed. Hence in a 

 fierce summer day, it will be desirable to interpose a 

 curtain of muslin, oiled-paper, or ground glass, which 

 will break the full power of the rays, without greatly 

 interfering with their illumination. 



Cost. — On this point a hint or two, the record of 

 my own experience, is all that I can give. If an 

 Aquarium of considerable dimensions be required. 



