COVERING — ASPECT. 257 



COYERIXG. — Within an inhabited room, or wherever 

 there is much liability to dust or soot, as there is 

 necessarily everywhere in cities and large towns, the 

 Aquarium may be protected by a cover. This may 

 be made of fine muslin, or, which is better, of plate- or 

 sheet-glass, according to the dimensions required. 

 The latter may simply be laid over the top of the 

 vessel, allowing the escape of gases under the edge. 

 It should be occasionally lifted for a moment, to 

 allow of a change of the superincumbent air : — the 

 necessity of this will be manifest, from the close 

 smell which is perceived on lifting the cover, espe- 

 cially if there be many sea-weeds in the tank. 



In ordinary circumstances, however, there is no 

 necessity for a covering of any kind. My own tanks, 

 though placed in an inhabited room, remain for 

 months together uncovered, in winter and summer, 

 without the least loss of transparency. The dust 

 speedily sinks, and is harmless. 



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 smface all over the vessel, and to see this 

 process going on uninten-uptedly as long as the rays 

 of the sun are uninterrupted. 



Now these globules consist of jpure oxygen^ elimi- 

 nated by the vegetation under the stimulus of light ; 

 and as this is the vivifying principle of animal life, 



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