258 THE TANK. 



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 draw down 

 the window-blind, or 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. 



On this subject, a suggestion made by Mr. George 

 Guy on in the " Zoologist " for March, 1856, is 

 worthy of attention. " Since photography has become 

 a popular science, it is pretty generally known that 

 the three principles existing in common light — lumi- 

 nosity, heat, and chemical action — are to a great 

 extent separable, and reside respectively in the yellow, 

 red, and blue rays of the spectrum. It is, moreover, 

 I believe, considered that growing plants decompose 

 carbonic acid, and liberate the oxygen under the 

 influence of the luminous or yellow rays : if this latter 

 opinion is correct, would not the interposition of a 

 screen of yellow glass, while giving free admittance to 

 the purifying influence, effectually prevent the water 

 from getting over-heated, by arresting the progress of 

 the red or heat-giving rays ? " 



