10 THE TANK. 



light ; and as tliis is the vivifying principle of 

 animal life, the importance of the process will be 

 readily acknowledged. 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. 



Heat. — 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 de- 

 sirable to draw down the window-blind, or to inter- 

 pose, a curtain of muslin, oiled-paper, or gTound 

 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 

 Guyon 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, — luminosity, 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 in- 

 fluence, effectually prevent the water from getting 

 over heated, by an^esting the progress of the red or 

 heat giving rays ? " 



