102 



COLORIFIC ABSORPTION. 



395. By having the two spectra side by side, and close to one another, they are 

 placed under circumstances most convenient for making a perfect comparative estimate 

 of the light which is lost. In this manner the following table has been constructed. 

 The specific gravity of the solutions is not given, as it is not supposed that any direct 

 connexion exists between the density of a solution and its absorptive power. Much 

 more depends on the shade of colour. 



TABLE OF COLORIFIC RAYS ABSORBED BY SOLUTIONS. 



Name. 



Rays absorbed. 



Bichromate of potassa blue, violet. 



Prussiate of potassa ....... extreme red, extreme violet, yellow. 



Sulphate of copper extreme red. 



Chloride of gold violet. 



Chloride of platinum extreme violet. 



Sulp. copper and ammonia .... red, yellow. 



Solution of tannin violet, indigo blue, orange, and a part of green. 



Solution of litmus orange, yellow, green, extreme violet. 



Chromate of potassa extreme red, blue, violet. 



Linseed oil violet, indigo blue. 



Hydro-sulphate of lime violet, blue. 



Decoc. logwood in alum-water . . . orange, yellow, blue, and green. 



Decoc. of Brazil wood violet, indigo blue, green, yellow, orange. 



Cochineal in cream of tartar solution . yellow and part blue. 



396. Some remarkable phenomena may be produced by taking double solutions ; a 

 beam which has passed through a stratum of solution of sulphate of copper and ammo- 

 nia, and then through a decoction of Brazil wood, becomes almost totally extinct. 

 On looking through such solutions separately at the noontide sun, he appears with 

 overpowering effulgence, but on using them together, only a very faint trace of a dirty 

 olive-green light indicates his position. The sulphate of copper and ammonia absorbs 

 the red rays, and the Brazil-wood decoction nearly all the remainder. 



397. Already have some of these phenomena of absorption, in the hands of Sir D. 

 BREWSTER, disclosed important facts respecting the colorific rays. The colour of the 

 sky, and of the clouds, and of the sea, has also been long attributed to an action of this 

 kind, exercised by thick masses of air, or vapour, or water. 



398. But this action is not alone confined to the rays producing vision, it extends to 

 the other elementary constituents of the spectrum. While the trough (b,fig. 43) is filled 

 with a solution of sulphate of copper and ammonia, if the prism and the metallic screen 

 be removed, and a very delicate thermometer be plunged in the ray, a new phenome- 

 non is discovered ; the ray is found to be, to a great extent, deprived of the power of 

 exciting heat, and the thermometer shows little disposition to rise. How is this ! is it 

 because the red ray is gone that the sunbeam has lost its power of exciting a sensa- 

 tion of warmth ? It was at one time supposed that as the violet ray had the power of 

 determining chemical change, so the red ray possessed the power of exciting calorific 

 impressions. 



399. Fill the trough next with a strong decoction of Brazil wood, analyze the light 

 which passes through it by the prism (391), and it will be found that all the rays have 

 been absorbed except the red. Now, in such a beam, if the red ray possess inherent 

 caloric, the thermometer should rise as much, or nearly as much, as if it were in the 

 direct solar ray ; if the colour passes unabsorbed. so too should the caloric ; but, place 

 the thermometer in the beam, and it does not rise. Or throw a concentrated column 



