gg APPLICATION TO SPECTRUM STAINS. 



come to consider their mode of origin, as thus connected with absorptive influence, 

 nothing is more plain or easy to understand. We must regard them as phenomena 

 of exactly an equivalent character to those of the different appearances exhibited by the 

 luminous spectrum when it is received on variously- coloured paper. For, in respect 

 of the tithonic rays, surfaces that have the same tint to our eyes act like surfaces of 

 different colours. A solar spectrum received on a surface of lampblack is scarcely 

 visible. On a piece of red paper the red and orange rays are copiously reflected, the 

 others more or less absorbed. On a yellow paper the yellow and orange are brilliantly 

 given, but the blues almost disappear ; on a blue surface the more refrangible rays are 

 brightest, the yellows have faded away. So, when sensitive surfaces are exposed to the 

 spectrum, they give us an expression of their particular action. Bromide of silver ab- 

 sorbs more uniformly than any other body that we know rays of every refrangibility. 

 In respect, therefore, of the tithonic rays, it acts as lampblack does to the luminous, 

 and might be regarded as a black body. Iodide of silver absorbs the blue, and reflects 

 the red, the orange, the yellow, and part of the green. To eyes, therefore, which could 

 perceive those invisible rays which it reflects, it would be seen as though acting as a 

 ruddy-coloured body, and giving forth rays like those which nitrous acid gas transmits. 



352. When, therefore, we find on a sensitive surface which has been exposed to the 

 spectrum a given stain, we infer that rays corresponding in refrangibility to the place 

 of the stain have been absorbed, and the rest reflected or transmitted. And if this be 

 true, our views will be greatly facilitated if we resort to some simple method of nomen- 

 clature, which shall be descriptive of the facts observed. It is for this reason that we 

 have proposed to recognise the phenomenon of coloration for these dark rays, and speak 

 of red tithonic, yellow tithonic, or blue tithonic rays, as pointing out in a general man- 

 ner the place in the spectrum of the ray, the properties of which we are discussing. 

 Extending these ideas to the physical characters of ponderable bodies, we would assert, 

 with MELLONI, that they have an invisible coloration of their own ; that bromide of sil- 

 ver is tithonic black, though as respects light it is white ; that iodide of silver is tithonic 

 red, though as respects light it is of a lemon yellow. 



353. Aided by these definite views of absorptive action, and the concomitant phe- 

 nomena of coloration, it becomes interesting to examine whether these principles are 

 applicable to cases in which chemical changes are brought about by the action of LIGHT. 

 It is with a view of showing that all these things hold in the decomposition of carbonic 

 acid by leaves under the light of the sun, that we have entered on this minute dis- 

 cussion. 



354. The law under which the discharge of vegetable colours in the solar spectrum 

 takes place has not escaped the penetration of Sir J. HERSCHEL, who has furnished us 

 with so much that is new in this department of science. " The rays effective in 

 destroying a given tint are in a great many cases those whose union produces a colour 

 complementary to the tint destroyed, or, at least, one belonging to that class of colours 

 to which such complementary tint may be referred." (Phil. Trans., 1842, p. 189.) 



355. Now this is nothing more than an expression of a particular case of absorptive 

 decomposition, in which light is the agent, and vegetable matter the substance involved. 



