472 Prof. Draper on the Interference Spectrum, 



sink to different depths, because the quality of coloration 

 which they possess enables them to absorb heat more or less 

 rapidly, and the calorific effect is determined by the optical 

 constitution. 



In the same manner, in another branch of actino-chemistry, 

 PHOTIC absorption; — 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 comple- 

 mentary to the tint destroyed, or at least one belonging to that 

 class of colours to which such a complementary tint may be 

 referred" (Phil. Trans., 1842, p. 189). 



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. The reason 

 that a yellow substance is bleached by the blue rays, is be- 

 cause it absorbs those rays', for the very same reason there- 

 fore that it looks yellow. A purple vegetable body is bleached 

 by the yellow and green rays, arid because it absorbs those rays 

 it looks purple. 



As respects light, the pha3nomena of coloration are obvious 

 to our organs of vision ; as respects the dark beam of heat, of 

 tithonicity, and of phosphorescence, they must be hypothetical 

 or ideal ; but in the same manner that Melloni has found the 

 admission of them for the calorific rays of such admirable ad- 

 vantage, so in this department of science similar and palpable 

 advantages arise. Who could for a moment doubt that light 

 and the tithonic rays were agents totally distinct, as soon as 

 he came to understand that to one of them iodide of silver is 

 yellow, to the other orange; to one chloride of silver is white, 

 to the other red; to one bromide of silver is white, to the 

 other it is black ? 



On this subject of ideal or imaginary coloration, I do not at 

 present propose to enlarge ; on a future occasion I shall re- 

 turn to it. 



Let us now investigate the changes which take place in the 

 sensitive body. The substance 1 have used is a mixture of 

 chlorine and hydrogen, one of the most sensitive combinations 

 known. 



When a mixture of chlorine and hydrogen is exposed in a 

 tithonometer to the rays of an Argand lamp, with due pre- 

 cautions to ensure a correct result, four distinct periods of 

 action may be traced. 



1st. For a brief space the mixture expands. 



