72 LIFE AND EXPERIENCES CHAP. 



origin to gaseous absorption of a similar character in the 

 solar atmosphere. Even now I think it likely that some of 

 the non-terrestrial lines in the solar spectrum may be of this 

 character, though after what Bunsen and Kirchhoff have done 

 I think it probable they are a minority. 



The idea of connecting the bright and dark lines by the 

 theory of exchanges had never occurred to me, and I was 

 greatly struck with it when I first saw it, which was in a 

 paper of Balfour Stewart's read before the Royal Society and 

 printed in the Proceedings. I was wrong in saying lines > for 

 B. Stewart considers only solids, the spectra of which don't 

 present such abrupt changes. Stewart's paper was inde- 

 pendent of, but a little subsequent to, Kirchhoff s, though the 

 same idea with reference to radiant heat occurs in two papers 

 of his printed in the Edin. Phil. Trans, and much anterior to 

 KirchhofFs paper. These papers I was not acquainted with 

 at the time when Stewart's paper on light came before the 

 R.S. 



We can by no means affirm from the theory of exchanges 

 that every dark line in the solar spectrum must be capable 

 of reversion. For it may be due to absorption by a 

 compound gas which is incapable of existing un-decomposed 

 at the temperature requisite for becoming luminous, or which 

 though not decomposed might yet have its mode of absorp- 

 tion completely changed, as we know that even a small 

 elevation of temperature is sufficient materially to alter the 

 absorption of light by NO 4 gas. 



As to the mention of the metallic lines in the invisible 

 region, I own to feeling a wish that the subject may be novel 

 when I bring it forward, and yet I can't help feeling that 

 that is mere selfishness and that I have no business to keep 

 it bottled up. When are your lectures to be given? If not 

 for some time perhaps I may draw up a note for the Royal 

 Society for publication in the Proceedings which would of 

 course set you quite free. 



Yours very truly, 



G. G. STOKES. 



In 1862 I induced Bunsen and Kirchhoff to visit 

 England. Kirchhoff had never done so, and Bunsen 

 only some thirty years before, when, as a young 

 man, Playfair took him to Alfreton to analyse, as 

 already stated, the gases of the blast furnaces. I here 



