48 Prof. Tyndall on the History of Negative Fluorescence. 



freely conceded to his insight what I deemed wanting in his 

 data. But now that he talks of my reasoning being the same as 

 his, save that it exhibits the defect of a missing link, let me 

 inquire what is his reasoning, and what is mine ? As far as I 

 can see, he does not reason at all. He founds his " opinion," 

 that the radiation from a hydrogen-flame is ultra-red, on the 

 defect of brightness which the flame exhibits. Now the flame 

 of hydrogen, and other gaseous flames, have been over and over 

 again compared, by chemists, with flames yieldingso/ic? products 

 of combustion, the defective brightness, in those cases where solid 

 matter was absent, being either referred to the extremely dilute 

 character of the radiation, or to the fact that intense combus- 

 tion generated rays of a refrangibility too high for the purposes 

 of vision. What has Dr. Akin done to prove this notion in- 

 correct ? Nothing. He never, to my knowledge, made a single 

 experiment on the radiation of a hydrogen-flame. Again, he 

 speaks of the probable paucity of rays of high refrangibility 

 in a hydrogen-flame, as one ground of his conclusions. On this 

 point let us hear Professor Stokes : — " It appears that the feeble 

 flame of alcohol is extremely brilliant with regard to invisible rays 

 of very high refrangibility. The flame of hydrogen appears to 

 aboundininvisible rays of still higher refrangibility^ (Phil. Mag. S.4. 

 vol. iv. p. 392). And again — " The effect of various flames and 

 other sources of light on sulphate of quinine, and on similar media, 

 indicates the richness or poverty of these sources with respect to 

 the highly refrangible invisible rays. Thus the flames .of alcohol, 

 hydrogen, fyc. were found to be very rich in invisible rays " (Pro- 

 ceedings of the Royal Institution, vol. i. p. 264). I have 

 italicized sentences which are diametrically opposed to the as- 

 sumed "poverty of rays of high refrangibility," on which 

 Dr. Akin, in part, founds his opinion. 



My " reasoning," such as it is, is contained in the December 

 Number of the Philosophical Magazine, p. 457, and at pp. 524 

 to 526 of the December Supplement. I start from the princi- 

 ple that, from the transparency or opacity of any homogeneous 

 medium to the radiation from any source, we may infer the dis- 

 cord or the accord of the vibrating-periods of the source and 

 medium. I illustrate this principle by a series of experiments 

 on carbonic acid, aqueous vapour, and water. First proving 

 the periods of water to be in accord with those of the ultra-red 

 waves, I give reasons for concluding that the periods of a 

 hydrogen-flame are in accord with those of water. Hence the 

 inference that the periods of a hydrogen-flame are ultra-red, and 

 that, when a platinum wire is raised to whiteness by the heat of 

 such a flame, we must have a change of period. 



Now these experiments, and the reasoning founded on them, 



