20 On the Radiating and Absorbing Powers of different Bodies. 



iect by M. Foucault to the Soc. Philom. in 1849 is reproduced 

 oy M. Verdet, from the Journal de Vlnstitut, in the April Num- 

 ber of the Ann. de Chim. et de Phys. 



M. Foucault* s observation appears to be regarded as essentially 

 the same as mine ; and for this reason I take the liberty of draw- 

 ing attention to the difference between the two. The observa- 

 tion of M. Foucault relates to the electric arch between charcoal 

 points, a phenomenon attended by circumstances which are in 

 many respects extremely enigmatical. My observation relates 

 to ordinary flames into which vapours of certain chemical sub- 

 stances have been introduced. By the aid of my observation, 

 the other may be accounted for on the ground of the presence of 

 sodium in the charcoal, and indeed might even have been fore- 

 seen. M. Foucault' s observation does not afford any explana- 

 tion of mine, and could not have led to its anticipation. My 

 observation leads necessarily to the law which I have announced 

 with reference to the relation between the powers of absorption 

 and emission; it explains the existence of Fraunhofer's lines, 

 and leads the way to the chemical analysis of the atmosphere of 

 the sun and the fixed stars. All this M. Foucault* s observation 

 did not and could not accomplish, since it related to a too com- 

 plicated phenomenon, and since there was no means of deter- 

 mining how much of the result was due to electricity, and how 

 much to the presence of sodium. If I had been earlier acquainted 

 with this observation, I should not have neglected to introduce 

 some notice of it into my communication, but I should never- 

 theless have considered myself justified in representing my ob- 

 servation as essentially new. 



3. Since the above communication was printed in Poggen- 

 dorfFs Annalen, I have learned in the course of a written corre- 

 spondence with Professor Thomson, that the idea was some years 

 ago thrown out, if not published, that it might be possible, by 

 comparing the spectra of various chemical flames with that of the 

 sun and fixed stars, in the manner I have described, to become 

 acquainted with the chemical constitution of the latter bodies 

 (an idea now demonstrated to be correct by the observations 

 and theoretical considerations above set forth). Prof. Thomson 

 writes : — 



" Professor Stokes mentioned to me at Cambridge some time 

 ago, probably about ten years, that Professor Miller had made 

 an experiment testing to a very high degree of accuracy the 

 agreement of the double dark line D of the solar spectrum with 

 the double bright line constituting the spectrum of the spirit- 

 lamp burning with salt. I remarked that there must be some 

 physical connexion between two agencies presenting so marked 

 a characteristic in common. He assented, and said he believed 



