446 Mr. J. Moser on the Spectra of 



ment whose lines we observe is at one time chemically com- 

 bined, and at another in the free state ; but these spectra do 

 not allow us to associate the presence of a distinct compound 

 with a definite temperature. If the temperatures at which 

 observations are made be varied, KirchhofF's fundamental pro- 

 position forbids us to suppose that the spectra should remain 

 unaltered. 



But the observation of absorption-spectra enables us, I 

 believe, to answer the question already proposed, and to 

 answer it in the affirmative. 



My experiments show that large alterations in the mass of 

 material and in the temperature are without material influence 

 upon the absorption-spectra examined by me ; hence the dif- 

 ferences noticed in these spectra cannot be attributed to either 

 of those causes. 



The question arises, whether absorption-spectra alone, with- 

 out the simultaneous observation of emission-spectra, are suf- 

 ficient to characterize chemical substances. This question 

 may, 1 think, be answered in the affirmative. The molecules 

 of some substance, iodine for instance, may be regarded as 

 vibrating so as to give forth light ; and at the same time they 

 may absorb a portion of the white light passing through them ; 

 if the temperature be decreased a point will be reached at 

 which the vibrations will not be sufficiently rapid to produce 

 light, but the molecules will nevertheless continue to absorb 

 light. But it cannot be supposed that at this point KirchhofTs 

 fundamental proposition should cease to hold good. 



The following experiments show an analogy between absorp- 

 tion- and emission-spectraj: the former behave themselves at 

 low temperatures analogously with the latter at high tempera- 

 tures ; changes in the intensity of the absorption-bands are 

 observable coincident with changes in mass and in tem- 

 perature. 



The experiments were carried out with iodine, with bromine, 

 and with nitrogen tetroxide. I shall first describe those ex- 

 periments in which mass of material underwent variation. 



The apparatus was arranged so that two spectra could be 

 simultaneously brought into the field of vision. Thalen's ab- 

 sorption-spectrum of iodine served as a scale with which to 

 compare the bromine-spectrum. The following numbers ex- 

 press the wave-lengths of the bromine absorption-lines. I 

 have numbered the lines, beginning with that corresponding 

 with D, +1, +2, &c. in the blue, and —1, —2, &c. in the 

 red: — 



