100 Lord Bayleigh — Spectroscopic Notes 



quality as well as quantity ; that is, this equality between' 

 the radiation and absorption must hold for every individual 

 description of heat "*. 



Surely this goes to the root of the matter, and it presents 

 the argument in its most natural form. KirchhofY's inde- 

 pendent investigation of a year and a half later f is more 

 formal and elaborate, but scarcely more convincing. 



No one in England or elsewhere disputes the great obliga- 

 tions under which Spectrum Analysis lies to KirchhofF. In 

 a passage quoted by Dr. Kayser {loc. cit. p. 92) from Lord 

 Kelvin — " To Kirchhoff belongs, I believe, solely the great 

 credit of having first actually sought for and found other 

 metals than sodium in the sun"" — the force of " solely" seems 

 to have been misunderstood. I have Lord Kelvin's authority 

 for interpreting this to mean that the entire credit of the dis- 

 covery mentioned belongs to Kirchhoff, not that he is 

 entitled to no credit in other directions. 



VI. Spectroscopic Notes concerning the Gases of the 

 Atmosphere. By Lord Rayleigh, F.R.S.% 



On the Visibility of Hydrogen in Air. 



MY first experiments upon this question were made in 

 July 1897. The sparks were taken between platinum 

 points in a small chamber through which dried air at 

 atmospheric pressure could be led, and the spectrum was 

 examined with a spectroscope of two prisms. The C-line 

 could be very nearly obliterated by careful drying. But if 

 ~ part by volume of hydrogen were added and the mixture 

 passed afterwards through the phosphoric anhydride, the 

 increased visibility of C was very marked. At that time I 

 was occupied with the density of carbonic oxide and 

 interested in the question as to whether it contained appreci- 

 able quantities of hydrogen §. When carbonic oxide, pre- 

 pared from prnssiate of potash and dried as for weighing, 

 was passed through the apparatus, the C-line became nearly 

 invisible ; but the test with carbonic oxide was thought to be 

 less delicate than with air in consequence of the proximity of 

 another bright line in the former case. 



I have lately resumed these experiments, induced thereto 

 principally by the remarkable results of M. Gautier. This 

 observer, working by chemical methods, finds that air normally 



* Edin. Trans, vol. xxii. p. 13, March 1858. 



t Moiiatsbericht der Akad. d. Wiss. zu Berlin, Dec. 1859. 



X Communicated by the Author. 



§ Proc. Koy. Soc. vol. 62, p. 205 (1897). 



