THE 

 LONDON, EDINBURGH, ,ixd DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



OCTOBER 1911. 



XLVIT. The Resonance Spectra of Iodine Vapour and their 

 Destruction by Gases of tKe Helium Group. By R. W. 

 Wood, Professor of Experimental Physics, Johns Hopkins 

 University, Adams Research Fellow of Columbia Uni- 

 versity *. 



[Plates IV. & V.] 



SHORTLY after the discovery of the resonance spectrum 

 of iodine vapour in vacuo excited by the radiations 

 from a mercury vapour lamp, the remarkable effect of the 

 presence of small traces of helium in transforming the 

 sharply defined resonance spectrum into the complete band 

 spectrum, such as is excited by white light, was described 

 by the author and J. Franck (Phil. Mag. Feb. 1911). It 

 was shown that the band spectrum appeared as a faint back- 

 ground when the helium was at a pressure of less than one milli- 

 metre, while a pressure of between 5 and 10 mm. destroyed 

 the resonance spectrum entirely, its place being taken by 

 the band spectrum. The total amount of light radiated in the 

 two cases was about the same, the helium appearing to act 

 as an agent in transferring the energy from the excited 

 system of electrons to all of the other systems, which 

 remained quiescent when the iodine vapour was in vacuo. 



In the present paper I propose to take up the further study 

 of the resonance spectra, and the effect of the other gases of 

 the helium group, as well as the remarkable polarization 



* Communicated by the Author. 

 Phil Mag. S. 6. Yol. 22. No. 130. Oct. 1911. 2 I 



