692 Resonance Spectra of Iodine by Multiplex Excitation. 



while line 7 of the iodine absorption is in coincidence with the 

 right-hand component of the mercury line (seeP, PI. XVIII.), 

 I imagine therefore that this line is responsible for the right- 

 hand member of the triplet removed by preliminary filtration 

 of the green line through bromine. Further experimentation 

 will of course be necessary before we can be at all sure of 

 this point ; and I have attempted an analysis merely to 

 indicate the line along which work is possible. The effect 

 of the bromine filter is shown by the small photographs of 

 groups 5525 and 5660 attached to spectrum Gr. 



Transformation of the Resonance into the Band Spectrum 

 by Gases of the Helium Group. 



A further study of the remarkable transformation of the 

 resonance spectrum into the band spectrum, which occurs 

 when the iodine vapour is in helium at a pressure of a few 

 millimetres, has been made, and some photographs superior 

 to those published with my first paper have been secured. 

 These are shown on Plate XVII. Spectrum H is a portion of 

 the resonance spectrum between the green mercury line 

 (to the left) and the two yellow lines (to the right), the 

 iodine being in a high vacuum. Spectrum I shows the 

 effect of introducing helium at 2 millimetres pressure, and 

 spectrum J the effect of helium at 10 millimetres pressure. 

 The other gases of the helium group behave in the same 

 way, and I have recently found that the band spectrum can 

 be brought out by the presence of air at one or two 

 millimetres, though the intensity of the fluorescence is 

 enormously weakened, and long exposures are required. It 

 is my plan to investigate this side of the subject during the 

 coming winter. One of my tubes which originally was 

 highly exhausted, and showed only the resonance spectrum, 

 after long use showed traces of the band spectrum. I have 

 therefore prepared some tubes containing only iodine vapour, 

 that is no solid crystals, to see whether prolonged illumination 

 eventually destroys the vapour. It is of course a very delicate 

 matter to speculate about the result of any such experiments, 

 and sufficient data are not yet at hand to make their discussion 

 worth while. 



Destruction of the Fluorescence by Temperature. 



It will be remembered that I found that the fluorescence of 

 mercury vapour is destroyed by raising the temperature 

 of the quartz flask with a blast-lamp. The same thing is 

 true of iodine, which however loses its luminosity at a 



