Photometric Study of Fluorescence of Iodine Vapour. 531 



this, on liolitino- the burner a flame of a most astonishing 

 brilliancy is at once formed. So rapid is the evaporation 

 of the chloride that clouds of smoke rise from the flame, 

 and the intensity, while at its maximum, appears to be as 

 great as that of the oxy-hydrogen sodium flame, which is 

 much more difficult to manage. The simplicity of this 

 method makes it immediately available in any laboratory. 

 The function of the scrap of mantle is of course to spread 

 the material over a large surface of very small heat capacity, 

 so that it can be brought to the temperature of the hottest 

 part of the flame. The bead melts and the mantle acts like 

 the wick of a lamp. 



LXII. A Photometric Study of the Fluorescence of Iodine 

 Vapour. By R. W. Wood and W. P. Speas*. 



TIIHE reduction in the intensity of the fluorescence of 

 JL iodine vapour caused by the admixture of air or other 

 foreign gas was studied by one of the present writers a 

 number of years ago (Wood, Phil. Mag. xxi. p. 309, 1911) ; 

 and subsequently Wood and Franck (Phil. Mag. xxi. p. 311, 

 1911) discovered that the gases which were strongly electro- 

 negative were the most effective in reducing the intensity of 

 the fluorescence. Of all the gases studied the least effective 

 was helium, the intensity of the fluorescence of the iodine 

 vapour, when mixed with helium, even at two or three centi- 

 metres' pressure, being almost as great as in vacuo. The 

 colour of the fluorescent light was changed, however, from 

 yellowish -green to orange-red by the presence of the helium ; 

 and the curves obtained showed that this resulted from the 

 circumstance that the helium reduced the intensity of the 

 radiations of shorter wave-length in the fluorescent spectrum 

 to a greater degree than the less refrangible radiations. The 

 extensive investigations of the remarkable resonance spectra 

 emitted by the vapour when excited by monochromatic light, 

 which have been carried on during the past two years by one 

 of us, made a further photometric study desirable ; for it 

 appeared probable that careful determinations of the variation 

 of the intensity of the radiations with the density of the 

 iodine vapour would throw some light upon certain obscure 

 points : for example, the circumstance that the faint band- 

 spectrum which accompanies the resonance spectrum is 

 more strongly developed when the iodine vapour is at very 



* Communicated by the Authors. 



