316 Mr. J. Franck and Prof. R. W. Wood on the 



One sees at once that helium, in spite of the fact that its 

 molecular weight is double that of hydrogen, is much 

 less detrimental to the fluorescence, and that the argon curve 

 runs nearly in coincidence with that of hydrogen, though its 

 molecular weight is twenty times that of hydrogen. Above 

 the air curve we have a point for nitrogen, and below it one 

 for oxygen, exactly as we should expect, for nitrogen is 

 nearly neutral, while oxygen is strongly electro-negative. 



The lowest curve of all is that for chlorine, with an atomic 

 weight of 70, much lower than the curve for ether with an 

 atomic weight of 75. It is clear that the affinity of a gas for 

 electrons is a powerful factor in suppressing the fluorescence. 



For helium, we have given two curves, one for the green 

 portion of the fluorescence spectrum, the other for the red. 

 The measurements were made in the two cases by suitably 

 selected colour filters. 



This separation was necessary in the case of helium, for it 

 was found that the colour of the fluorescence changed rapidly 

 from green to reddish orange, as more and more helium was 

 added, which made photometric measurements with a standard 

 of fixed colour impossible. 



As is apparent, the two curves cross, resulting from 

 the circumstance that in vacuo the green portion of the 

 fluorescent spectrum is much stronger than the red, but that 

 it is rapidly reduced in intensity by the addition of helium, 

 while the red portion is reduced to a much less degree. 



A similar effect, though to a much lesser degree, is 

 observed with hydrogen and argon. This change of colour 

 is probably a pure collision effect, for with electro-negative 

 gases, such as chlorine, it is not noticeable. The very feeble 

 fluorescence observed when we have 3 or 4 mms. of chlorine 

 in the bulb has practically the same colour as w r hen the iodine 

 is in vacuo. The cause of the colour change will be given in 

 a subsequent paper dealing with the resonance spectrum of 

 iodine recently discovered by Wood. It seems probable that 

 when an iodine molecule is near enough to a chlorine 

 molecule to be influenced at all, its fluorescence is practically 

 destroyed. The fluorescence observed when chlorine is 

 present probably comes from those iodine molecules which 

 at the moment happened to be beyond the sphere of action of 

 any chlorine molecule. Their number will be fewer and 

 fewer as the pressure of the chlorine increases. 



We have also investigated qualitatively the effects of these 

 gases upon the fluorescence of mercury vapour, which has 

 already been extensively studied by Wood *. Mercury 

 * Phil. Mag. vol. xviii, p. 240 (Aug. 1909). 



