Study of the fluorescence of Iodine Vapour. 53a» 



molecules, when an equal number of similar molecules has 

 been interpolated. We raise the temperature to 7°, the 

 pressure doubles (*06 mm.), and the intensity increases to 3G.. 

 We have, however, measured the radiation from all of the 

 molecules, and we are concerned only with that which is 

 emitted by the original group, which contributes one half of 

 the measured intensity ; consequently we must divide the 3& 

 by 2, which gives us 18. The intensity of the radiation of 

 the group has been reduced from 24 to 18 by an increment 

 of pressure equal to *03 mm. 



At a temperature of 11° the pressure is '09 mm. and the 

 intensity is 47, one third of which, or 15*7. is contributed by 

 the original group ; consequently the intensity is reduced 

 from 24 to 15' 7 by a pressure increment of '06 mm. 



We can in this way construct a curve showing the decrease 

 in intensity resulting from the interaction between the 

 molecules. 



It is to bo noted, however, that we must choose, for the 

 original group, a mass of vapour at a pressure below that at 

 Avhich the action of one molecule upon the radiation from a 

 neighbouring one is appreciable. 



If we take as our starting-point the intensity 12, at a 

 pressure of *015 mm. we find that at '03 mm. the intensity 

 is 24 : one half of this is 12, our original value — in other 

 words, no reduction in intensity lias resulted from an incre- 

 ment of pressure of *015 mm. At *045 mm. the intensity is 

 30, one third of which is 10, a slight reduction having 

 occurred. 



In this way curve D was computed, the values calculated 

 being multiplied by 8-t so as to make the intensity of the 

 radiation from the vapour at the lowest pressure equal to 100. 

 This curve shows us the extraordinary effect of iodine vapour 

 upon its own fluorescence, the vapour at 1 mm. pressure 

 reducing the intensity from 100 to 5. 



It is interesting to compare the action of iodine vapour 

 with that of the gases and vapours studied in the earlier 

 work. In these other cases a constant temperature was 

 employed, with the result that the iodine vapour density 

 remained unchanged, and the intensity of the fluorescence 

 was measured when various gases at different pressures were 

 introduced into the bulb. 



The intensity is reduced from 100 to 19 by hydrogen at 

 24 mm., by air at 11 mm., by COo at 7 mm., by ether vapour 

 at 3 mm., by chloride of iodine at 1*8 mm., and hy iodirn 

 vapour at a pressure of only 0*4 mm. 



