•536 Prof. Wood and Mr. Speas : A Photometric 



It is probable that chlorine would be still more effective 

 than iodine, as it is more strongly electro-negative. The 

 value given above for chloride of iodine is the value given 

 in the earlier paper for chlorine, the fact having been over- 

 looked that the vapours unite to form the compound when 

 mixed. There was always an excess of iodine in the bulb, 

 so that there is little doubt but that the correct interpretation 

 of the experiment is to consider the active vapour iodine 

 chloride instead of iodine. 



Bromine vapour is more electro-negative than iodine, and 

 though it has an absorption spectrum similar to that of 

 iodine, its action in destroying its own fluorescence is so 

 powerful that it is only possible to observe fluorescence at 

 pressures probably in the vicinity of *001 mm., the intensity 

 then being so small that it is only with difficulty that the 

 phenomenon can be detected. Sunlight must be focussed at 

 the centre of the exhausted bulb, and the bromine vapour 

 condensed by applying solid C0 2 to the exterior : just before 

 the last trace of vapour is condensed, there is a very feeble 

 green fluorescence, of about the intensity of that shown by 

 iodine vapour at — 30°. 



The results obtained with iodine vapour at varying 

 pressure emphasizes the following general statement made 

 in the earlier pnper : — 



"In order to obtain a visible fluorescence we must have 

 a sufficient number of molecules present : their number 

 must not, however, be so great as to cause them to disturb 

 each other. The pressure at which maximum fluorescence 

 occurs depends upon the electrical character of the molecule." 



Absorption of the Fluorescent Lic/ht by Iodine Vapour. 



It is obvious that, for a correct interpretation of the 

 results found with the photometer, determinations of the 

 absorbing power of the vapour for the fluorescent light must 

 be made, since in all of the experiments the fluorescent light 

 is obliged to traverse a greater or less amount of absorbing- 

 vapour. 



The colour of the fluorescent light is distinctly red with 

 dense vapour, orange-yellow at room-temperature, and yellow 

 with a suggestion of green at the lowest temperatures. 

 While this change is due in part to absorption of the green 

 portion of the spectrum of the emitted light, there is un- 

 doubtedly another factor at work. In the earlier investiga- 

 tion it was found that the colour was changed very markedly 



