Dissociation of Iodine Vapour and its Fluorescence. 193 



galvanometer-mirror m occupied the centre of the field of 

 vision. Thus we realized something like the Lummer- 

 Brodhun photometer. 



The experiments were also performed using a mercury- 

 quartz lamp (Westinghouse, Cooper-Hewitt); in this case 

 the screen, painted in orange-red, was illuminated separately 

 by a little incandescent lamp. 



The oven for the heating was an electrical one ; in the case 

 of the mercury lamp the observations were made "end " on, 

 and a gas-furnace supplied with suitable cuttings for the 

 passage of light was adapted. 



The pressure of the iodine vapour was kept constant by 

 placing the tube B (fig. 1) in a water-bath of the temperature 

 of 20° C. 



The results obtained were generally the same, whether 

 we used the incandescent or the mercury-lamp, and whether 

 the vapour was saturated or unsaturated. A typical curve 

 is shown in fig. 3: in this case, which corresponds to the 

 mercury light excitation, saturated iodine vapour at a pres- 

 sure of 0*25 mm. was used. In this figure the observations 

 taken with rising temperature are marked by points ; the 

 observations obtained when lowering the temperature by 

 crosses. 



Fig. 3. 



. 5 

 Sin. c< 



O, 2 



0,1 _. 



300° e 



100 200 



It will be seen that the intensity of the fluorescence dimin- 

 ishes with rising temperature,' but the diminution is not as 

 great as might be supposed. At 360° C. the fluorescence is 

 still very intense*. 



* We find in the Fortschrdte d, Physik, ii. p. 385 (1915) an abstract 

 from a paper of McLennan (Proc. Royal Society of London, A. xci. 

 pp. 23-26 (1919)). It is said there that the author has found a dis- 

 appearance of the resonance radiation at the temperature of 326° C. It 

 will be seen that our observations are opposite to that, as we did observe 

 the resonance radiation very clearly even above 700° C. It was im- 

 possible for us to obtain the" original paper of McLennan (see footnote 

 p. 191). 



