Study of the Fluorescence of Iodine Vapour. 53? 



to red by the admixture of helium with a constant amount of 

 iodine vapour, the same effect being observed in decreasing 

 degrees with argon, hydrogen, and nitrogen. No change of 

 colour was, however, observed when the intensity was reduced 

 by chlorine. The suggestion was made that a foreign gas 

 reduced the intensity of the fluorescence in tw T o ways — by its 

 electro-negative quality (the reduction in this case being 

 unaccompanied by change of colour), and by collisions, which 

 reduced the intensity of the short waves more than that of 

 the long. It is quite possible that the collisions weaken what 

 have been termed the resonance radiation lines more than 

 the lines of increased wave-length. 



In the case of the weakening of intensity by iodine vapour,, 

 the change of colour is probably largely due to absorption, 

 since measurements showed that the fluorescent light was 

 more strongly absorbed by iodine vapour than light of the 

 same colour obtained by filtering the light of the Welsbach 

 light through suitable colour-filters. This results from the 

 circumstance that the fluorescent spectrum is discontinuous^ 

 some of its lines coinciding with absorption-lines. If the 

 two fields of the photometer were matched, one being illumi- 

 nated with the fluorescent light, the other with the filtered 

 white light, the balance was destroyed if a bulb containing 

 iodine vapour was held between the eye and the photometer. 

 Measurements were also made by restricting the length of 

 the illuminated column of iodine vapour by means of screens,, 

 illuminating first the end of the tube farthest from the 

 photometer, and then the nearer end. 



The actual intensity of the fluorescence was the same in 

 the two cases, but in the former, owing to the greater 

 thickness of the layer of iodine vapour traversed by the 

 emitted light, the measured intensity was less. 



The results indicated that the portions of the illuminated 

 column nearest the photometer contributed more to the 

 intensity than the portions farther away. 



It was found that the absorption was much stronger for 

 the fluorescence of the vapour at 0° than at room -temperature, 

 amounting to 43 percent, in the former case and 29 percent, 

 in the latter, for a layer of iodine vapour at 23° 14 cm. in 

 thickness. The absorption was brought about by inserting 

 an exhausted bulb, 14 cm. in diameter, containing iodine 

 crystals, between the fluorescent tube and the photometer. 

 Instead of removing the bulb, to determine the intensity 

 without absorption, the iodine vapour was condensed by the 

 application of cotton wet with liquid air. In this way 



