.Dissociation of Iodine Vapour and its Fluorescence. 191 



the most prominent role. The light cannot penetrate dense 

 layers of vapour; the light of the fluorescence is also absorbed 

 on its way to the eye of the observer. 



When our experiments were terminated, we learned that 

 W. H. Westphal* had also observed below 400° C. the effect 

 of the temperature on the fluorescence of iodine vapour, kept 

 at constant pressure. But as that author has accomplished 

 only few and rough measurements (he attributes a certain 

 sense only to the mean value), we give here a short description 

 of our experiments, which seem to us more complete and 

 more conclusive. 



In all our experiments we used iodine carefully purified by 

 distillation in vacuo. The apparatus joined to the pump is 

 shown at fig. 1. Purified iodine crystals were put in part G; 



( =2 



Fig. 



JV 



M 



K^J 



the Graede mercury pump working, the whole apparatus was 

 strongly heated during 1-2 hours, excepting the lower part 

 of C, containing iodine crystals; the U-tube immersed in 

 solid C0 2 served to exclude mercury vapour from the 

 apparatus and iodine vapour from the pump ; the apparatus 

 was then sealed at M. During 24 hours some of the iodine 

 sublimated from A to B; then the apparatus was closed at N. 

 The bulb A, which served for observations, was from Jena 

 glass (called "durobax 5 "' — with red stripe), as ordinary 

 glass was found to be attacked by iodine vapour at high 

 temperatures. 



The photometer we used was the same as is described by 

 Wood and Speas (I. c.) with little improvements. Through 



* Verhandlungen d. Deutschen Phi/sik. Ges. ]914, p. 829. 



During the war (and even now) it was impossible to receive regularly 

 scientific journals at Warsaw ; the authors excuse themselves for having 

 perhaps omitted some papers concerning the question treated. 



