190 Messrs. St. Landau and Ed. Stenz on the 



spectra, inherent to the atom or to the molecule ? In this last 

 case we should suppose that the vibrating mechanism lies in the 

 bond of: the atoms. The experiments described below show 

 that the atom o£ iodine is incapable of fluorescence, at least 

 in the visible part of the spectrum. It seems interesting to 

 notice that the result obtained is in accordance with the work 

 of Dunoyer* on the fluorescence of sodium vapour. This 

 author showed that the fluorescence of. pure sodium vapour 

 is reduced to the D-lines ; he admits that the complicated 

 fluorescence spectrum hitherto observed with sodium vapour 

 is due to the aggregates of molecules of sodium with the 

 molecules of the impurities, perhaps hydrocarbons. Thus in 

 the case of iodine and sodium, as well as in the case of 

 mercury, one would think that the structure of the atom is 

 not of an excessive complication. 



It was impossible to foresee the results obtained, as the 

 absorption spectrum of the monatomic iodine is yet unknown. 

 Konen j", who did the most complete work on the spectra of 

 iodine, states that he only observed that the absorption 

 spectrum of iodine becomes less marked with increasing 

 temperature, but he never remarked its complete vanishing ; 

 he also never observed a separate absorption spectrum 

 belonging to the dissociated iodine vapour (/. c. p. 259). 



The plan of our work was very simple. We raised the 

 temperature of iodine vapour at a known low pressure and 

 observed its fluorescence, the degree of dissociation being 

 calculated. Three factors come then into account : tempe- 

 rature, pressure, and dissociation. The pressure remained at 

 a constant value in the majority of our experiments. It was 

 important to study separately the effect of the temperature 

 and the effect of the dissociation. The separation of the two 

 factors can be only partially done, as will be seen in the 

 following. 



II. The effect of the temperature', observations below 400° C. 



R. W. Wood and W. P. Speasj have made a photometrical 

 study on the fluorescence of saturated iodine vapour between 

 — 30° and +75° 0. In that research it was found that 

 the maximum of the fluorescence corresponds to 20°-25° G. 

 With higher temperatures the fluorescence decays rapidly ; 

 at 75° 0. the fluorescence is quite invisible. It is clear 

 that in these experiments it was vapour density that played 



* D. Dunoyer, Le Radium, 1912, p. 177. 



■1- Annalender Physik, vol. lxv. p. 257 (1898). 



X Phil. Mag-, vol. xxvii. p. 531 (1914). 



