196 Dissociation of Iodine Vapour and its Fluorescence. 



Thus dissociation and disappearance of the fluorescence 

 are parallel to each other. 



One more experiment may be mentioned, which seems to 

 confirm our idea. The unsaturated iodine vapour, heated 

 while the volume remains constant, is less dissociated at a 

 given temperature than the saturated vapour kept at a constant 

 pressure. This is obvious without any calculation, as the 

 raising of the pressure causes a diminution of the dissociation. 

 We isolated the quartz bulb containing iodine vapour from 

 the tube with the crystals. A quartz rod was sealed to the 

 bulb and the last placed in the iron pipe. In that case we 

 observed the fluorescence at 825° C. 



To be quite conclusive the experiment should be arranged 

 as a differential one. We were impeded by material diffi- 

 culties from accomplishing this, but we intend to do it next 

 time. The calculation shows that the experiment is a hopeful 

 one, as is shown in the table below containing the fractions 

 of dissociated molecules in the case of a vapour kept at a 

 constant pressure of j mm. mercury, and in the case of 

 vapour kept at constant volume (quartz vessel), having 

 originally (20°) a pressure/of J mm. mercury. 



Temperature 500° C. 600° 700° 800° 900° 



Fractions of diss. mol. at const, press. 0*1 0'36 076 095 099 

 „ „ „ „ volume 0-06 0-20 047 0-76 0-90 



IV. Conclusions. 



A. The raising of temperature does not produce as much 

 effect on the fluorescence of the iodine vapour as has hitherto 

 been thought. We have observed the fluorescence even 

 above 800 6 C. 



B. Dissociation destroys the fluorescence and the resonnnce 

 spectra. 



Thus the complicated vibrating system, corresponding to 

 the thousands of absorption lines in the visible part of the 

 spectrum, is not inherent in the atom, but in the molecule. 

 The structure of the atom should be relatively simple. That 

 idea based on our observations is perfectly in accord with 

 other facts from the domain of the fluorescence of vapours. 

 The monatomic mercury vapour gives a simple resonance; 

 the complicated fluorescence spectrum of the sodium vapour, 

 as shown by Dunoyer, is due to impurities ; pure sodium gives 

 D -lines only. 



It seems also nearly certain that the absorption lines, which 

 are so characteristic for the diatomic iodine and so sensitive 



