532 Prof. Wood and Mr. Speas : A Photometric 



low density, the tube being cooled by ice. Moreover, it is 

 of considerable interest to determine to what extent the 

 luminosity of an iodine molecule is diminished by the 

 proximity of other iodine molecules ; in other words, to 

 determine the effect of iodine vapour at different pressures 

 upon the intensity of the iodine fluorescence, for comparison 

 with the effects of the various other gases determined in the 

 earlier work. 



In the present case, however, the matter is complicated by 

 the circumstance that an increase of pressure increases the 

 number of fluorescing molecules. 



It has been found possible, however, to allow for this 

 circumstance and construct a curve showing the destructive 

 action of iodine vapour upon the fluorescence of iodine 

 vapour, precisely analogous to the curves constructed for 

 helium, argon, nitrogen, hydrogen, &c. in the earlier 

 investigation. 



The iodine vapour was contained in an exhausted glass 

 tube of the same form as those used in the study of the 

 resonance spectra. The image of a quartz mercury arc 

 formed along the axis of the tube by a large condenser 

 excited a fluorescence of very constant intensity, which was 

 measured by a photometer viewing the fluorescent vapour 

 column " end-on." 



The photometer was of the same type as that used in the 

 earlier work, the comparison source being a white screen 

 illuminated by the light of a Welsbach manfle passed through 

 suitable filters for the purpose of matching the yellowish- 

 green colour of the fluorescence. The temperature of the 

 tube was raised by a water-bath, or lowered by the immersion 

 •of a small lateral tube in a bath of alcohol contained in a 

 small Dewar cup and cooled to any desired temperature by 

 the addition of liquid air. The density of the iodine vapour 

 is determined by the temperature of the coldest part of the 

 system, so that when working below room-temperature it 

 was necessary to vary only the temperature of the small 

 lateral tube. 



It was found that a measurable fluorescence was obtained 

 even with a density corresponding to — 30° G. It was im- 

 possible, however, to obtain an absolutely black background, 

 even with the end of the tube painted black for a distance 

 of 10 cm. To determine the small amount of diffused and 

 reflected light sent out by the background, it was only 

 necessary to immerse the lateral tube in liquid air, which 

 removed every trace of iodine vapour from the observation 



