Dissociation of Iodine Vapour and its Fluorescence. 195 



other parts were of glass. Glass and quartz Mere joined 

 together hy chalk sealing-wax ; a compact layer of clean 

 asbestos interposed between the quartz and glass tubes 

 prevented the iodine vapour from coming in contact with 

 the sealing-wax. The iodine was introduced in the apparatus 

 in the way described above. 



In an experiment the tube with iodine crystals was held in 

 a water- or air-bath of suitable temperature. An intense 

 light-beam was directed on the bulb, and then the bulb heated 

 in the flame of the Meker burner. The fluorescence disappears 

 gradually; the bulb seems optically empty. After removing 

 the flame, the fluorescence reappears. The phenomenon 

 was observed as well with carbon-arc as with mercury-arc 

 light, and employing iodine vapour of different density. The 

 temperature of the flame exceeds 1000° 0.: using iodine 

 vapour of the pressure of j mm., Ave had complete dis- 

 sociation. 



It seemed to us to be of interest to determine the tempe- 

 rature of extinction of the fluorescence. For that purpose 

 we employed a large iron pipe of 70 cm. length and 8 cm. 

 diameter. That pipe was vertically set ; the large flame of 

 the Meker burner was introduced into the lower part of the 

 pipe. The pipe was covered by a piece of asbestos provided 

 with an opening, the size of which could be changed. In 

 that manner the draught and also the temperature in the 

 different parts of the iron pipe were regulated. The walls of 

 the pipe scarcely emitted any light, although the tempe- 

 rature near the axis in the middle part of the pipe reached 

 800-900° C. The middle part of the pipe was protected 

 externally by a thick layer of asbestos. Four openings were 

 made, two of a diameter of 9 mm. for the passage of the 

 light ; a large window of a diameter of 36 mm. served for 

 the observation of the fluorescence, it was placed somewhat 

 above the openings mentioned ; a fourth little opening per- 

 mitted the introduction of a thermocouple platinum-platino- 

 rhodium. The three larger windows were covered with thin 

 mica sheets. The joint of the thermocouple was placed 

 very near to that place on the quartz bulb where the light 

 came out. The lateral tube containing iodine crystals was 

 kept at a constant temperature of 20° C. by a stream of 

 water. 



Using the light emitted by a little carbon arc-lamp we 

 were able to state, that the fluorescence is very distinct still 

 at a temperature of 700° (■. ; at 780° C. when the fraction 

 of dissociated molecules was equal to about 0*9, we were 

 able to distinguish a faint glimmer of fluorescence. 



