﻿and Tempered are Spectra of Iodine Vapour. 335 



On the theory proposed, this indicates that the bromine 

 molecule has a greater capacity for storing energy than the 

 molecule of iodine : in other words, it takes care of the absorbed 

 energy, without re-emission, during the interval between 

 collisions. 



The magnetic rotatory power appears to be less easily 

 affected by molecular impacts. 



The Temperature Emission Spectrum. 



The temperature omission of iodine vapour can be most 

 beautifully shown by means of a quartz bulb provided with a 

 short tubular stem. A small flake of iodine is introduced 

 into the bulb, which is then highly exhausted and sealed with 

 an oxy-hydrogen flame. The bulb is supported with a stiff 

 iron wire, and heated with a blast-lamp as shown in fig. 1. In 



Fit?. 1. 



a dark room it will be observed that, even at the high tempe- 

 rature of the blast-lamp, the quartz bulb radiates little or no 

 light, on account of its small absorption coefficient. If now a 

 small flame is applied to the stem containing the iodine, the 

 vapour enters the white-hot bulb, which instantly appears 

 filled with a highly luminous red cloud, which flashes into 

 view like a red flame. The spectrum of this light appeared 

 almost continuous in the three-prism spectrograph. There 

 was, however, a faint suggestion of bands in the orange region, 

 which made it appear probable that the continuity of the 

 spectrum might be due to too great depth of the radiating 

 layer. A.s is well known, the theory of radiation shows that 

 an absorbing vapour, when radiating a discontinuous spectrum, 

 will emit a continuous spectrum, if the thickness of the 

 radiating layer be sufficient. On heating the stem as well as 

 the bulb and projecting its image on the slit of the spectro- 



