78 SCIENCE PROGRESS 



able parallelism (already indicated by F. H. Gee and the writer) 

 between the phenomenon of photochemical inhibition and that 

 of resonance investigated during recent years by R. W. Wood. 

 Gee and the writer 1 comment on this coincidence in the follow- 

 ing terms : 



" Concerning the mechanism of the photochemical changes 

 under consideration, our own view is briefly this. Chlorine, 

 when it is absorbing light, preserves, for a time, the transformed 

 light energy in efficient forms which are gradually changed and 

 finally become the ordinary heat energy of the system, the rate 

 of degradation being considerably greater in the presence of 

 certain impurities. This efficient energy confers on the gas the 

 property of reacting with other substances for which it possesses 

 an affinity and therefore the presence of those impurities 

 which hasten the degradation of energy is a circumstance that 

 can only result in a reduction in the rate of a possible photo- 

 chemical change. 



" It might be urged that if efficient energy is accumulated in 

 the chlorine in the manner assumed and that if consequently 

 the light is not instantly degraded to the state in which it 

 exists in the unilluminated system at the same temperature, it 

 ought to be possible to demonstrate the existence of this energy 

 in the illuminated gas by some physical means. The work of 

 R. W. Wood on the resonance spectra of the elements would 

 appear to have a direct bearing on this aspect of the question. 

 Five years ago it was shown by this investigator that iodine — 

 an element allied to chlorine — in the state of vapour emits a 

 green light when the rays from an arc-lamp act on it and that in 

 the presence of small quantities of oxygen the fluorescent light 

 is considerably reduced in intensity. At that time an unsuccess- 

 ful attempt was made to show that chlorine would fluoresce 

 under similar conditions. Quite recently Wood has returned to 

 the subject 2 and his latest results are such as to strengthen the 

 conviction that there is a close relationship between the phenc- 

 mena investigated by him and those observed in the study of 

 photochemistry. He has now shown that, when the pressure 

 is sufficiently low, bromine vapour can be made to fluoresce, 

 a fact which very considerably increases the probability that 

 chlorine, exposed to light rays, will ultimately be shown to be 

 capable of retaining the absorbed energy in an efficient form for 

 a sufficient length of time to give rise to the phenomenon of 

 fluorescence. What is still more significant is the influence of 

 impurities on the fluorescence of iodine vapour. When the 

 vapour is excited by monochromatic light — the green light of 



1 Trans. Chem. Soc. 191 1, 99, 1727. 



2 Phil. Mag. 191 1 [vi], 21, 261, 309 and 314. 



