Mr Whiddington, Note on the electrical behaviour, etc. 189 



Note on the electrical behaviour of fluorescing iodine vapour. 

 By R, Whiddington, B.A., Hutchinson Student of St John's 

 College. (Communicated by Prof. Sir J. J. Thomson.) 



[^Received 4 August 1909.] 



In view of the fact that no satisfactory theory of fluorescence 

 exists, it was thought likely that a study of the electrical be- 

 haviour of iodine vapour when fluorescing might give valuable 

 results. 



The apparatus employed consisted of a wide glass tube into 

 which a cylinder of copper gauze slipped tightly. The light 

 could be concentrated within the tube through a hole cut in the 

 gauze. 



The electrodes were discs of copper placed parallel and at a 

 distance of about three centimetres apart. One electrode was 

 earthed and connected to the gauze sheath, the other being 

 connected to a sensitive Wilson gold leaf electroscope. 



In the first trials ebonite insulation was used, but as the 

 iodine — which was liberally sprinkled within the vessel — was 

 found to attack it, quartz was substituted in its stead. 



The vessel was connected to a mercury pump and evacuated 

 to the limit set by the vapour pressure of the iodine {-^ mm. 

 about). 



When a beam of arc light was focussed between the electrodes, 

 the fluorescence was plainly visible but no trace of ionization 

 could be detected even when the applied electric field was almost 

 enough to produce a discharge. 



To be quite certain that the ionization chamber and electro- 

 scope were in good working order, a test was made with a weak 

 sample of radium. Even at this low pressure there was a very 

 rapid deflection of the gold leaf 



This experiment conclusively shows that ionization is not 

 necessarily an accompaniment of fluorescence. 



