64 The Light from Mercury Vapour. 



positive ions as well as by the electrons. Consequently, with 

 160 volts a condition was being approached where both 

 positive and negative ions were able to ionize by impact. 



The appearance of the photographs favours the view that 

 there was not a displacement of the light, but an actual pro- 

 duction of new centres of luminosity. When no field is 

 present, the luminosity gradually fades out as the gas passes 

 from the opening. When the field is applied, it does not in 

 any case gradually fade out. In some cases the whole region 

 between the electrodes becomes luminous. In all cases the 

 region near the positive terminal is luminous. In photograph 

 No. 11, for example, which was a photograph of mercury 

 vapour, the luminosity first decreases as the gas leaves the 

 opening, and then increases as the positive terminal is 

 approached. It is very difficult to see how the luminosity 

 could increase, unless new centres of radiation were formed. 

 The ions do not give out more light as they move from the 

 opening, as is shown in the photograph where there is no 

 field. The application of an electric field to this region 

 causes the ions to move out of the region, so that there are 

 fewer ions per unit volume. Moreover, the strength of the 

 field will become greater in the neighbourhood of the elec- 

 trodes, and the ions will move faster, so that the number of 

 the ions per unit volume will be less, unless new ions are 

 formed near the electrodes. If, then, the photograph repre- 

 sents correctly the appearance of the glow, there must be new 

 centres of illumination formed near the positive electrode. 

 It is difficult to see how this can occur, unless there is 

 ionization by impact near the electrodes. 



Again, the increase in luminosity in the neighbourhood of 

 the negative electrode is most prominent when the density 

 of the gas is least. This is also what would be expected if 

 the luminosity is due to ions which are produced by positive 

 ions. It would only be with the more rarefied gas and 

 longer mean free path that the positive ions would be able to 

 ionize. When both the positive ions and the electrons are 

 able to ionize, it only requires a small increase in voltage 

 to start an arc; and this, in fact, was what was found to occur. 

 It would, therefore, appear possible that the luminosity 

 observed by Strutfc was caused by recombination of positive 

 and negative ions, and not by any action of the positive or 

 negative ions alone ; and that further experiments are 

 necessary before a definite conclusion is reached concerning 

 this phenomenon. 



Colgate University. 

 Sept. 1918. 



