322 Mr Henry, Experiments on Ultra-violet Light, etc. [May 24. 



vapour is present or not, these experiments do not prove that 

 there is absolutely no conduction of the charge through the 

 vapour due to the light on it, but they do prove that if this 

 conduction exists at all, it must be very small. 



In testing amyl nitrite, the vapour was allowed to rise round 

 an insulated copper disc connected to an electrometer as before, 



Fig. 2. 



but when the disc was charged positively, the arc-light caused no 

 discharge whatever, whether the vapour was present or not. 



In another experiment on amyl nitrite, a beaker containing 

 some of the liquid was placed in coil " a " (fig. 2), and a bulb for 

 giving the electrodeless discharge in coil " b " which was in series 

 with " a," and the number of turns in the coils adjusted, until a 

 substance of very small conductivity placed in " a " destroyed the 

 discharge in " b." 



This arrangement showed the conductivity produced by the 

 Rontgen rays in the air in coil " a," yet it gave no indication of 

 conductivity in the vapour of amyl nitrite, when the light from 

 an electric arc placed over the open beaker fell on the vapour. 



In conclusion, I have to thank Professor J. J. Thomson for 

 his many valuable suggestions in the course of these experiments. 



