40 ROYAL SOCIETY OF CANADA 



of readings taken with hydrogen, air, oxygen and carbon dioxide is as 

 follows : 



Millivolts. 



Hydrogen 205 



Air 216 



Oxygen 220 



Carbon Dioxide 233 



Experiments were also made with lead by varying the diameter 

 of the inner cylinder. With cylinders 19, 13 and 4 cms. in diameter 

 respectively no difference was observed in the limiting potential, but 

 the time required to reach it slightly increased with the usfe of cylinders 

 of smaller diameter. 



The experiments were also varied by placing the lead cylinders 

 in a tank filled with water, the surrounding layer of water having a 

 thickness of 13 cms. Under these conditions the limiting potential 

 was found to be unchanged. 



It was also observed that this metal effect was produced when 

 Kontgen or Becquerel rays were allowed to traverse the cylinders. 

 With these radiations, however, the limiting negative potentials were 

 reached in the course of a few seconds and varied but little from the 

 values obtained for the different metals under ordinary conditions. 

 A small quantity of radium chloride of activity 1,000 confined in a 

 glass phial was used as the source of Becquerel rays, and was placed, 

 in the experiment, on a support close to the outside of the cylinder 

 B. The effect was also produced with a quantity of uranium oxide 

 placed near the cylinder, but a much longer time was required than 

 with the radium to reach the steady state. 



The following are the values of the limiting negative potentials 

 obtained under the different conditions: — 



VoUa Effect. It is evident that, with cylinders of different metals, 

 the arrangement shown in Fig. I afforded a means of exhibiting the 



