346 



Messrs. McLennan and Burton on the 



temperature of the room are set forth in fig. 2, where curves are 

 drawn with the time expressed in hours for abscissae, and the 



' ^ 3 ^ ^ e 



negative potential attained by the inner cylinder expressed in 

 millivolts for ordinates. The limiting values for the different 

 metals are as follows : — 



Millivolts, 



Aluminium 178 



Zinc 160 



Lead 216 



Tin 05 



Copper 73 



In experimenting with lead cylinders the pressure of the 

 air in B was reduced from 750 mms. to 20 mms. of mercury, 

 and a slight drop in the limiting potential of about 3 or 4 

 per cent, was observed. 



Measurements were also made with different gases at atmo- 

 spheric pressure between these cylinders, and it was found 

 that the limiting potential slightly increased with the density 

 of the gas used. A set of readings taken with hydrogen, 

 air, oxygen, and carbon dioxide is as follows : — 



Millivolts. 



Hvdrogen 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 ob- 

 served in the limiting potential; but the time required to 



