[m'lennanI radioactivity OF METALS 43 



volts, the potential differences for the different pairs of metals are as 



follows : — 



Difference of Potential 

 (millivolts) 

 Aluminium'! 



y 35 



Zinc j 



Lead } « ^^ 



Tin } 37 



Copper 



I 306 



values wliich are considerably below those generally adopted. 



Conclusions. The gas between the two cylinders always contains 

 'a number of ions, and, on account of the greater rate of diffusion of 

 the negative ions, it is possible that an excess of these would impinge 

 upon the inner cylinder in a given time and thus leave it negatively 

 charged. The resulting potential, however, should be the same for 

 all metals on this hypothesis. 



Again, it is possible that the very penetrating radiation which is 

 present in ordinary air may consist of negatively charged matter and 

 that the negative charge taken up by the inner cylinder represents the 

 amount of this radiation intercepted by it. But the high value 

 obtained for aluminium, together with the results obtained with 

 Eontgen rays, is against this conclusion. 



It seems rather that a process is going on at the surface of the 

 metal, whereby an excess of positively charged corpuscles is being con- 

 tinually emitted, and that the steady state attained represents a con- 

 dition of equilibrium in which the current between the cylinders is 

 equal to rate of efflux of the positive charges. 



