Difference of Potential of Distilled Metals. 345 



Pringsheim, who did not take such elaborate precautions in 

 preparing their vacuum. They found that the long wave- 

 length limit of freshly distilled zinc was in the ultra-violet, 



Amount of gas absorbed 



but as time went on it moved slowly into the violet and 

 towards the red. Presumably this was due to a slight 

 evolution of gas inside the apparatus and to its diffusion up 

 to the zinc. The admission of air caused the long wave- 

 length limit to move back to the violet. We may therefore 

 conclude that both the long wave-length limit of the photo- 

 electric effect of zinc and its electropositive character (as 

 measured by its contact potential) alter in the same way with 

 the amount of gas absorbed in the surface. Results of this 

 kind are to be expected from Richardson's interpretation of 

 V in equation (1). Any process which makes a metal more 

 electropositive (t. e. V less) should, according to equation (1), 

 be accompanied by a movement of the long wave-length 

 limit of the photo-electric effect towards the red end of the 

 spectrum and vice versa. 



A number of experiments, similar to those already described, 

 were made using bismuth instead of zinc. The contact 

 difference of potential between the platinum film and the 

 newly distilled bismuth altered with the pressure of the 

 air in the vacuum in the same way as when zinc was used. 

 In some experiments the bismuth and zinc were distilled 

 simultaneously, and their contact difference of potential 

 relative to platinum and to each other could be studied under 

 identical conditions. The results of these experiments may 

 be summarized by saying that bismuth is initially electro- 

 negative to zinc by about '1 volt, and that the change in it 



