Gases on Contact Potential Difference between Metals. 167 



This is probably due to the time required to attain equi- 

 librium between the gas and the metal. 



Theory of the Platinum Displacements. 



Whilst a complete test of the theory of these effects will 

 require a more elaborate experimental investigation than the 

 present, the results are in some respects definite enough to 

 merit a theoretical discussion on broad lines. A theory of the 

 connexion between contact electromotive force and electron 

 emission from contaminated surfaces has been developed by 

 one of the authors *, and is discussed in connexion with 

 H. A. Wilson's theories of the effect of hydrogen on the 



* Richardson, Roy. Soc. Proc. A, vol. xci. p. 524 (1915). 



