SURFACES OF DISCONTINUITY 679 



famous controversy will be found in Ostwald's Elektrochemie, 

 Ihre Geschichte und Lehre, or in briefer guise in the first few 

 pages of a paper by Langmuir, " The Relation between Con- 

 tact Potentials and Electrochemical Action" (Trans. Am. Eledro- 

 chem. Soc, 29, 125 (1916)). The great temporary success of 

 Nernst's "solution pressure" hypothesis still further intensified 

 the neglect of Volta's ideas. It was the essence of Volta's 

 theory that the contact P.D. between two metals is the differ- 

 ence between two quantities, each one being a characteristic 

 of one metal only, and Volta recognized that such an assump- 

 tion fitted very simply with the fact that in a closed chain of 

 different metals in series no current flows. It must be admitted 

 that the great discrepancies between the different experimental 

 attempts to measure Volta potentials militated against the 

 success of the theory as a working hypothesis, and led people 

 generally to believe that such potentials, if they existed, were 

 the result of chemical actions at the surfaces of metals and not 

 characteristic of the metals purely and simply. 



But today investigation of thermionic and photoelectric 

 phenomena has greatly altered the status of Volta's ideas just 

 when the validity of Nernst's hypothesis is being seriously ques- 

 tioned by the physical chemists themselves. The work initiated 

 by Richardson on thermionic emission, and the great power 

 which experimentalists possess in producing high vacua and 

 maintaining scrupulously the cleanliness and freedom from con- 

 tamination of metal surfaces, has demonstrated beyond question 

 that electron emission from metals is an intrinsic property of 

 pure metals, and that for each metal there is a characteristic 

 quantity, viz., the energy absorbed when an electron escapes 

 from the metal across the surface. If this be postulated it 

 follows as a logical result that when two metals are in electric 

 equilibrium there must be a P.D. between them if their "electron 

 affinities" are different. (The electron affinity is defined as 

 the quantity cf), where e4> is the characteristic energy of escape 

 referred to, e being the numerical value of the electron charge.) 

 Further, the experimental work of Langmuir, Millikan and 

 others has placed the existence of this P.D. beyond the pale 

 of doubt. To demonstrate the logical dependence of contact 



