Prof. Franz Exner on Contact Electricity. 285 



thirdly, it can be shown that two pieces of one and the same 

 metal evolve electricity immediately they are placed each in 

 a differently acting atmosphere. 



The second and third methods are followed in the following 

 pages ; and I shall now describe the experiments which show 

 that the difference of potential between two metals in air is 

 really measured by their heats of combustion. 



In the first place it will be well to obtain a distinct con- 

 ception with reference to the mode of dependence of the 

 tension on the heat of combustion. Though the whole process 

 of the production of electricity is ; according to the present 

 state of things, by no means clear, the theory contained in the 

 following pages] will be found to correspond in all im- 

 portant parts with the facts. 



It is well known that in galvanic cells any chemical process 

 gives rise to a difference of potential proportionate to its heat 

 value ; and in the case of the oxidation of a metal in air one 

 must conclude that the difference of potential between the 

 metal and the oxide formed is proportionate to the heat of 

 combustion of the former. Thus any metal which, being in- 

 sulated, is oxidizing in air must contain a certain amount of 

 positive and of negative electricity separated from one another. 

 That these must be inactive towards surrounding objects is 

 obvious. Furthermore, the difference of potential of such 

 separated electricities cannot exceed a certain limit; for the 

 observed tension is always the same whether the oxidation 

 continue or not. It appears therefore that the electricities 

 evolved by further continued oxidation recombine with 

 liberation of the corresponding heat equivalent. 



If, for example, a piece of zinc has, by oxidation in the air, 

 received the potential + E, but the oxide film or perhaps the 

 enveloping air-film the potential— E, so that the difference of 

 potential is 2E, then this amount 2E is measured by the heat 

 of combustion of the zinc. If now we connect the zinc with 

 any metal unaffected by air, e. g. platinum, a part of the 

 electricity of the zinc will pass to the platinum till both 

 metals have the same potential, which we may call + P. The 

 free tension at the zinc becomes now— E + P, that at the 

 platinum + P ; consequently the difference of potential 

 between zinc and platinum is — E, and is thus measured by 

 half the heat of combustion of the zinc. 



If, accordingly, the heat-value of Daniell be A * and the 

 combination of the zinc be B, the difference of potential be- 

 tween zinc and platinum in air must be equal to ~-r. If the 



* The heat-values must all of course be referred to the chemically 

 equivalent quantities of the substances. 



