292 Prof. Franz Exncr on Contact Electricity'. 



silver is 92,400 calories. Therefore the calculation for the 

 difference of potential between silver in air and silver in chlo- 

 rine gives the value '543 Daniell. This extraordinary agree- 

 ment, considering the small number of experiments, can pro- 

 bably only be ascribed to chance. Finally, the movable 

 silver plate was exposed to the action of chlorine till perfectly 

 black, and again examined. There resulted the value '54, as 

 before ; but this value decreased very rapidly when the plate 

 continued exposed to the air ; and experiments immediately 

 following one another gave the values *46, -42, '38, *36 ; and, 

 after standing twelve hours, no difference of potential was 

 observable, although the one plate was bright and the other 

 perfectly blackened by chlorine. 



I believe the results of the foregoing experiments bear elo- 

 quent testimony in favour of the chemical theory. It is not 

 only that the qualitative relations correspond, without excep- 

 tion, to this hypothesis, but the quantitative determinations 

 agree so well with the calculated values that the truth of the 

 theory under consideration hardly admits of any further doubt. 

 The only substance with which I have not succeeded in ob- 

 taining any positive result was lead. Not that it failed, 

 when in contact with platinum, to give values which agreed 

 with calculation, but it was impossible to obtain two observa- 

 tions that would agree together. The reason of this was ap- 

 parently the great rapidity with which a bright lead surface 

 is attacked by the air, and, secondly, the impossibility of 

 giving it a good polish; this latter, however, is imperatively 

 necessary when working with a solid insulating layer in the 

 condenser, in order that the distance between the plates may 

 be the same at each measurement. 



In reference to the value of the last experiments with silver 

 in chlorine,, I may make the following remarks : — If one could 

 prove, with regard to all substances, that their differences of 

 potential are proportionate to the differences of their respec- 

 tive heats of combination, this would still be no direct proof 

 against the contact theory ; such, however, is given by the 

 experiments with silver. 



The contact theory evidently loses its basis so soon as it is 

 shown that two heterogeneous metals in contact assume no 

 difference of potential. This De la Hive has already shown 

 by his experiment with different metals in vacuo, an experi- 

 ment which has unfortunately fallen into oblivion. The same 

 result can, however, be attained more easily and in a more 

 obvious manner. 



