184 Prof. Franz Exner on Contact Electricity. 



only by the condensed water-vapour, as is now generally 

 supposed, but also in dry air directly by the oxygen, and, 

 moreover, that electricity is produced by any kind of chemical 

 action, in proportion to the intensity of the chemical affinity. 

 I take it for granted that the nature and method of his 

 deduction of the voltaic phenomenon from this hypothesis is 

 well known. 



If there had existed at that time exact quantitative measure- 

 ments of the electromotive forces at contact, and also a 

 measure for the intensity of chemical affinity such as we now 

 have in the heats of combustion, the truth of this view, which 

 is supported also by other writers *, would have been at once 

 apparent. 



Since such direct proof was not forthcoming, the opponents 

 held fast the old Voltaic hypothesis with an unvarying bitter- 

 ness, more especially Pfafff, who sought, by experiment, to 

 reduce the chemical theory to an absurdity. He arranged, 

 inter alia, the fundamental experiment in vacuo instead of in 

 air, also in indifferent gases such as hydrogen, but found 

 always the same production of electricity, where none should 

 have been expected. De la Rive has, however, proved the 

 cause of this to be in the difficulty of removing a gas-film 

 from the surface of a metal even in vacuo, and shown J, at the 

 same time, that the experiment, when quite fairly carried out, 

 was in favour of the chemical theory. 



Nevertheless the chemical theory has in course of time 

 been more and more supplanted by the contact theory, since 

 an exact proof could not be found for either one or other ; and 

 so the above-mentioned proposition of Helmholtz (originally 

 expressed as a well considered hypothesis, and as such not to 

 be undervalued) came gradually to be regarded as the expres- 

 sion of a fact. 



Such was the position of matters when I was driven by my 

 experiments on galvanic polarization to the conclusion that 

 the so-called contact electricity had likewise a chemical origin. 

 One can undertake the proof of this view in different ways: — 

 first, by showing that two heterogeneous metals give no 

 evolution of electricity when in a chemically indifferent 

 environment. This proof has already been given by De la 

 Rive in very careful experiments; and I consider it super- 

 fluous to repeat an experiment for which De la Rive is re- 

 sponsible. Secondly, it can be shown that the differences of 

 potential assumed by any two metals in air have a direct 

 connexion with the heats of combustion of the metals ; and, 



* Compare for example E. Becquerel, Commit. Rend. xxii. 

 t Ann. de Chim. et Phys. xvi. 

 \ Ann, de Chim, et Phys, xxxix. 



