ELECTROMOTIVE FORCES IN THIS VOLTAIC CELL. 485 



von Zahn, the little too-mixed-up observation of Mr. Hart (described 

 later), tbe measurements of Schultze-Berge, and tbe more decided ex- 

 periments of Brown. It may indeed be readily beld that the -weight of 

 experimental evidence tends the other way, since most experimenters on 

 the subject — Pellat, Schultze-Berge, von Zahn, and, I may add, Sir 

 William Thomson — have left off just as pure contact theorists as they 

 began. I would attempt an experiment myself, save that I am so pro- 

 foundly impressed with the difficulty of making one in which no fault 

 or loophole can be found, and which will by everyone be deemed satis- 

 factory and final ; so I prefer to base my views on a general survey, 

 and on fairly conclusive reasoning, rather than on a crucial but almost 

 impossible experiment. 



5. Perhaps this is now the place to refer to the somewhat erratic 

 series of papers by Professor Franz Exner, of Vienna. 1 He sets him- 

 self to disprove the existence of contact force in the most straight- 

 forward and obvious manner, and to establish the fact that there is no 

 electrical evolution without definite and actual chemical action. To this 

 end he announces the following propositions : (1) that two metals in a 

 chemically indifferent medium show no electricity ; (2) that the potential 

 difference of two connected metals in air is exactly half the difference of 

 their heat combustion energies ; and (3) that two pieces of the same metal 

 produce contact electricity as soon as they are put into chemically 

 different atmospheres. 



The experiments by which he supports these assertions have, every 

 one of them, been elaborately and severely criticised by Beetz, Hoorweg, 

 Julius, Schultze-Berge, von Zahn, Ayrton and Perry, Pellat, and Wiede- 

 mann ; and his numerical determinations of contact force ajipear to be 

 unique. 2 



It is not necessary for me to enter into a discussion on the merit of 

 his experiments, inasmuch as the mere fact of the existence of so great a 

 body of hostile opinion is sufficient to show that they are not of a kind best 

 qualified to produce conviction. The theoretical views which led Professor 

 Exner to formulate his second statement above, that the potential dif- 

 ference of two connected metals is equal to half the difference of their heats 

 of combustion per equivalent, are, I am sorry to say, quite unintelligible 

 to me. They depend on the hypothetically necessary existence of films 

 of oxide, between which and the metal there is supposed to be a consider- 

 able difference of potential. Perhaps a few quotations from Professor 

 Exner's first paper on contact electricity will render his position clearer/' 



1 Exner: Sitzb. tier Altai, der Wisnenseh. Wien : July 1S78, ' On the Nature of 

 Galvanic Polarisation'; July 1879, 'On the Cause of the Production of E. by the 

 contact of Heterogeneous Metals ' ; Dec. 1879, ' On the Theory of Inconstant Galvanic 

 Elements'; May 1880, ' On the Theory of Volta's Fundamental Experiment ' ; July 

 1880, ' On the Theory of Galvanic Elements ' ; Nov. 1880, ' On the Nature of Galvanic 

 Polarisation ' ; July 1882, ' On some Experiments relating to Contact Theory.' 



2 Beetz: Wiedemann's Annalen, xii. 290; Hoorweg, ibid., xi. 133, 1880, and xii. 

 p. 90; Julius, ibid., xiii. 276 and 290; Schultze-Berge, ibid., xv. 440, as well as xii. 

 307 and 319 ; von Zahn, p. 41 and Preface, of his Memoir; Ayrton and Perry, Phil. 

 Mag. 1881, p. 43 ; Pellat, Paris, Tliises, No. 461, p. 17; Wiedemann, Elclitrieittit, ii. 

 992-995. 



* I quote from Mr. J Brown's translation (Phil. Mag., Oct. 1880) of a paper by 

 Exner in Wiedemann's Annalen of the same year, with some abbreviations. ' An inves- 

 tigation concerning the nature of galvanic polarisation has led me to a quite distinct 

 view of the origin of the so-called contact electricity, a view which will be supported 

 by experiments following. I have shown that the original cause of the polarisation 



