48G BEPOfil — 1884. 



His views are but little really different from those of De la Rive and other 

 older ' chemical theorists,' but they are (especially in later papers) ex- 

 pressed in so definite and decided a manner that they have excited a 

 sharper controversy than vaguer and more hesitating writings could. 

 This indeed may be regarded as their special merit. The main objection 

 which can be taken to them relates to the quantitative statements : these 

 are vigorously made, but they seem unwarranted by facts accumulated by 

 all other observers, though iudeed some of his own experiments certainly 

 seem to support them. It has also been objected that he misinterprets 

 some of his experiments. 



He has got hold of the notion that the heat of combustion has some 

 sort of relation to the "Volta effect, and there I am heterodox enough to 

 ao-ree with him. But what the relation is, and how it acts, and what 

 sort of potential difference you ought to expect in accordance with theory, 

 concerning all these things I am utterly at variance with him; and I 

 deem it prudent not to attempt to represent views which I am unable to 

 understand, because it is unlikely that I should do them justice. 



Professor Exner to strengthen his position adduces a large number of 

 very simple experiments (such as connecting first one Daniell and then 

 two Daniells to an electrometer, and observing that in the second case the 

 deflection is double the first), and from them he obtains equations p:-ov- 

 ing algebraically that Zn/Cu = 0. Considered as conundrums these 

 equations are ingenious, but it is a waste of time seriously to discuss 

 them as Herr Julius has done in an elaborate manner. To suppose that 

 such everyday experiments as these are in direct contradiction of the 

 contact theory is scarcely complimentary to the great men who have held, 

 and who still hold, that view. 



Dr. C. G. Knott in 1879 ' examined the contact force between plates of 



cm-rent is to be sought for not at the contact of the electrodes with ions liberated 

 mi tl. em, but in the recombination of the latter, and the E.M.F. of the current so 

 produced is measured by the heat value of this combination, just as the E.M.F. of 

 any galvanic cell is measured by the heat value of the chemical process going on in 

 it." With a so-called contact action the existence of the polarisation current, and 

 obviously of even- other current, has nothing whatever to do. The idea then 

 ugg sfced itself to seek for the cause of the production of electricity in the experi- 

 ment of Volta, not in the contact of two metals, but in previous chemical actions of 

 the surrounding media on their surfaces. I have expressed the opinion that so- 

 catyed contact electricity is produced by the oxidation of the metnl in contact by 

 the oxygen of the air just as in galvanic cells it is evolved by oxidation of zinc. If the 

 supposition prove true — and it has proved true— the E.M.F. of his metal in contact in 

 air must be measured and expressed by their heats of combustion.' 



Exner then point- out how all Volta tension series are in oxidation order, and 

 relates approvingly De la Eive'.- view that metals in air were attacked not only by 

 water vapour, but by dry oxygen, and that electricity is produced by any kind of 

 chemical action in proportion to the intensity of the chemical affinity. Then he 

 gives his numerical theory and supporting experiments, and finally concludes : ' I 

 believe we are entitled to say that no ScJieidungsltraft exists at the contact of two 

 metals.' The following must take the place "of Volta's law of the evolution of 

 electricity: 'The difference of electric potential between two metals in contact is 

 measured by tie algebraic sum of the heat value of the chemical action going on at 

 each.' 



In his theory and experiments, and all through the rest of the paper, Exner con- 

 siders the difference of potential equal to half the difference of heat values, so the 

 above last statement must be a slight numerical slip. 



The above extracts are among the most favourable I have been able to find. It 

 would be easy to select passages from this, and from his other memoirs on the subject, 

 of a more surprising character. 



1 Knott: Proo. /?. S. Edin. 1S79-80, No. 105, p. 362. 



