48G 



RKroKT — 1884. 





, •' S 



His views arc but little really different from those of Do la Hive and other 

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

 pressed in so definite and decided d 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 bo taken to them relates to tlio quantitiitive statements : those 

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

 :dl other observers, though indeed 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 llie notion that the heat of combustion has some 

 sort of relation to tiie \ olta effect, and there I am heterodox enough to 

 agree with him. B it what the re'ation is, and how it acts, and what 

 sort i>r potential dilTerence you oug^ t to expect in accordance with theory, 

 concerning all these things I am utterly at variance with him; and 1 

 det'Ui it prudent not to attempt to represent views which I am unable to 

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



J*rofessor 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 seccmd case tlie 

 detlcction is double the first), and from them he obtains equations prov- 

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

 eqi;ati(ms 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 

 j^Kjli everyday experiments as these arc in direct contradiction of tlio 

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

 ai.d who still hold, that view. 



Dr. C. G. Knott in 1871> ' examined the contact force between jilates of 



I'uni'iit is to bi' sought for not at the contact of the ch-ctrodcs with ions hhi'iatal 

 on them, but in the recombination of tlic hitter, and the K.M.F. ol the current so 

 |i!0(hicc(l is measured l)j- the lieat vahu' of thi.s (Hjml)iiiation, just iio the K..M.F'. of 

 .•my uidxanic cell is measured by the heat vahte of the eheniieal process jjfoiiitr on in 

 it. W'itli a so-called contact action the existence of the polarisation cm-rent, and 

 obviously of every other current, has nothinf> whatever to do. The idea then 

 >U!,'iire-teil itself to seek for the cause of the i)roduction of electricity in the e.xperi- 

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

 the .surrounding' media on their surfaces. I have express' d the opinion that .so- 

 called contact electricity is jiroducod by the oxidation of the metal in contact In 

 the oxysjen of the air just as in oalvanic cells it is evolved by oxidation of zinc. If tlic 

 supposition prove t''ue — and it has pi"o ed true-- the E.5I.F. of his metal in contact in 

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



Kxner then ])oints out how all Volta tension series are in oxidation order, mv\ 

 relates approvin^ily Di^ la ItiveVs view that metals in air were attacked not only by 

 water vapour, but by ilry oxyji^cn, and that electricity is produced by any kind of 

 (■hemie.'d action in jirojiortion to the intensity- of the chemical affinity, 'i'hcnlu' 

 i,dves ins nunierii^al theory and supporting experiments, and finally concludes: 'i 

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

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

 (■lectricity : ' The dilference of electric potential between two metals in contact i> 

 measured by the 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 ecpud to /itilf thv dilference of heat values, .so tlit' 

 above last statement nuist be a slight numerical slip. 



The aViove extracts are among the most faxourable I have been able to tiixl. 'l 

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

 of a more surprising character. 



' Knott : J'rnc. 11. S. L'diii. 1879-80, No. lOo, p. :562. 



