40 Messrs. Ayrton and Perry on Prof. Exner's 



says, in the ' Electrician ' for Dec. 4th, he does not follow. 

 If, however, he will refer to Prof. Exncr's own description of 

 the measuring-apparatus employed, he will, by applying the 

 ordinary mathematical laws, see that the method of attaching 

 a DanielFs cell to the zinc and platinum plates in Volta's 

 condenser, first with its poles one way and then reversed, 

 measures, in terms of the Daniell's cell, the difference of 

 potential between a point in the air close to the zinc plate and 

 a point close to the platinum; but cannot possibly give any 

 measurement of the actual electric charge in the zinc oxide 

 or in Mr. Brown's " condensed compound gas-film," or in the 

 platinum plate itself, unless we know the actual distance 

 between the plates forming the condenser.] 



III. We are justified, then, in concluding that he means 

 that there is neither a constant difference of potentials between 

 zinc and its oxide, nor a constant quantity of separated elec- 

 tricity. What, then, is his assumption ? As far as an atten- 

 tive study of his paper on our part can lead us, it is simply 

 this : — The potential-difference between two metals in con- 

 tact in air is measured by half the difference of the heats 

 of combustion — an assumption which, in spite of the sem- 

 blance of reasoning employed in the previous sentences, our 

 knowledge of electricity gives us no basis for making. But 

 Prof. Exner's sets of experiments I., II., III., consisting of 

 direct measurements with Kohlrausch's apparatus of the elec- 

 tromotive forces of contact of zinc and platinum, copper and 

 platinum, and iron and platinum, combined with J. Thomsen's 

 determinations of the heats of combustion, appear to lend a 

 most powerful support to Prof. Exner's assumption. Are we 

 not justified, then, in accepting it as a conclusion proved by 

 experiment, although it could not have been previously arrived 

 at from our existing knowledge of electricity ? Now it is 

 unfortunate for this conclusion that Prof. Hoorweg, in the 

 Annalen der Phydk und Chemie, No. 9, Band xi. Heft I. 

 pp. 133-155, 1880, has taken exception to Prof. Exner's expe- 

 riments themselves ; and, to make this plain, he gives the fol- 



lowing table: — 







Ayrton and Perry 





Kohlrausch*. 



Hankel. 



Exner. (1880). 



Zn 



Pt . 



. 0-984 



0-984 



0-881 0-981 



Cu 



Pt . 



. 0-184 



0-184 



0-367 0-238 



Fe 



Pt . 



. 0-384 



0-312 



0-704 0-369 



* Prof. Exner says that since the time of Kohlrausch " only isolated 

 and untrustworthy contact experiments have been made." As, however, 

 those even carried out by Kohlrausch himself with metals appear to dis- 

 agree with those of Prof. Exner, while they agree with the results obtained 

 by other experimenters, Prof. Exner might consistently have included 

 Kohlrausch's name in his sweeping condemnation. 



