294 Prof. Franz Exner on Contact FAectricity. 



chemical theory, but very weighty reasons in its favour and 

 against the voltaic theory. 



I believe we are perfectly entitled to say that a develop- 

 ment of electricity by the contact of heterogeneous metals and 

 an electrical parting force (Scheidungskraft) at the surface of 

 contact of two heterogeneous metals does not exist. The 

 following must take the place of Volta's law of the evolu- 

 tion of electricity : — " The difference of electric potential be- 

 tween two metals in contact is measured by the algebraic 

 sum of the heat-value of the chemical action going on at 

 each." This law holds equally good for every galvanic cell 

 as well as for galvanic polarization and Volta's fundamental 

 experiment. 



That Volta's law of tensions is not prejudiced thereby, but, 

 on the contrary, is an immediate consequence of the chemical 

 theory, requires no explanation. 



It is evident that such a transformation of the theory of the 

 production of electricity cannot be without effect on a large 

 number of phenomena, to only one of which, however, I shall 

 here refer, viz. thermo-electricity. There has been proposed 

 by Le Roux *, and defended by many others, a theory that 

 the origin of the thermo-electric effect is to be found in an 

 alteration of Volta's contact force caused by the variation in 

 temperature. This view has latterly been supported, among 

 others, by Edlund f, who endeavoured to refer the voltaic- 

 tension between two metals to their thermo-electric relations ; 

 also by Avenarius J and by Gaugain § . This view, however, 

 according to the foregoing experiments is highly impro- 

 bable, if not untenable. We can on no account accept the 

 totally unfounded proposition that the heats of combustion 

 of the metals are materially altered by even the smallest 

 variations of temperature, and, secondly, that the difference 

 of such alteration varies considerably in different metals. 

 Furthermore, the thermo-electric force of the two metals 

 must vary considerably according to the medium in which 

 they are immersed, a phenomenon which, if it existed at all, 

 could hardly have escaped observation till now. It seems to 

 me, therefore, far more probable that the thermo-electric force 

 has nothing whatever in common with the so-called voltaic 

 force. Moreover it has already been shown by Kohlrausch || 

 that the assumption of such a connexion is entirely unneces- 



* Compt. Rend, lxiii. 



+ Pogg. Ann. cxliii. p. 404, 1871. 



% Pogg. Ann. cxix. p. 406, 1863 ; exxii. p. 193, 1864. 



§ Ann. de Chim. et de Phys. [6] yi. p. 193, 1864. 



|| Pogg. Ann. clvi. p. 601, 1875. 



