Prof. Franz Exner on Contact Electricity. 283 



ruling theories of Volta's experiments. Still they give but a 

 partial test for the same. It is known that Volta himself 

 assumed the existence of a heterogeneous action of the touching 

 metals on the electricity contained in them, in consequence 

 of which a redivision of it resulted. This view, so far as I 

 know, was first by Hehnholtz brought into a determinate 

 form, in which it is now generally accepted. 



Helmholtz * says : — " All phenomena in conductors of the 

 first class may be referred to the assumption that the different 

 chemical substances have different affinities for the two 

 electricities The contact force would therefore con- 

 sist of the difference of the attraction forces which the mole- 

 cules lying near the surface of contact exercise on the elec- 

 tricities at that place." 



Against this position a protest, as is knowu, has been raised 

 by Clausius f , at least so far as concerns the statement that 

 " all phenomena in conductors of the first class " may be 

 deduced from the contact force, referring principally to the 

 phenomena of thermo-electricity. 



After all, however, this hypothesis is at present commonly 

 accepted ; and it cannot be denied that through this concise 

 theory any investigation in this field has rendered good service. 

 It is another question whether, in the present state of things, 

 we are compelled to hold with it, or if there may not be found 

 a less forced explanation of voltaic phenomena, an explanation 

 which shall accord with a series of other fully investigated 

 phenomena. 



And such is, as appears to me, the view advanced by the 

 opponents of the contact theory, the founders of the chemical 

 theory, viz. : — that two metals in contact are in a position to 

 evolve electricity only when they are undergoing at the same 

 time chemical change. This view endeavours, accordingly, 

 to refer the production of electricity in Volta's experiments to 

 the same causes as those at work in galvanic cells. 



In fact, the correspondence between Volta's tension-series 

 and the series of oxidation of the metals is so striking, that 

 one is led at once to the supposition that the electric tension 

 produced has its origin in the oxidation of the metals. This 

 view was first perfectly developed by De la Rive ; and he 

 supported it by numerous experiments. How it was never- 

 theless subjected to numerous attacks, and how it gradually 

 fell into oblivion, is well known. 



De la Rive t assumes that a metal in air is attacked, not 



* Ueber die Erhaltung der Kraft. 

 f Abhandlung. xii. ; and Pogg. Ann. xc. p. 513, 1853. 

 X Traite de VlZelectricite ii. j and Pogg. Ann. xv. p. 98, 1828 ; xxxviii. 

 1836; xl. p. 515, 1837. 



X2 



