Electromotive Forces in a Voltaic Cell. 377 



of Prof. Wiedemann is rather fuller of hypothesis than most 

 other contributions to the subject with which I am acquainted ; 

 for it postulates 



1. An unexplained difference of potential between metals in 



contact. 

 2 K A similar difference of potential between the elements of 

 a compound. 



3. An unequal attraction of bodies for + and — E. 



4. An unequal attraction of metals for chemical elements. 



5. A chemical contact-force between metals and liquids, 



depending on No. 4, not obeying Volta's law. 



6. A physical contact-force, depending on No. 3, obeying 



Volta's law. 



7. Proportionality of the chemical contact-forces and the 



heats of combination. 



8. A mechanical falling together of atoms accounting for 



heat-production at electrodes; and 



9. That electrical actions go on between the molecules of a 



liquid, and between those molecules and metals im- 

 mersed in it, according to the ordinary laws of electro- 

 statics. 



Not all of the above hypotheses, however, are to be regarded 

 as independent : the dependence of Nos. 5 and 6 is stated, and 

 it is possible that some of the others are intended to be likewise 

 dependent, though I do not clearly see how. 



In some of these hypotheses, for instance Nos. 4 and 5, 

 and, with considerable differences, Nos. 6 and 7, Prof. Wiede- 

 mann's theory bears a resemblance to the views set forth by 

 the present writer in the paper to which this communication is 

 a sequel. 



One obvious objection may be taken to the theory on a 

 question of fact. According to it, zinc half immersed in water 

 is negative to copper half immersed in water (see diagram 

 above) : but experiment has failed to exhibit anything of the 

 kind ; and it is well known that Sir W. Thomson, Prof. Clifton, 

 and others have shown that zinc and copper immersed in water 

 or dilute acid are electroscopically at the same, or nearly the 

 same, potential. 



Theory of Von Helmholtz, 1847 and later. 



There is another theory which I did not indeed ignore, but 

 which I passed over with very insufficient mention, in the 

 paper referred to (see footnote to § 9, Phil. Mag. April, p. 258); 

 viz. the theory of Professor von Helmholtz, which he first stated 

 in his great memoir of 1847, Die Erhaltung der Kraft, and 



