THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



MARCH 1889. 



XXV. On the Divergence of Electromotive Forces from Thermo- 

 chemical Data. By E. F. Herroun, Professor of Natural 

 Philosophy in Queen's College, and Demonstrator in King^s 

 College, London*. 



rflHE fact that with many voltaic cells the electromotive 

 X forces observed experimentally do not accord with 

 values calculated from the thermochemical equations repre- 

 senting the reactions occurring within the cells, has been the 

 subject of remark by several investigators, including Favre, 

 Julius Thomsen, Braun, Helmholtz, Willard Gibbs, Wright 

 and Thompson, and others. But although, at the present 

 time, we are in possession of a knowledge of the actual and 

 computed values of the electromotive forces of a large number 

 of voltaic cells, anything approaching a complete explanation 

 of the reason why certain cells should give electromotive 

 forces in excess, others in defect, others again according 

 almost exactly with the calculable values, is still wanting. 



It was in the hope of deciding certain questions and of 

 going some way on the road to a final solution of the problem 

 that the present research was undertaken. 



Certain cells, like the ' Grove ' or ' Bunsen,^ in which nitric 

 acid is reduced by the evolved hydrogen, are said to furnish, 

 with nitric acid of certain degrees of concentration, electro- 

 motive forces somewhat in excess of the values calculated 

 from the thermal effects accompanying the chemical changes ; 



* Commiinicated by the Physical Society : read January 26, 1889. 

 Pkil. Mag. S. 5. Vol. 27. No. 166. March 1889. P 



