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XLY. On the Contact Potential Differences determined by 
means of Null Solutions. By S. W. J. Smith, M.A., and 
H. Moss, B.Sc, Royal College of Science, London *. 
§ 1. Introduction. 
| 4. Palniaer's " null solutions.'' 
§ 7. A method of search for other null solutions. 
§ 9. Null solution of KCN. 
§ 12. Effect of Na.,S upon the p.d. between Ha- and KC1. 
§16. Null solution of KOI. 
§ 17. Effect of oxygen upon Paschen's relation. 
§ 18. Inferences from results obtained -with null solutions 
of KCN andofKCl. 
§ 19. Null solution of KI. 
§ 20. Null solution of KOH. 
§ 21. Summary of conclusions. 
§ 1. TNTRODUCTIOJS 7 .— The processes which occur during 
■^ the polarization of electrodes are cf considerable theo- 
retical and practical importance. Being largely surface effects, 
they can be followed, in the case of mercury, by observation 
of the changes of the surface-tension which result from 
polarization. Mercury electrodes have been the subject of 
much study in this way, but while the interpretation of some 
of the results seems clear, there are others which have been 
the subject of frequent discussion. Chief amongst these is 
the significance of the maximum surface-tension between 
mercury and the electrolyte which occurs at some particular 
degree of polarization in almost every solution that has been 
examined. The maximum was for long supposed to indicate 
zero potential-difference between the mercury and the solu- 
tion ; but the adequacy of this hypothesis, as a general 
interpretation of the significance of the maximum, is now 
very doubtful. 
§ 2. Evidence was given by one of us (Phil. Trans. A. 
1899, pp. 47-87), in discussing the validity of the Lippmann- 
Helmholtz theory of electrocapillarity, that the maximum 
surface-tension between mercury and certain electrolytes is 
not reached when the potential-difference is zero. It was 
shown that, for equally concentrated solutions of the parti- 
cular electrolytes KCl and KI, the potential-differences at 
the respective maxima differ to such an extent that if one is 
assumed to be zero the other must be nearly a quarter of a 
volt (L c. pp. 70, 71). The assumption of zero potential- 
difference in either case is arbitrary. It is possible that the 
potential-difference at the maximum surface-tension is zero 
in some electrolytes ; but the evidence from the electro- 
* Communicated by the Physical Society : read February 28. 1908. 
