424 



The Contact Theory of Voltaic Action. [Mar. 13.. 



must have been introduced by former experimeDters using a moist 

 blotting-paper surface instead of the surface of the liquid itself. 



A large number of discordant results were obtained in March, 1878, 

 and their explanation led to the interesting result that the apparent 

 contact difference of potentials between a metal and mercury, as 

 measured inductively, varied much with small additions of tempera- 

 ture. The accidental difference of temperature in the different ex- 

 periments arose from the mercury having been redistilled in the 

 laboratory between every two experiments to remove all possible traces 

 of impurities, and probably in some cases it had not become perfectly 

 cold before a new experiment was made. The investigation of this 

 apparent change of contact difference of potentials with temperature 

 led to a consideration of the contact difference of temperature of 

 mercury with air, since, of course, in all these inductive experiments 

 two air contacts are included in the result. The results thus obtained 

 will form part of the substance of the next paper. 



Next follow a number of checks of the accuracy of the results 

 based on the well known law that in any compound metallic circuit at 

 uniform temperature there is no electromotive force. This is followed 

 by some considerations regarding the measurement of the difference of 

 potentials between substances and the air in contact with them, and of 

 measurements of the Peltier effect. 



It has usually been thought that the differences of potential of liquids 

 in contact with one another were so small as to be almost inappreciable 

 in comparison with the differences of potential of metals in contact ; 

 but the authors have ascertained, among other results, that strong 

 sulphuric acid in contact with distilled water, solutions of alum, copper 

 sulphate, and zinc sulphate has a measured difference of potentials of 

 1*3 to 1*7 volts, an electromotive force more than twice as high as that 

 of zinc and copper in contact. And hence the importance of on 

 apparatus that can directly measure the difference of potentials of two 

 liquids. 



March 20, 1879. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table and thanks ordered for 

 them. 



The following Papers were read : — 



