Mercury-dropping Electrodes. 391 



Among these may be mentioned Ostwald^s Stvdien zur 

 Kontaktelehtricitdt, as referred to above. 



J. Moser^ Ueher die Zerlegiing der electromotorischen Kraft 

 der Elemente in ihre Potentialdifere7izen*, where von Helm- 

 holtz^s dropping electrodes are employed in testing the poten- 

 tials. 



Pellat, Mesure de la diference de potential vraie de deux 

 metaiuv en contact t- Much of the above reasoning on the 

 cause of the current from the dropping-electrode arrangement 

 applies also to the experimental work of this paper. 



Konig, Ueher die Beziehung zwischen der galvanischen 

 Polarization und der Oherjlachenspannung des Quecksilbersl^. 

 Some doubt must arise as to the correctness of the conclusions 

 arrived at in this paper, since they are found, as stated 

 towards the end of it, to agree with those obtained by dropping 

 electrodes. 



The hypothesis adopted in the main part of the paper may 

 be briefly referred to. 



From a mathematical discussion of the electrical forces 

 supposed, on more or less speculative grounds, to be present 

 in a double layer at contact of mercury and electrolyte, it is 

 concluded that at maximum tension of the surface between 

 mercury and another liquid no difference of potential exists 

 between these substances. 



Quincke expressly states (p. 204) that the electromotive 

 force has no relation to the capillary constant at the common 

 surface of mercury and liquid. 



Exner and Tuma consider (p. 8) that maximum surface- 

 tension occurs simply when the mercury surface is clean, i. e. 

 free from oxide &c. If polarized with either or H, the 

 surface-tension is diminished. No experimental evidence in 

 support of this view is given, but it seems a not improbable 

 assumption. 



It seems to me difficult to suppose that the surface of clean 

 mercury can by any means, so long as it remains clean, be 

 brought to the same potential as that of the surface of dilute 

 acid in contact with it. If we assume, as seems generally 

 admitted, that there is a natural contact electromotive force 

 of an electrolytic kind at the dividing surface, this local 

 " Potent iaUprung " would seem a necessary consequence, and 

 would, if the potentials of any other portions of the liquid and 



* Wiener Anzeiger, No. 19, p. 232 (1887). 

 t Comptes RptuTus, civ. ]). 109l» (1887). 

 i Wif'd. Ann. xvi. p. 1 (l882j. 



