384 Mr. J. Brown on Helinholtz's Theory of 



above difficulties is added the one of the diamond-point con- 

 tinually breaking down. For this reason Professor Rowland 

 has ruled only three glass gratings, one of which has been lost, 

 and the other two are kept in his own laboratory. These two 

 were used by Dr. Bell in his determination of the absolute 

 wave-length of the D lines. 

 Baltimore, March 27. 



XLVI. On Helmholtz's Theory of Mercury-dropping Elec- 

 trodes, and the Difference of Potential between Clean Mercury 

 and Electrolytes. By J. Brown, Belfast"^. 



WITHIN the last few years considerable attention has 

 been attracted on the continent by a hypothesis put 

 forward by von Helinholtz f, regarding mercury-dropping 

 electrodes as a means of testing potentials of liquids. 



The hypothesis is given in § 8 below. Researches based 

 on it have been undertaken by several authors, but chiefly by 

 Ostwald, whose short communication j, published in 188(5, 

 first drew my attention to the subject. His complete work § 

 gives tables of contact potential-difference for many metals 

 and liquids ; and the conviction, on electrochemical grounds, 

 that these were incorrect led to an examination of the hypo- 

 thesis on which they were based, and to a conclusion respect- 

 ing the action of mercury-dropping electrodes different from 

 that of von Helmholtz. 



The recent publication of a paper || by Bxner and Tuma, in 

 which these authors come to the same conclusion, encourages 

 me to give a resumd oi the arguments on both sides, beginning 

 with a very much condensed translation of the portion of von 

 Helmholtz^'s paper referring to the subject (p. 933j, of which 

 1 shaU endeavour to give as correct a rendering as is possible 

 in a few words. The quotation-marks are for convenience, 

 and are not to be taken as implying verbal exactness. 



2. " The current which occurs on unequal [successive] 

 immersion of similar electrodes in the same liquid is due to 

 the alteration produced by local action on the surface of the 

 first-dipped electrode. A similar action occurs when mercury 



* Communicated by Dr. Oliver Lodge, being a contribution to the 

 Electrolysis Committee of the British Association. 



t Wissenschaftliche Abhandlungen, i. p. 925 (1382). 



X Phil. Mag. xxii. p. 70 (1886). 



§ Zeitschnft fiir physih. Chejnie, i. p. 581 (1887). 



II " Studien zm* chemischen Theorie des galvanischen Elementes," 

 Sitzungsberichten d. kais. Akad. d. Wissenschaften in Wien, xcvii. part ii. 

 p.l. 



