Measurement of Electrode-Potentials in Liquids. 21 



by which name I propose to indicate the drop of electro- 

 motive force on the boundary between electrode and electro- 

 lyte, and the rate of these reactions is proportional to the 

 current strength, this may be expressed in the following- 

 manner : " When the electrode-potential at which a reaction 

 begins has been exceeded by a finite amount, the current 

 strength becomes independent of it, being only limited by 

 the velocity of convection and diffusion." The instance just 

 referred to of the deposition of copper conforms to this rule, 

 for it seems very probable that when a sufficient drop of 

 potential is maintained on the boundary between electrode and 

 electrolyte, the rate of deposition of copper only depends on the 

 rate of renewal of the metal-salt at the electrode. Examples 

 of a similar kind have been studied by Brunner *, viz., the 

 rate of reduction of iodine to hydriodic acid with definite 

 cathode-potentials and the rate of liberation of hydrogen 

 from weak solutions of acids under similar conditions. 



As has already been indicated, Nernst and Brunner are 

 plainly of opinion that all cases of electrochemical action may 

 be ultimately considered from the same point of view. It is 

 not the object of the present paper to enter very fully into a 

 thermodynamical discussion of the principle involved. It 

 will suffice to point out that the latter seems sufficiently 

 invalidated for a priori application to actual cases owing to 

 the impossibility of stating that the atoms chemically reacting 

 on the boundary of two phases are in infinite proximity t- 

 In fact the results of several recent researches, notably 

 those of Haber i, stand in opposition to conclusions drawn 

 from the principle under discussion. The object of all these 

 researches is the investigation of the dependence of electrode- 

 potential as defined above on current-density. In Haber's 

 work the reduction of nitrobenzene is examined ; but instead 

 of finding the current-density to be independent of the 

 potential of the cathode, his results point to a logarithmic 

 dependence of the latter on the former. In fact he has* 

 partially explained his results, starting from the not improb- 

 able assumption that the variations of potential are brought 

 about by the absorption in varying concentrations of hydrogen, 

 or other reducing material, by the electrode. In the further 

 elaboration of this view, his results may all be foretold if we 



* ZUchft. Phys. Chcm. xlvii. p. 06 (1904). 

 that 



t Xernst himself points out that the boundary of two phases is not to 

 be considered mathematically discontinuous, but apparently attaches 

 little importance to this fact. 



t Ztschft. Phys. Chem. xxxii. p. 193 (1900): also IJuss, ZUchft. Phys. 

 Chem. xliv. p. 641 (1903). 



$4649 



