BANCROFT. POTENTIAL OP METALS. 105 



equal concentrations of the two sulphates would not correspond to 

 equal concentrations of zinc and copper ions, and this would affect the 

 potential difference between the solutions. The concentration of the 

 K2SO4 should have no effect, and it was shown in Tables II. and V. 

 that this was the case. It is clear that in measurements made with 

 two-liquid reversible cells, there are two sources of error besides those 

 due to the surface conditions of the electrodes. These are differences 

 of concentration and differences of wandering velocities. The effects 



of these two errors are that the terms log ^-^ and z do not disappear. 



The determinations made with single-liquid cells are free from these 

 sources of error ; but the difficulties due to polarization are so great 

 that the variations are apt to be much larger than in measurements 

 made with two-liquid reversible cells. In Table VIII. I give some of 

 the results obtained with the two styles of cells. In the first four col- 

 umns are the measurements of Paschen,* myself, Overbeck and Edler,t 

 Ostwald,! all made with single-liquid cells. In the next three are 

 the figures of Wright and Thompson, § Neumann, || Braun,^ with re- 

 versible cells. In the eighth are the data of Magnanini,** and in the 

 ninth those of Regnauld,tt the former being for polarizable, the latter 

 for non-polarizable cells. The agreement is not as striking as one 

 might wish ; but it is sufficient. The values marked with a star are 

 not properly comparable, because the two solutions were not of the 

 same concentration. 



Nernst's formula for the cells we have been considering is 



TT = — log ^ X 10-" volts. 



It is therefore necessary to discuss the nature of log P. Nernst has 

 not made any direct statement, so far a>i I know, about a possible con- 

 nection between log P and the negative ion of the salt solution. 

 Ostwald J J and his pupils look upon log Pas a function of the electrode 



* Wied. Ann., XLIIL 590, 1891. 



t Ibid., XLII. 209, 1891. 



t Zeitschr. f. ph. Cliem., I. 583, 1887. 



§ Phil. Mag., [5 ], XIX. 1, 1885. 



II Zeitschr. f. ph. Chem., XIV. 193, 1894. 

 1 Wied. Ann., XVI. 575, 1882. 

 ** Rend. Ace. Lino., VI. 182, 1890. 

 tt Wiedmann, Elektricitat (2 Aufl.), I. 792. 

 tt Lehrbuch, II. 855. 



