BANCROFT. — POTENTIAL OF METALS. 103 



experiments I found only the smaller values. Repeating the measure- 

 ments with other solutions I obtained chiefly the figures given in the 

 first column, although now and then the values for the other series 

 appeared. As I could not discover the reason for these sudden 

 clianges of 0.036 volt, I give both series, though I am inclined to re- 

 gard the larger values as the more probable. 



The first glance at the table reveals the striking regularity of the 

 phenomena. Where the electrodes are any two of tlie metals Mg, 

 Zn, Cd, Sn, Pb, or Bi, the negative ion has no effect ; the electro- 

 motive force remains unaltered when the concentration changes from 

 10 to 1,000 litres. The influence of the negative ion is very marked 

 in the cells where mercury forms one pole ; but there is no sign of any 

 effect due to the concentration. This appears first in the combina- 

 tions with platinum, and then only in the solutions of the halogens. 

 This abnormal behavior of platinum is, as Ostwald* has pointed out, 

 probably due to a tendency to go over into chlor-, brom-, or iod- 

 platinates, where the platinum is no longer present as ion. It is worth 

 noticing that a similar result was not detected with mercury, though 

 here there is a tendency to a partial formation of complex salts. 

 Whether the greater accuracy of the measurements with mercury in 

 solution of halogens is connected with the possible existence of complex 

 salts, 1 do not know. 



The relation between these single liquid jwlarizable cells and the 

 corresponding, constant, reversible cells of the Daniell type must next 

 be considered. According to the theory of Nernst, the potential differ- 

 ence between a metal and a solution of a salt of that metal is given by 

 the expression 



P T P 



TT = -^ log — X 10"^ volts, t 



where tt is the potential difference, n the valency of the kation,/) its par- 

 tial osmotic pressure, e the quantity of electricity transported by a gram 

 equivalent, and P the solution pressure of the electrode. The electro- 

 motive force of a cell of the Daniells type Mil/)jMiX|;>2M2X|M2 will 

 be the algebraic sum of the two potential differences between the 

 metals and the solutions plus the difference of potential between 

 the liquids. I leave out of account a possible potential difference be- 



* Lehrbuch der AUgem. Chem. (2 Aufl ), II. 897. 

 t Nernst, Zeitschr. f. ph. Chem., IV. 148, 1889, 



