BANCROFT. POTENTIAL OF METALS. 



97 



Edler we can calculate the values for the combinations of zinc, cad- 

 mium, tin, lead, and bismuth in different salt solutions. I have done 

 this, and I give a few of the figures in Table I. The electromotive 

 forces are expressed in volts, as will always be the case when nothing 

 is said to the contrary. The first column gives the electrode, the 

 raetal to the left being the anode. The concentrations of the different 

 solutions are not comparable, and are therefore omitted. I have raised 

 the value given by Overbeck and Edler for Piilllg in Nal by 0.025 

 volts, and that for Bi| Hg in NaCl by 0.075 volt. There is no doubt 

 in my mind that these two values had been affected by secondary 

 causes, and that this correction was necessary iu order to make them 

 comparable with the other observations. 



TABLE L 



The result is surprising. The hifluence of the negative ion dis- 

 appears, and, if we considered this table alone, we should conclude that 

 the electromotive forces of the cells MilRXIMg depended on the nature 

 of the electrodes only. The experiments from which this table was 

 compiled showed that, when Mo = Hg, the negative ion is of impor- 

 tance. It is clear that the metals separate into two classes, the distinc- 

 tion being in respect to the behavior towards the negative ion. From 

 the figures of Overbeck and Edler it appears further that zinc and cad- 

 mium, for instance, give the same value in all solutions of halogens ; 

 but that this is not the case for solutions of sulphates and nitrates. 

 The question arises whether this difference is due to errors of obser- 

 vation, or whether we have to do with a special property of the halo- 

 gens. The question cannot be settled by a reference to any existing 

 experiments. Gore * has made a series of observations recently ; but 



* Phil. Mag., [5.J, XXXIII. 28, 1892. 



VOL. XXXI. (n. S. XXIII.) 7 



