BANCROFT. — POTENTIAL OP METALS. 119 



nium sulphate and potassium sulphate solutions, between calcium 

 chloride and ammonium chloride. This weakens the conclusions 

 which one would like to draw from these experiments; but, making 

 allowance for a large experimental error, it is very curious that 

 ZulZnS04|Hg should give so nearly the same value as the cells with 

 indifferent sulphates, and that zinc chloride should be indistinguishable 

 electrically from calcium chloride. The experiments of Hockin and 

 Taylor * may be interpreted either way. They found that the com- 

 bination of zinc and another metal in sulphuric acid gave a higher elec- 

 tromotive force than the same two metals in a saturated solution of 

 zinc sulphate. This is not so convincing as if they had used potassium 

 sulphate instead of sulphuric acid, because in all except dilute solutions 

 free acids do give a higher value than the corresponding salts. The 

 reason for this variation is unknown. When it comes to the absolute 

 values in the zinc sulphate solution, matters are no better. With 

 some of the metals, notably cadmium and mercury, the zinc sulphate 

 appears to give the same value as any other sulphate ; with others, 

 there is qualitative agreement with Nernst's theory. The same re- 

 marks hold true of the work of Lindeck.f I have not access to the 

 original paper of Wolff, and the review of it | is too meagre to be of 

 much assistance. He investigated, among other things, the effect of 

 changing the concentration of the zinc sulphate in a one-liquid cell. 

 His results are given in Table XIX. The first column shows the 

 electrodes and the electrolyte ; the second, the concentrations of the 

 latter in specific gravities ; the third, the corresponding electromotive 

 forces. 



In all cases there is a qualitative agreement with the theory ; that 

 is, the electromotive force increases with decreasing concentration of 

 zinc sulphate. The quantitative agreement is not so satisfactory. In 

 the second, third, sixth, and last cells given in the table, the variations 

 are much too small, while in the other cases they are too large. The 

 ratio of the strongest solutions to the weakest in the experiments of 

 Wolff lies between 100 and 1000 to 1, which corresponds to a change 

 of electromotive force of 0.05-0.09 volt owing to the bivalence of zinc. 

 Some experiments winch I made with the cell CdlCdCUIHg, the 

 strength of the solution being unknown, gave me 0.815, 0.821, 0.814, 

 average 0.817 volt, the same value which I had already found for 

 the KCl solution. 



* J. Tel. Eng, VIII. 282, 1879. J Beibl., XII. 700, 1888. 



t Wied.Ann., XXXV. 311, 1888. 



