BANCROFT. — POTENTIAL OF METALS. 121 



the different solutions, they do not behave exactly alike. With solutions 

 of pure copper sulphate, or from mixtures containing copper sulphate 

 in any quantity, the maximum value is obtained at once, and is very 

 constant. With solutions of pure zinc sulphate or mixtures containing 

 only traces of copper sulphate, the maximum Value can be obtained 

 only by vigorous stirring, and is very inconstant. There is, of course, 

 nothing surprising about this, as it is what one would have predicted. 

 It is a very striking and curious fact, that the Nernst formula, though 

 deduced from apparently erroneous assumptions, should yet give the 

 effect of changes of concentration in a two-liquid cell with such sur- 

 prising accuracy. 



It will be noticed that the electromotive forces of the non-reversible 

 cells have nothing to do with the heats of reaction. This has always 

 been known ; but it acquires new significance since it has been shown 

 that the non-reversible cells are to be considered, as far as the electro, 

 motive force is concerued, as limiting cases of the reversible two-liquid 

 cells. In the cell ZnlHaSO^lAg there is not much doubt what reaction 

 takes place ; but it has nothing to do with determining the electromotive 

 force. An interesting example of this, which also brings up another 

 point, is the cell CulCuS04lPt. Here the reaction consists in the re- 

 phicement of copper by copper. What happens experimentally, on 

 closing the circuit, is that copper is dissolved from the copper electrode 

 and precipitated on the platinum until this latter becomes, electrically 

 considered, an electrode of pure copper, when further action becomes 

 impossible. Overbeck * made some experiments a few years ago to 

 determine what thickness of copper made a platinum electrode behave 

 like a piece of pure copper. His method was to deposit copper on 

 platinum electrolytically, and was open to the objection that it was 

 almost impossible to be certain that the copper was deposited uni- 

 formly over the surface of the platinum. By using the cell 

 CulCuSO^IPt, it would seem that this difficulty might be avoided, as 

 the plating is stopped automatically as soon as the minimum thickness 

 is reached. Suppose now we balance this cell, to some extent, by an 

 electromotive force less than its own. There will still be a tendency 

 for copper to be deposited on the platinum ; but it cannot be deposited 

 to the thickness corresponding to pure copper, as it must then dissolve 



special communication. Since tiiis paper was written there has appeared an 

 article by G. Meyer, Wied. Ann., LIIL 848, 1894, which confirms my views, 

 tliough with certain exceptions. 

 * Wied. Ann., XXXL 337, 1887. 



