ELECTROLYSIS AND ELECTRO-CHEMISTRY. 243 



vanic polarisation. He finds that, at the decomposition point in a 

 solution from which a metal is deposited at the kathode, the electro- 

 motive force of polarisation at this electrode is equal to the electrolytic 

 solution pressure of the metal in the solution, and is independent of the 

 nature of the electrode, provided it is not attacked. The numerous 

 apparent exceptions to this rule are referred to secondary effects, such as 

 the development of gases at the electrodes, which cause the electromotive 

 force necessary for their liberation to depend on the nature and condition 

 of the electrode. This, for example, makes the decomposition limit of 

 water rise to about 1-6 volt ; but when these effects are eliminated, it is 

 found that the true value comes out as 1"03 volt. Now 1"03 volt is the 

 maximum electromotive force of the oxy-hydrogen gas battery ; and thus 

 the decomposition of water is a reversible process at 1*03 volt. 



Freudenberg ' has applied the theory to the electrolytic separation of 

 metals, and finds that metals are separated from a solution, through which 

 a constantly increasing current flows, in the revei'se order of their ' decom- 

 position pressures.' They can often be thus separated for quantitative 

 chemical analysis. The influence of the solvent on the solution pressure 

 of metals has been investigated by H. C. Jones,^ who examined cells 

 whose electrodes were silver in solutions of silver nitrate of equal strength, 

 the solvent round one electrode being water, and round the other ethyl 

 alcohol, methyl alcohol, or acetone. In all cases the water solution was 

 negative to the other. The ionisation of the salt in ethyl alchohol being 

 known, the ratio of the solution pressures can, in this case, be calculated, 

 and comes out 0'024. 



Much discussion has taken place about the exact significance of the 

 ' solution pressure ' of a metal — the property represented by P in Nernst's 

 equations. Following Nernst, Ostwald considers that P is a function 

 of the metal and temperature only, and consequently independent of the 

 nature of the negative iron. Measurements of the potential differences 

 at single reversible junctions — i.e. when the kation is of the same metal as 

 th§ electrode — have been made by Le Blanc ^ and Neumann.'' The latter 

 measured the electromotive forces of cells made up with the junction in 

 question at one electrode, and mercury in a normal potassium chloride 

 solution with an excess of calomel at the other. The normal mercury 

 calomel electrode has a potential difference of 0"560 volt, and thus the 

 value of the other contact could be found, the potential difference between 

 the liquids being assumed to be small. Neumann found that at great 

 dilution the potential difference was in general independent of the anion ; 

 but Paschen, Bancroft, and other observers, working with metals in. 

 solutions not of their own salts, which there is reason to suppose form 

 limiting cases of the reversible electrodes and are subject to the same laws, 

 have found that the potential difference does, when the metal is copper, 

 platinum or mercury, depend on the anion. Many experiments on cells 

 containing non-reversible electrodes have been made to determine the 

 influence of the nature of the ions and of concentration. Among these 

 experiments we may mention those of Paschen,* Ostwald,*^ Oberbeck and 



' Zeits. pJiysikal. Chem. vol. xii. p. 97 (1893). 

 " Zeits, physihal. Chem. vol. xiv. p. 346 (1894). 

 ' Zeits. physikal. Chem. 1893, vol. xii. p. 345. 



* Zeits. pliygikal. Chem. 1894, vol. xiv. p. 225. 



* Wied. Ann. 1891, vol. xliii. p. 590. 



' Zeits. j}/njdJial. Chem. 1887, vol. i. p. 583. 



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