244 REPORT— 1897. 



Edler,^ Bancroft,^ and A. E. Taylor.^ Taylor finds reason to suggest that 

 the difierences found by some of the observers on changing the anion 

 may be due to large potential differences at the surface of contact of the 

 two liquids in the cells. He finds that such differences arise in cases 

 where there is a tendency to form complex salts. Moreover, it has been 

 found by Gouy,* Rothmund/ and Luggin'' that the maximum surface 

 tension of mercury is not the same in all solutions, as Lippmann's law- 

 supposes, but varies in cases where complex salts might be formed. Now 

 the values taken for the potential difference at a contact between mercury 

 and solution depend on this result, and so an error is introduced into 

 many of the observations which depend on the subtraction of this 

 potential difference from the total electromotive force of a cell. Such con- 

 siderations may possibly explain exceptions to the rule that the potential 

 difference between a metal and an electrolyte is independent of the nature 

 of the anion. More experiments on the subject, particularly with rever- 

 sible electrodes, would be of great value. 



In a general review of the results of this theory of the migration of 

 the ions, the agreement between calculation and observation is most re- 

 markable. The experimental measurements of the absolute velocities of 

 various ions, which have been described, fully confirm the general truth 

 of the theory, and leave no doubt that the values calculated from the con- 

 ductivities and migration constants give the real average speeds Avith 

 which the ions travel. 



The ability of Kohlrausch's theory to represent the facts being thus 

 established, it must follow that, in dilute solutions, the motion of one ion 

 is independent of the nature of the other ion present. This suggests that 

 the ions are free from each other for, at any rate, the greater part of their 

 time, and this idea is, as we have seen, confirmed by the fact that the con- 

 ductivity of a dilute solution is proportional to the concentration, whereas, 

 if the ion were free only at the instants of collision between dissolved 

 molecules, it would vary as some higher power of the concentration. 



Further evidence, pointing in the same direction, is furnished by the 

 general success of Nernst and Planck's theory of the diffusion of electro- 

 lytes, and of the contact difference of potential between solutions. The 

 numerical deductions from this theory, which agree in general with the 

 results of experiment, involve (i.) the specific ionic velocities, as determined 

 by Kohlrausch, and (ii.) the freedom of the opposite ions to migrate inde- 

 pendently of each other until the electrostatic forces prevent further 

 separation. Thus we seem obliged to accept the idea, originally suggested 

 and strongly supported by other phenomena outside the scope of this 

 section of the Report, that the ions not only enjoy perfect freedom of 

 interchange, as Ohm's law demands, but are actually dissociated from 

 each other for, at any rate, the greater part of their existence. It must 

 be particularly noticed that this freedom from each other does not at all 

 prevent the ions from forming chemical combinations with the solvent 



* Wied. Ann. 1891, vol. xlii. p. 209. 



' Zeits. physiMl. Chem. 1893, vol. xii. p. 289; Physical iZmew, 1896, vol. iii.p.250. 

 3 Journal Physical Chemistry, 1896, vol. i. pp. 1, 81. 



* Compt. Bend. 1892, vol. cxiv. pp. 22, 211, 657. 



* Zeits. physihal. Chem. 1894, vol. xv. p. 1. 



* Zeits. physihal. Chevi. 1895, vol. xvi. p. 667. 



