

[ 454 J 



XLII1. Oh the Controversy concerning Voltas Contact Force. 

 By Professor Oliver Lodge, JJ.Sc, LL.B., F.R.S. 



[Continued from page 383.] 



Chapter IV. (continued) . 



Osmotic Pressure or Di fusion Views of a Voltaic Cell. 



1\ELE osmotic pressure hypothesis, strictly speaking, perhaps 

 hardly belongs to my present subject, since whatever 

 controversy or question has gathered round it is of a different 

 kind from that concerning the seat of the E.M.F. The osmotic 

 pressure hypothesis does not concern itself apparently with the 

 Volta electrostatic effect, but with the mechanism of propulsion 

 in a voltaic cell : it hardly cares to discriminate between the two 

 cases, for they appear in many respects much the same; and 

 it locates the seat of the E.M.F. quite definitely at a junction 

 on one side of which at least there is a liquid or substance with 

 loco-mobile atoms, and not at a metal-metal junction: it 

 deals entirely with moving ions, such as cannot exist in true 

 solids. Moreover it is not a hypothesis really distinct from that 

 of the recognized chemical combination associated with a cur- 

 rent-producing cell, only it regards that chemical combination 

 from a different point of view — from the dissociation or 

 ionisation point of view. Instead of saying that zinc is 

 oxidised or that zinc sulphate is formed, it professes to regard 

 atoms of zinc as shot off into the liquid in a non-combined or 

 free form : ultimately, no doubt, to enter into real combination, 

 but existing for a time, and always in a certain proportion, in 

 the form of practically free ions. Thus the full discussion of 

 the osmotic pressure hypothesis is essentially chemical in its 

 nature, and need not here concern us. At the same time so 

 many people are interested just now in this remarkable view 

 of chemical combination, that I refrain from passing it by 

 altogether ; though, as it is not a hypothesis specially and 

 long familiar to me, my remarks on it will not have any par- 

 ticular weight. Fortunately there are several writings to 

 which a student of such matters can be referred — the treatise 

 of van t' Hoff, the work of Arrhenius and of Planck, and 

 the textbooks of Ostwald and of Nernst, especially of Nernst ; 

 and likewise the excellent Report on these subjects presented 

 to the British Association in 1897 at Toronto by Mr. W. C. 

 D. \Yhetham, as well as to his book on ' Solution.' Further- 

 more, I may refer to the works of Dr. J. Larmor, especially 

 Phil. Trans, vol. cxc. p. 270; and to a paper by Prof. 

 Poynting in Phil. Mag. vol. xlii. p. 289 (1896), where it is 

 shown that the dissociation hypothesis need not be pressed 



