concerning Volte? s Contact Force. 461 



calculations of this kind. The success of this brilliant concen- 

 tration theory of batteries with one metal and two liquids, or 

 rather one liquid with different strengths, lends weight to the 

 supplementary suggestions of the same great chemist*, that 

 the E.M.F. of batteries containing one liquid and two metals 

 can likewise be calculated on similar lines. So long as the 

 metals are the same at either side, and only the solutions have 

 to be considered, this diffusion theory clearly represents a 

 great step in the direction of the truth, even if it cannot be 

 asserted at present to be the whole truth. 



But when the so-called solution-pressure of a metal itself has 

 to be taken into consideration, the ground becomes less secure 

 and satisfactory. 



When zinc dissolves in an acid this hypothesis assumes not 

 that the zinc is eaten away by combination with the acid 

 radical, but that it as it were evaporates into the liquid, giving 

 rise to free zinc ions having a certain osmotic pressure. 



If asked why r zinc should evaporate any more freely into 

 liquid than into air, it may be answered (as Mr. F. H. Neville 

 suggests), because the great cohesive force, Laplace's K, would 

 resist evaporation into air, but would be far less effective in 

 resisting evaporation into a liquid where the change of 

 density is much less sharply emphasised at the boundary. 

 Hence though a certain repugnance may naturally at first be 

 felt to this evaporation theory, it may turn out a reasonable 

 mode of expressing facts, and it demands careful con- 

 sideration. 



The above mere cohesion consideration would tend, however, 

 to make all liquids too much alike in their solvent influence on 

 metals : the form given to the theory by Mr. Whetham and 

 Prof. Poynting, that the ions though free of each other are 

 chemically attached to the solvent, seems far more plausible, 

 and overcomes many difficulties. On either view the follow- 

 ing statements hold good. 



When a metal dissolves in a liquid there is no interchange 

 of ions, cations pass into the liquid, but no anions necessarily 

 pass out of it. Sometimes there is an interchange of cations- 

 one set going in and another set coining out — as when iron 

 displaces copper, or zinc displaces hydrogen. Probably some 



* Meaning either Ostwald or Nernst. I cannot pretend to discriminate 

 the portion belonging to each, and I hope it is not necessary. Both have 

 done admirable work in connexion with this subject. l"find that the 

 first theory of concentration batteries, based upon thermodynamic and 

 vapour-tension considerations of a less simple kind than those here 

 quoted, and not depending on any ionisation hypothesis, was given by 

 yon Helmholtz in 1878 (Wied. Ann. vol. iii. p. 201). 



