450 Mr. J. Brown on the Role of 



Thermochemical data for some of the reactions involved, and 

 some doubt as to exactness of those available and degree 

 of their applicability to the circumstances. 



Amount of some possible thermoelectric force at contacts 

 of solids and liquids. 



2. It does not therefore seem surprising that Wright and 

 Thompson should be led to conclude from their extensive 

 series of experiments that the electromotive force stands in no 

 simple relationship to the chemical action or the heat evolu- 

 tion taking place in the cell during the passage of the current 

 or to the heats of formation of the two substances electrolysed *. 



It would have been perhaps more correct to say chemical 

 action assumed to take place, and to conclude that, whether 

 the relationship be simple or not, the chemical action is 

 certainly not simple. 



Laurie has shown f that in the case of an aluminium plate 

 we have not to deal with the metal at the surface but partly 

 at least with its oxide. The case of " passive " iron is also to 

 the point. 



3. Notwithstanding all this, however, there is, in any given 

 cell, evidently a certain relationship (more or less masked by 

 the conditions of the case) between the electromotive force 

 and the relative chemical attractions of the positive and nega- 

 tive plates for the anion of the electrolyte — these attractions 

 being measured by the combining heats of the attracting 

 substances. In this connexion Lodge thus replies to the 

 question, Why is the force of a zinc-iron battery less than 

 that of a zinc-copper or zinc-platinum ? : — 



" If we picture to ourselves the actual forces in action we shall 

 get a kind of answer indicated to us. In a zinc-iron cell the 

 E.M.F. is due to the zinc pulling at oxygen harder than the 

 iron does; but, since the iron does pull too, with no incon- 

 siderable strength, the balance of force is not so great as if 

 the iron were replaced by copper, which pulls less, or by 

 platinum, which barely pulls at all until it is coated and 

 alloyed with hydrogen. This answer cannot be considered as 

 complete. . . ." { Then follows a discussion of some of the 

 interfering conditions. 



We note here that the electromotive force of the cell is 

 decreased by a mere attraction of the copper for the anion 



* Phil. Mag. [5] xix. p. 208. 



f Phil. Mag. [5] xxii. p. 213. Compare also Herroun, Phil. Mag. [5] 

 xxvii. p. 209; and Braun, Czapski, Jahn, and others in recent numbers of 

 Wied. Ann., who refer to points, however, rather beyond the scope of the 

 present paper, which treats mainly a more elementary \iew of the question. 



% Phil. Mag. [5] xix. p. 344. 



