232 On Electromotive Forces. 



that I conclude that since the hydration and solution of a 

 newly formed salt will, to a great extent, occur simultaneously, 

 the heat of hydration is partly absorbed in the process of 

 solution and dilution, and therefore to that extent cannot 

 furnish electrical energy. On the other hand, when a salt is 

 used which does not form hydrates, and consequently can 

 only absorb heat during solution, the energy due to the for- 

 mation of the solid salt determines the E.M.F., which cannot 

 be subsequently decreased by a process which has no chemical 

 continuity with it, but which merely absorbs sensible heat. 



As Helmholtz has already remarked, the exact reversibility 

 of these processes indicates the constancy of the partition of 

 " free " and " bound," or electrogenic and thermogenic energy, 

 and negatives the earlier supposition of Braun that the pro- 

 portion of transformable energy was the accident of circum- 

 stances. 



Helmholtz has further shown that, regarding the energy 

 and entropy of a system undergoing a restricted change as 

 represented by the differential coefficients of an integral 

 function of the system or substance, under the usual con- 

 ditions the performance of external work takes place at the 

 expense of the "free energy," while loss of heat at the expense 

 of the " hound energy." But whether thei'e subsists a definite 

 relation between the freely transformable and the bound 

 energy on the one hand, and on the other hand between that 

 portion of the energy set free by the process of hydration of 

 a salt which is convertible into electric energy, and that por- 

 tion which runs down into heat, is a question which appears 

 to demand careful experimental as well as mathematical con- 

 sideration ; especially with reference to the influence of tem- 

 perature and various degrees of concentration of the salt. 



The fact that the E.M.F. of a cell, like zinc-cadmium 

 chloride, does not increase with greater dilution as rapidly as 

 it should from thermochemical considerations, may be taken 

 as indicating that the degradation of energy of the system 

 increases as the number of water-molecules in which the salt 

 is dissolved is augmented. 



I conclude, therefore, that : — 



I. The primary factor in determining the electromotive 

 force of a voltaic cell is the relative heat of formation of the 

 anhydrous salts of the two metals employed. 



II. That the E.M.F. may set up chemical changes of a 

 different direction and character from those predicable from 

 the heat of formation of the dissolved salts. 



III. That the E.M.F. set up by (I.) may be, and usually 



