Electromotive Forces from Thermochemical Data. 231 



in respect to the main reaction, we see that such portion 

 may be supplied either at the expense of the free or of 

 the hound energy. If performed by the free energy, 

 obviously the amount of that energy which could appear in 

 other forms must be proportionately reduced ; -while if per- 

 formed at the expense of the bound energy, merely thermal 

 effects would fol]ow_, without affecting the course or amount 

 of the free-energy stream. It follows that in certain cases, 

 as for instance in the case of lead nitrate, or silver nitrate, 

 if the heat of solution is a large negative quantity, the free 

 energy, in my opinion, is greater than the total energy, 

 since the bound energy is negative in sign ; but, assuming 

 that this negative bound enei gy is not supplied at the 

 expense of the free energy, it can be readily made up by 

 thermometric heat. Hence, while thermal measurements 

 indicate 27,600-10,880 = 16,720 as the heat of forma- 

 tion of silver nitrate in solution, the voltaic cell shows that 

 at least 27,600 thermal units must be regarded as trans- 

 formable free energy, the 10,880 negative heat-units being 

 abstracted not from it, but from sensible heat. On the whole 

 there must be an increase of entropy, but there may be an 

 absorption of heat, as when a lead-copper nitrate cell sends a 

 current, or a copper-silver cell is w^orked backwards. 



The partial independence of thermal effects due to con- 

 centration of solutions and electromotive forces, is recognized 

 by Helmholtz when he states in the case of a Latimer Clark 

 cell with saturated zinc-sulphate solution, that " the zinc sul- 

 phate, newly formed by the current, can no longer be dis- 

 solved, and its latent heat of solution is saved ; accordingly 

 there is a more poioerful development of heat in the cell in spite 

 of the weaker electromotive forced His theoretical mathe- 

 matical treatment disposes of many difficulties, but it did not 

 prevent him from the error of believing that mercury, when 

 deposited by zinc, even from its solid salts, gave rise to an 

 E.M.F. largely in excess of the thermal values, which, accord- 

 ing to my view, would be quite impossible, as it would 

 amount to a conversion not merely of " bound energy " into 

 "free energy," but, under special circumstances of the 

 thermometric heat, into higher forms of energy with the pro- 

 duction of mechanical effect. 



It might be thought that the whole of the heat liberated 

 in the formation of the fully hydrated salt was a measure of 

 the available energy, and that the negative heats of dilution 

 were supplied solely at the expense of sensible heat, but this 

 supposition is entirely negatived by the actual electromotive 

 forces of several cells [ex. gr. zinc-cadmium bromide cell); so 



