ELECTROCHEMICAL THERMODYNAMICS. 407 



reciprocal convertibility, and not in the loose and often misleading 

 sense in which we speak of heat and work as equivalent when there 

 is only a one-sided convertibility.) Therefore the rendement of a 



t" t' 

 perfect or reversible galvanic cell would be J0e - '- units of electrical 



t' t 



work, with #677-, units of (reversible) heat, for each unit of electricity 



which passes. 



You will observe that we have thus solved a very different problem 

 from that which finds its answer in the Joule- Helmholtz- Thomson 

 equation with term for reversible heat. That equation gives a 

 relation between the E.M.F. and the reversible heat and certain other 

 quantities, so that if we set up the cell and measure the reversible 

 heat, we may determine the E.M.F. without direct measurement, or 

 vice versa. But the considerations just adduced enable us to predict 

 both the electromotive force and the reversible heat without setting 

 up the cell at all. Only in the case that the reversible heat is zero 

 does this distinction vanish, and not then unless we have some way 

 of knowing d priori that this is the case. 



From this point of view it will appear, I think, that the pro- 

 duction of reversible heat is by no means anything accidental, or 

 superposed, or separable, but that it belongs to the very essence of 

 the operation. 



The thermochemical data on which such a prediction of E.M.F. and 

 reversible heat is based must be something more than the heat of 

 union of the radicles. They must give information on the more 

 delicate question of the temperature at which that heat can be 

 obtained. In the terminology of Clausius they must relate to entropy 

 as well as to energy a field of inquiry which has been far too much 

 neglected. 



Essentially the same view of the subject I have given in a form 

 more general and more analytical, and, I fear, less easily intelligible, 

 in the closing pages of a somewhat lengthy paper on the " Equilibrium 

 of Heterogeneous Substances " (Conn. Acad. Trans., vol. iii, 1878), of 

 which I send you the Second Part, which contains the passage in 

 question. My separate edition of the First Part has long been 

 exhausted. The question whether the " reversible heat " is a negligible 

 quantity is discussed somewhat at length on pages 510-519.* On 

 page 503t is shown the connection between the electromotive force 

 of a cell and the difference in the value of (what I call) the potential 

 for one of the ions at the electrodes. The definition of the potential 

 for a material substance, in the sense in which I use the term, will be 

 found on page 443 j of the synopsis from the Am. Jowr. Sci., vol. xvi, 

 which I enclose. I cannot say that the term has been adopted by 



* [This vol. , pp. 339-347. ] t [Ibid. , p. 333. J J [Ibid. , p. 356. ] 



