ON ELECTROLYSIS IN ITS PHYSICAL AND CHEMICAL BEARINGS. 389 



the cell at all. Only in the case that the reversible heat is zero does this dis- 

 tinction vanish, and not then unless we have some way of knowing a priori that 

 this is the case. 



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

 ■sible 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 rever- 

 sible 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 Equihbrium 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 pp. 510-519. On page 50.3 is shown 

 the connection between the electromotive force of a cell and the difference in the 

 value of (what I call) the ]}ote7itial 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, wiU be found on page 443 of the synopsis from the ' Am. Jour. Sci.,' vol. xvi., 

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

 It has, however, received the unqualified commendation of Professor Maxwell 

 (although not with reference to this particular application — See his lecture on 

 ihe Equilibrium of Heterogeneous Substances, in the science conferences at South 

 Kensington, 1876) ; and I do not see how we can do very well without the idea in 

 certain kinds of investigations. 



Hoping that the importance of the subject will excuse the length of this 

 letter, I remain 



Yours faithfully, 



J. WiLLARD GiBBS. 



Note by the Editor. — It is perhaps hardly wise to comment on the letter of 

 so great an authority without further consideration, but it naturally occurs to one to 

 ask provisionally whether he is not regarding a galvanic cell as too simply a heat 

 ■engine ? Surely if the union of certain elements can generate 0e units of heat when 

 heat-production is all that is allowed, they can, under favourable circumstances, do 

 JOe units of (say) electrical work instead, quite independently of any considerations 

 of entropy or of the temperature at which the heat might have been generated 1 In 

 other words, is Professor Gibbs not assuming that in a cell the union of elements 

 primarily produces heat, and secondarily propels a current, instead of (as may well be 

 the case) primarily generating a current, and secondarily producing heat when that 

 current is given nothing better to do 7 To this Professor Gibbs will doubtless reply : 

 JsTo, the highest temperature at which the heat could reversibly be produced, viz., the 

 temperature of complete dissociation of the compound formed, is of the essence of 

 the question, whatever be the mode of exciting the current. It is needless to point 

 out the extreme interest and importance of such a view. O. L. 



On the Migration of Ions and an Experimental Determination 

 of Absolute Ionic Velocity. By Dr. Oliver Lodge. 



What may be considered as the greatest step in advance since the 

 time of Faraday in the subject of electrolysis is due to Professor F. 

 Kohlrauscb. His idea of specific ionic velocity is obviously most im- 



