48 president's address — section a. 



Kelvin had appealed as the probable origin of the " volta force " at 

 a zinc-copper junction — helped in the same direction. Though his 

 method of working is open to some degree of criticism, his results 

 suffice to prove that the energy set free in the alloying of zinc and 

 copper is far too small to account for the volta force. If Gait did 

 not obtain quite reliable measures for the heats of formation of the 

 brasses, he at any rate inflicted a severe blow on the contact theory. 

 There seems now to be a tacit agreement to identify the seat, or, rather, 

 seats, of electromotive force with the various sources and sinks of 

 energy in the circuit. The metal-metal, metal-liquid, and liquid- 

 liquid boundaries all have a share ; even the body of the liquid plays 

 a part in virtue of ionic migration. The battle has ended in a draw, 

 somewhat in favor of the chemical theory. 



It is unnecessary to go into any detail respecting the work of 

 Helmholtz and his contemporaries and immediate successors, as such 

 matters were dealt with in my report (/) to this association on the 

 thermodynamics of the voltaic cell, and are now common property. 

 Suffice it to say that the chief results of work on their lines, since the 

 date of that report, ha\e taken the form of utilisation of an established 

 theory, rather than of additions to it. To this statement there is, 

 however, one very important exception. 



The distinguished American chemist, Richards, (m) has called 

 attention to a remarkable parallelism between the bound energy of a 

 cell and the change in thermal capacity of its contents which takes 

 place during its working. He has shown that the algebraic signs of 

 these two quantities are opposed, and their numerical values of the same 

 order of magnitude, in every one of the numerous cases which he was 

 able to consider ; at the same time he calls attention to the differences 

 between the measured values of the two. 



In order to see the bearing of Richards' suggestions on voltaic 

 theory, we must revert for a moment to Helmholtz's position. The 

 latter had shown that the bound energy is necessarily represented by 

 the reversible thermal effects within the cell, and his successors — 

 Jahn (n) and Gill (o) — had proved, by direct experiment, that these 

 thermal effects are identical with the sum of the Peltier effects at the 

 various junctions. In accordance Avith their work, Helmholtz's equa- 

 tion takes the form 



E + 2(n) = — f - = H 

 9q 



The comparison of this formula with Richards' figures — though 



Richards does not make it {'p) — is, to my mind, of the highest order of 



significance, inasmuch as it throws a flood of light on the nature of 



thermo-electricity. There seems to me no room for doubt that part, 



if not the whole, of the Peltier electromotive force at a metal-liquid or 



(I) Loc. cit. (m) Zeitsch. Phys. Chem.. XLII. (n) Loc. cit 



(o) Loc. cit. 

 (p) In fact he does not refer to the formula, and seems to be unaware of its 



importance. 



