340 REPORT— 1887. 



alone. Further experimental evidence is still being obtained, however, 

 and perhaps Mr. Shaw has something to communicate on this head. 



Among several communications received by the Committee from non- 

 British philosophers is an exceedingly suggestive one by Professor WUlard 

 Gibbs, which raises a very interesting point. 



It is perfectly well known that in 1851 our present chairman, Sir 

 Wilham Thomson, reasoning from some experiments of Joule, taught us 

 how to calculate the E.M.F. of a cell from thermo-chemical data — 



B = S(j£0); 



2 0" 

 or ^^gO^ volts. 



Strictly speaking he hedged with regard to reversible heat effects in a 

 way equivalent to the complete equation 



E=2(j£«)-S(jn) . . . . (1) 



where TTi is the heat developed at junction 1 per unit quantity of elec- 

 tricity conveyed across it, IT, the same at the second junction, and so on. 



But the value of IT, in any given case, is extremely difficult to mea- 

 sure, especially at metal-liquid and liquid-liquid junctions. Bouty has 

 attempted it with but small success. 



Fortunately Helmholtz has thought of applying the second law of 

 thermodynamics to the subject, and shown that it was only necessary to 

 know the rate at which the E.M.F. of a cell varied with temperature in 

 order to know the sum of the IT. For, quite analogous to Professor 

 James Thomson's freezing-point relation — 



is the following E.M.F. relation : — 



cZE8Q=J^SH, 

 or 



vn— ^^— ^^ r9\ 



SQ~JdT • ■ ■ • ^^) 



Putting the two equations together we get 



E=JT£|^ (3) 



which we may say is certainly true. 



But now Professor Willard Gribbs suggests a novel mode of applying 

 the second law or doctrine of entropy. 



He takes into account the temperature of dissociation, or temperature 

 at which the reaction could reversibly take place ; and, calling this Tq, 

 he writes the E.M.F. at any actual temperature T thus : — 



E=Jdel^ (4) 



■■-0 



This he gives as the complete expression; wherein, therefore, JOe is the 



T 

 chemical portion of the total E.M.F., and J0t — the thermal portion of 



-'■0 



the whole E.M.F., equal to J2n. Equations (3) and (4) are plainly 



