492 report— 1884. 



Bosscha examines and develops all these matters in a series of inter- 

 esting papers published about 1857. * He attributes the development of 

 local beat, at a cathode against which hydi'ogen is liberated, to the change 

 of hydrogen from the nascent condition to the ordinary one— in other 

 words, to the energy of the molecular combination H, H. He finds the 

 electro-motive forces exhibited by this local generation of heat at the 

 surface of different metals in acid to have the following values in volts : — 

 Pt Fe Cu Sn Hg Zn 



•45 49 -64 -86 12 1-2 



One more memoir I must mention before closing this historical sketch 

 and discussion thereon ; a valuable communication by Bouty to the 

 ' Journal de Physique,' 2 ' On thermo-electric force at contact of metals and 

 liquids, and on the Peltier effect thereat.' He finds the Peltier co- 

 efficient at a junction of copper with salts of copper eighty times as 

 great as at an iron-zinc junction, and eleven times as great as bismuth- 

 copper. He also measures the metal-liquid thermo-electric E.M.F. at 

 different temperatures, and shows that Thomson's thermo-dynamic formula 



j u=1A e 



d T 

 is perfectly true and in agreement with experiment in these cases also. 

 He endeavours to see if this Peltier, or, as we had better call it for dis- 

 tinction, Joule or Bouty effect can be calculated from the energies of com- 

 bination. After tabulating his results alongside of heats of oxidation and 

 heats of solution, he decides that it is hopeless, and that we must give up 

 trying to establish a relation between these quantities. Chemical action, 

 he concludes, only disturbs the effect by altering the surfaces, and by 

 developing parasitic heats. They may mask, but they do not produce, 

 the true Bouty phenomenon, which he believes is probably physical. 



The difficulties of making these measurements are exceedingly great, 

 and, notwithstanding the ingenuity and skill displayed, it seems to me 

 possible that some error or unexpected source of disturbance may have 

 modified the results. So far as I know, they have not yet been repeated, 

 and I can hardly regard the experimental method used as perfectly safe. s 



7. The result of our survey in regard to the special subject of discussion 

 may be summed up thus : (1) that there is certainly an E.M.F. at the 

 junction of two different substances, or even of the same substance in 

 two different states ; and (2 ) that the total E.M.F. of a circuit is the 

 algebraic sum of all such contact forces at every junction in the circuit. 



I do not know that these two propositions could be passed nem. con., but 

 I believe that, provided they were properly understood, the dissenting 

 minority would be a very small one. It is probable that Professor Exner 

 would be in the minority, but I am unable to be sure of anyone else. 



We can also make a negative proposition which will command 

 almost universal assent — viz., that if in the above second proposition, 



1 Bosscha: Poffff. Ann. vols, ci., ciii., cv., cviii. 



2 Bouty : Journal de Physique, 18T'J, viii. p. 341 ; ix. p. 229, and p. 306 ; csp. p. 300. 



3 I find that a method exactly like that used by Bouty was suggested by Clerk 

 Maxwell, Elementary Electricity, p. HG, 



Various observations regarding the E.M.F. of different cells are made in the series 

 of papers still appearing in the Phil. May., by Dr. Wright and Mr. Thompson, ' On 

 the determination of Chemical Affinity in terms of E.M.F.' 



