Electromotive Forces from Thermochemical Data. 213 



electromotive force are spoken of as doing so " at the expense 

 of sensible heat/' They have not, however, given any reason 

 for such differences of action, and they do not appear to have 

 considered all the consequences which the adoption of the 

 latter part of this view would entail. 



Helmholtz^ in his papers " Zur Thermodynamik che- 

 mischer Vorgange,'' * suggests the treatment of a voltaic 

 cell as a reversible thermodynamical process, and that the 

 nature of its working may be detected by the difference of 

 E.M.F. brought about by change of temperature. The cell 

 selected for examination is his zinc-mercurous chloride 

 battery, the E.M.F. of which is very sHghtly increased by 

 rise of temperature ; and the chief discussion is the relation 

 of the vapour-tension of the zinc-chloride solution, the " free 

 energy of the salts/"" and the electromotive force dependent 

 on differences of concentration. 



But that part of his conclusion in which he states that 

 " the electromotive force between the metals increases with heating, 

 i. e. the calomel battery belongs, as 1 have already mentioned, to 

 the batteries tchich fix heat, which in part ico7'k at the exjyense of 

 the thermometric heat of surrounding bodies,'' is based either 

 purely on thermochemical data or without due consideration 

 of what the adoption of that ^-iew would entail in the case of 

 the strictly comparable zinc-mercurous sulphate cell of 

 Latimer Clark, which decreases in E.M.F. by a considerable 

 proportion as its temperature is raised, although this also is 

 a cell giving an E.M.F. in excess of the supposed thermo- 

 chemical effect. 



Now \'iewing a cell as a strictly reversible thermic engine, 

 if a portion of its E.M.F. be due to the absorption of sensible 

 heat, then, when worked backwards, an exactly equivalent 

 amount of heat would be evolved in the cell. Therefore, 

 taking two Latimer Clark cells, joined by their like poles, 

 one at a high, the other at a low relative temperature, the one 

 at the low temperature having the higher E.M.F. will send 

 a current against the E.M.F. of the one at the higher 

 temperature. 



But the current through the cold one will be direct, while 

 that through the warm one will be inverse, so that heat will be 

 absorbed in the cell at low temperature and evolved in the 

 cell at high temperature ; and since for unit quantity of 

 electricity flowing the same amount of chemical work will be 

 performed in each cell, but reversed in one, we should have an 

 instance of a thermodynamical arrangement working by means 



* Sitzungshericht. d. Akad. d. Wissensehaft. zu Berlin. Translated and 

 pub. in Physical Memoirs, by the Phys. Society, vol. i. pt. 1. 



