Chemical Affinity in terms of Electromotive Force. 23 
cell must be close to 1:1775—'175 =1-0025 volt, since the 
heat of displacement of silver from silver-sulphate solution by 
lead forming solid lead sulphate is 73800—20390=538410 
gramme-degrees per gramme-molecule, equivalent to 11775 
volt. On trying the experiment, the following values were 
observed with a number of similar cells :-— 
Maximum cell-value=1-:008 volt. 
Minimum a —— (1 bon ee 
Average as == 03) eae 
Probable error = 00S... 
the average observed value being thus sensibly identical with 
the calculated one. 
V. Tron. 
175. A number of cells were examined set up with electro- 
copper plates immersed in copper-sulphate solution, opposed 
to plates of brightened sheet iron of as high a degree of purity 
as could be obtained, immersed in freshly prepared ferrous- 
sulphate solution ; and, similarly, numerous observations were 
made with analogous cells containing electro-cadmium plates 
immersed in cadmium-sulphate solution, amalgamated-zine 
plates immersed in zinc-sulphate solution, or electro-lead plates 
immersed in amagma of lead sulphate suspended in zinc-sulphate 
solution. ‘The following average values were obtained: with 
soiutions of molecular strength throughout ‘1 MSO,100 H,0, 
the cells exhibited far greater want of permanence and much 
wider fluctuations in H.M.F’. than were noticed with cells of 
strength 1:0 MSO, 100 H,0; so that, whilst the probable 
error in the former class of cells ranged from +°003 to +‘U05, 
that in the latter class was only from +001 to +°002. The 
negative sign attached to the values for the iron-cadmium 
cells indicates that iron was the metal acquiring the higher 
potential, contrary to what might @ priori be anticipated trom 
the heats of formation, viz. Fe, O, SO; aq. =93200 and Cd, O, 
SO; aq. = 89880 *. 
Becme. | Memeaee | Mie we 
ms —— e 
Le ees | 399 “445 
Tron-Cidmium .. ......... —-035 —'073 
PR R-LCAG...6o5 02 00sseoves ‘112 120 
Tron-Copper ..........0+0s ‘715 "685 
* This peculiarity of iron-cadmium-sulphate cells has been already 
noticed by Braun (Annalen der Physik, xvi. p. 561). 
