Chemical Affinity in terms of Electromotive Force. 209 
thermovoltaic couple, in virtue of which a difference of 
potential is set up expressible by the algebraic difference 
between two numerical values, or thermovoltaic constants, 
assignable to each metal respectively, the which values vary pari 
passu with the variations in the corresponding voltaic con- 
stants. This thermovoltaic action being expressible by k,— kz, 
and the first source of potential-difference being indicated by 
Hn, the total difference of potential set up is 
H=Hyt+hk,—hky, 
where, if k;—k, be materially different from zero (which is most 
frequently the case), the value of H will differ materially from 
that of Hy, exceeding or falling short of it according as ky — ky is 
positive or negative in sign wath respect to Hy. In the latter 
case, if Eh, be numerically greater than Hy, the direction 
of the current actually generated is the reverse of that predi- 
cable from the relative heats of formation of the electrolytes. 
208. The following Table furnishes a brief epitome of the 
mean values of the voltaic and thermovoltaic constants deduced 
above for various metals and fluids at a temperature within a 
very few degrees above or below 18° C.; the solutions surround- 
ing both plates being of equal str ength (m zine-salt molecules 
per 100 water molecules), the plate-surfaces being freshly amal- 
gamated ones for zinc (standard), bright fused metal for iron, 
magnesium, and aluminium, and freshly electro-coated plates 
in all other cases. 
. . Voltaic Thermovoltaic 
Metal. Surrounding Fluid. mM. Pe tag 13 constines 
°1:114 inva-/|1°105 inva- } 4-009 
MOPrPER ...... SPU MIATC Say. ss aacnnswee's "1 to 2:25 : 
riable. riable. 
INSEL NOM ciate Ree Asian sicais ss -25 to 8:0 |1:066 to 1:109,1:105 to 1:100) —-039 to +005 
PROP EALON si oteasn on 95 50/afo seis "25 1-091 (SDE — ‘020 
Chloride: Cu,Cl, suspen- 
ded in CuCl, solution ; 
or CuCl, solution alone ‘25 1-099 ( +°061 
Chloride : Cu,Cl, suspen- | 
ded in CdCl, solution. "25 1-001 1038 + —°037 
Chloride : Cu,Cl, suspen- | 
ded in ZnCl, solution. Ay) 988 y —°050 
SAmMEUM ...| Sulphate ...........0e0.00 "1 to50 | 362 to 357 | -357 to 367 |+°005 to —-010) 
INGLE: ©. o0..e reser. clesdaaoens 25 "B52 307 —°005 
Chilorides02)..5 Ane -25 to 8:0 | °330 to -262 | °366 to 240 |—-036 to +°045 
Bromides 4. .i.coeaewere. "25 "315 "338 —°023 
ode <i seen csinnoseb eres "25 322 ‘279 +043 
