Chemical Affinity in terms of Electromotive Fore. 115 
forces considerably below the value of Ey pertaining to that 
class of cell, lead-silver-chloride cells possess electromotive 
forces considerably above the corresponding value of Hy. 
On substituting lead-chloride solution for zinc-chloride 
solution as the medium in which silver chloride is suspended, 
but little change is produced in the H.M.F. Thus a number 
of lead-silver-chloride cells containing solution of strength 
°25PbCl, 100H,O throughout gave values ranging from 
"453 to ‘522 volt, and averaging near to ‘480. 
Some lead-copper-chloride cells were examined; but for 
some unknown reason they failed to give concordant results, 
the observed H.M.F. always falling short of that deduced from 
the voltaic constants for lead and copper in chloride-solution 
(viz. 1:099—-591="508). In some cases the falling off was 
small; in others it exceeded ‘050: these cells, moreover, exhi- 
bited no constancy even for half an hour after setting up, the 
H.M.F. rapidly falling as time elapsed. In all cases, how- 
ever, higher electromotive forces were observed than corre- 
spond with the difference between Pb, Cl, aq. =75970 and 
Cu,, Cl, =65750, viz. 10220 gramme-degree ='225 volt. 
On the other hand, numbers closely agreeing with those 
calculated from the above-described thermovoltaic constants 
were obtained with cells set up with electro-lead immersed in 
lead-chloride opposed to mercury in contact with mercurous 
chloride suspended in zinc- or cadmium-chloride solution, the 
strength being ‘25 MCI, 100H,O throughout. The value 
of Hy in these cells is +°145 volt, corresponding with 
82550—75970=6580 gramme-degrees, the metal giving the 
smaller heat-value, and consequently acquiring the higher 
potential in consequence of the chemical action, being lead. 
Since, however, the thermovoltaic constant for lead is —+222, 
and that for mercury in contact with mercurous chloride sus- 
pended in zinc-chloride solution +°458, and when suspended 
in cadmium-chloride solution +°473, the value of k,—z, is 
in each case largely negative, and considerably greater nume- 
rically that Hy. Hence the calculated electromotive forces 
for the two cells are respectively *145+(—‘222)—-458 
= —‘535, and 145+ (—:222) —-4738= —:550 volt. The ob- 
served values were —*539 and —*549 respectively, i. e. electro- 
motive forces of °539 and °550 volt were observed, the mer- 
eury, and not the lead, acquiring the higher potential. These 
cells are remarkable not only for the large negative values of 
k,—kp, but also for the high negative values of Ey +h,—hy, 
exceeding in this respect all other cells examined with the 
exception of zinc-aluminium-sulphate cells (§ 178), which gave 
the numerical value *537. 
