Chemical Affinity in terms of Electromotive Force. 41 
axis of the tube and therefore parallel to one another, and the 
anterior portions of the plates and the wires being thickly 
coated with gutta-percha. The tube was then filled, for in- 
stance, with concentrated zinc-sulphate solution, with plates 
of zinc at an accurately known distance apart, and was kept 
at a temperature sensibly uniform. A series of currents of 
var'ous strengths was then passed through the cell, and the 
difference of potential subsisting between the plates deter- 
mined in each case. These values represented the numerical 
values of e x + CR, where e x is the counter E.M.F. set up during 
the electrolysis, C the current, and Rthe resistance of the cell; 
and from them the values of this expression for definite values 
of C (50, 100, 200 microamperes, &c.) -were readily calculated 
by interpolation. The + zinc electrode was then removed, 
and a copper plate exposing exactly the same area placed in 
precisely the same position. The observations were then 
repeated, the temperature being the same as before, and a 
new series of values, e 2 + CR, calculated, e 2 being the counter 
E.M.F. now set up for a given value of C. Since R is con- 
stant throughout, it is evident that the difference between the 
two values for a given current obtained, first with a zinc, and 
secondly with a copper + electrode, represents e 2 —e 1 . Now 
necessarily both e 1 and e 2 increase with the value of C in 
accordance with the general law to that effect deduced from 
all the previous observations (§ 133); but if it be true that a 
less production of heat instead of electricity is brought about 
when nascent sulphion is liberated in contact with zinc than 
when in contact with copper, e 1 must increase less rapidly with 
the current than e 2 , and hence the value of e 2 — e ± must rise 
with the current-strength. Precisely this result was observed 
in every case: for example, the following numbers were ob- 
tained in a pair of sets of observations carried out as described, 
the area of the plates being 0"50 square centim. throughout. 
+ Zinc electrode. 
+ Copper electrode. 
C.G.S. 
current. 
Observed 
potential- 
difference. 
C.G.S. 
current. 
Observed 
potential- 
difference. 
•00000436 
•00000866 
•00001432 
00002130 
•00004160 
•018 
•039 
•062 
•091 
•174 
00000466 
00000883 
00001460 
00002275 
00004450 
1-073 
1-089 
1-127 
1-159 
1-251 
