Chemical Affinity in terms of Electromotive Force. 128 
mercuric nitrate, and contained free nitric acid to an amount 
representing almost exactly one eighth of that present as mer- 
curous nitrate, and was finally made of strength ‘25 Hg.(NOs)., 
‘03 H,(NO3), 100 H,0. On setting up cells with pure mer- 
cury and this fluid, opposed to amalgamated zinc, electro- 
copper, electro-lead, and electro-silver respectively in con- 
tact with solutions of their nitrates of constant strength 
‘25 M(NOs). 100 H,O, the following values were obtained. 
It is noticeable that with the last cell the current sometimes 
passed from mercury to silver and sometimes in the opposite 
direction, the average H.M.F. being negative,—. e. mercury, 
and not‘silver, acquiring the higher potential, contrary to the 
result predicable from the relative heats of formation of mer- 
curous and silver nitrates (viz. 47990 and 16780 respectively 
—Julius Thomsen), which corresponds with an E.M.F. of 
688 volt, silver acquiring the higher potential. The pro- 
bable error amounted to +:003 to +:004 in each instance. 
Maximum. | Minimum. Average. 
Zinc-Mercury ......... 1535 1:476 1:500 
Copper-Mercury ...... ‘470 ‘410 433 
Lead-Mercury ......... ‘944 "896 ‘O17 
Silver-Mercury ......... — 041 +019 —°004 
Practically identical values for the voltaic constant for 
mercury in contact with mercurous-nitrate solution result in 
all four cases : — 
Mare MeLeHby: rts eens ht. Sains Pe Ye OOO 
Zinc-copper + Copper-mercury . ‘ noe == 1-499 
Zinc-lead + Lead-mercury . . ‘ eae —1°497 
i. 1 oe ae 
Zinc-silver — Mercury-silver { Sng pee 1°499 
Midget cig mee) cpl ee 
The average value is thus but slightly below that found for 
mercury in contact with mercurous sulphate suspended in weak 
zinc sulphate, viz. 1°514 (§ 179). 
Since Ey represents a distinctly less amount (1°202 volt), 
it results that HK —Ey=1°499 — 1-202= +°297; 7. e. the thermo- 
voltaic constant for mercury in contact with mercurous- 
K 2 
