Chemical Affinity in terms of Electromotive Force. 111 
up in the two ways with the mercury opposed to cadmium 
(or zinc) giving values as nearly identical as the considerably 
wide range of fluctuation in H.M.F. observed with different 
cells would permit to be deduced. In each case the values 
ranged between °89 and ‘95 volt (cadmium), and between 
1°21 and 1:28 (zinc). Combining the two sets of cells, the 
following values were obtained, the solution being ‘25 MCI, 
100 H,0 throughout. 
Zine-Merecury. | Cadmium-Mercury. 
Maximum ......... 1-282 948 
Minimum ......... 1:207 "892 
Average ....... aise 1-256 ‘929 
Probable error ... + ‘0056 +:0053 
From these two values nearly the same voltaic constant is 
deduced for mercury in contact with mercuric-chloride solu- 
tion (or with a magma of mercuric chloride suspended in 
mercuric-chloride solution) of strength ‘25 HgCl, 100 H,0. 
eemePecEGUEy cf 7 6) AEE et LS BDAY EAGO 
Cadmium-mercury + Zinc-cadmium . { a3 == 12209 
——_— 
Meat re vce lye d mee 
Since Hg», Clh=82550, the value of Ey corresponds with 
112840—82550=30290 gramme-degrees for zinc-mercury- 
chloride cells, and with 96250 —82550=13700 for cadmium- 
mercury-chloride cells, or *668 and ‘302 volt respectively ; 
whence evidently both classes of cells resemble lead-copper- 
sulphate cells (§ 174) in actually giving rise to an H.M.F. 
greatly superior to that corresponding with the net chemical 
change. The thermovoltaic constant deduced from the above 
mean voltaic constant is 1°257—°668= +589; so that mer- 
cury in this class of cells resembles iron, aluminium, and 
magnesium in their sulphate cells, possessing a large positive 
thermovoltaic constant. 
Some observations were next made with cells containing 
calomel suspended in zinc-chloride solution in contact with 
mercury on one side, and amalgamated zinc immersed in the 
same zinc-chloride solution on the other side. Like the 
analogous zinc-silver-chloride, zinc-lead-sulphate, and zinc- 
mercury-sulphate cells above described, the E.M.F. of this 
combination was found to decrease as the solution-strength 
