Chemical Affinity in terms of Electromotive Force. 201 
III. Lead. 
199. Two sets of cells were examined containing electro- 
lead immersed in saturated solution of lead bromide of strength 
‘037 PbBr, 100 H,O opposed to amalgamated zinc and electro- 
cadmium immersed in their respective bromide solutions of 
the same strength. The following values were obtained :— 
Zine-Lead. Cadmium-Lead. 
i foc) 5 fad) ‘257 
i D607 "255 
BYMEEMTO) oe 2sssoc..1i.ss. ‘571 256 
Probable error ......... +:0014 +:0001 
These figures lead to the following valuations for the vol- 
taic constant of lead in saturated lead-bromide solution :— 
_Picclendl, oe seen nai wen naan mmE tty ct ier ys 715) 
oe S150 a 
Zinc-cadmium* + Cadmium-lead . 4 4.956 ‘ = 571 
Meany ae ere 571 
Julius Thomsen finds Pb, Br, aq. =54410; whence Ey =806, 
corresponding with 36550 gramme-degrees. Hence 
H—Ey = —:235; 
1. e. the thermovoltaic constant for electro-lead in saturated 
lead-bromide solution is a large negative quantity, slightly 
greater numerically than that for lead in saturated lead- 
chloride solution, viz. —°222. 
IV. Mercury. 
200. When mercuric-bromide solution is agitated with 
metallic mercury, mercurous bromide is rapidly formed, so 
that in a very short time practically all mercury and bromine 
are removed from solution, just as is the case with mercuric 
chloride (§ 183). Cells were therefore set up containing 
mercury in contact with a magma of mercurous bromide 
(freshly precipitated and well washed), suspended in zinc- 
bromide solution, and opposed, firstly, to amalgamated zine 
immersed in zinc-bromide solution, and, secondly, to electro- 
* Zinc-cadmium assumed to give the same value for this solution- 
streneth as that found above for the somewhat stronger solution 
°25CdBr, 100 H,0. 
Phil. Mag. 8. 5. Vol. 19. No. 118. March 1884. je 
