206 Messrs. Wright and Thompson on the Determination of 
III. Mercury. 
205. Cells were set up with mercury in contact with freshly 
precipitated mercurous iodide suspended in zinc-iodide solu- 
tion, and opposed, firstly, to amalgamated zine in zinc-iodide 
solution, and, secondly, to electro-silver immersed in a magma 
of freshly precipitated silver-iodide and zinc-iodide solution, 
the solution-strength being uniformly °25 ZnI, 100 H,0. 
The following values were obtained :— 
Zine-Mercury. | Mercury-Silver. 
Wilbobaribhiie Sqgeesegn sess 806 — ‘092 
Muniimimma te) dea. ase 797 — 101 
INV ELAR OM OR EI Asante a ‘800 — ‘096 
Probable error ......... +:00138 +:0014 
the current passing in the latter case in the direction opposite 
to that predicable from the relative heats of formation of silver 
and mercurous iodides. From these values the following 
valuations result for the voltaic constant of mercury m contact 
with mercurous iodide suspended in *25 ZnI, 100 H,O :— 
eee See try ol) 
Zinc-silver — Mercury-silver J Me "802 
y +096 
Mean OU 
Julius Thomsen finds that Hg, 1,=48440; whence Hy= 
267 volt, corresponding with 12100 gramme-degrees. Hence 
K—Hy= +°584; 2. e. the thermovoltaic constant for mercury 
in contact with a magma of mercurous iodide suspended in 
‘25 ZnI, 100 H,O is a large positive quantity, as similarly 
found for mercury in. contact with mercurous-bromide and 
zinc-bromide solution and with mercurous-chloride and zinc- 
chloride solution of the same solution-strength, these latter 
two values being respectively +°472 and +:458. So that 
the bromide value is intermediate between the values for 
chloride and iodide; whilst the direction of variation with the 
three halogens is the same as that observed both with cadmium 
and with silver, viz. that the iodide value is algebraically the 
greatest. 
ZiAG-MCLCULY — . Ante 
IV. Lead. 
206. The very sparing solubility of lead iodide rendered it 
impracticable to obtain good readings with cells set up with 
