202 Messrs. Wright and Thompson on the Determination of 
silver in contact with a magma of silver bromide suspended 
in zinc-bromide solution, the solution-strength being :25ZnBr, 
100 H,O throughout. The following values were obtained, 
the current passing, in the latter case, in the direction oppo- 
site to that predicable from the relative heats of formation of 
mercurous and silver bromides :— 
Zinc-Mercury. Mercury-Silver. 
Wiaxamannin yo-ea-ceeaceecee ‘O74 — 062 
Mirmaimauan: 2ceeesnce aeacee ‘969 — 073 
MVEA Ce ie orien ine tanec ‘972 — 066 
Probable error ......... +0006 +0019 
These figures lead to the following valuations for the voltaic 
constant of mercury in contact with mercurous bromide sus- 
pended in *-25ZnBr, 100 H,O :— 
PANCAMMCT OOTY. re ye eee eek ee 
serie aes 972 
es a "906 ; 
Zine-silver—Mercury-silver . . { 4-066 (= 972 
Mean 2° ae 979 
Julius Thomsen finds Hg,,Br,=68290; whence Hy=*500 
volt, corresponding with 22670 gramme-degrees. Hence 
K—-—Ey= + “472 5 
that is, the thermovoltaic constant for mercury in contact with 
this bromide magma is a large positive quantity, somewhat 
greater than that observed with the corresponding chloride 
magma, viz. +°498. 
It is noticeable that the heat of formation of mercurous 
bromide exceeds that of lead-bromide solution (Hg,, Bry 
= 68290, Pb, Br, aq.=54410, difference 13880, corresponding 
with *306 volt); whilst the value of 4,—k, for lead-mereury- 
bromide cells is 
—'235—(+°472)=—-707 ; 
so that Eq+’,—, 1s negative relatively to Hy, being 
=—'401. On examining various such cells (solution-strength 
='037MBr, 100 HO), the E.M.F. was found to be actually 
of negative sign, the average observed value being —-402. 
V. Iron. 
201. A series of cells was examined containing plates of 
nearly pure sheet iron (bright) opposed to amalgamated zinc 
