Chemical Affinity in terms of Electromotive Force. 205 
Julius Thomsen finds the heat-values for this strength of 
solution are Zn, I,,aq.=60540, and Cd, I,,aq.=47870; whence 
Ey= "279 volt, corresponding with 12670 gramme-degrees. 
Hence H—HEy= +:'048; 7. e. the thermovoltaic constant for 
cadmium in iodide solution of strength :25 CdI, 100 H,O is 
positive in sign but not large numerically. It is noteworthy 
that, with the corresponding bromide and chloride solutions, 
the values were negative, viz. —'023 and —-086 respectively; 
the bromide value being thus intermediate between the chloride 
and iodide values. 
II. Silver. 
204. The following numbers were obtained with cells set 
up with electro-silver immersed in a magma of freshly preci- 
pitated well-washed silver iodide suspended in zine iodide 
solution, and opposed to amalgamated zinc or electro-cadmium 
in their respective iodide solutions, the solution-strength being 
uniformly °25 MI, 100 H,0. 
Zinc-Silver. Cadmium-Silver. 
oSco ee 713 393 
MUR RGNANATHS |... 25s wouesiese -696 374 
BEVERAGE .2 62.5 ..002 + eee "7055 "384 
Probable error ......... +0028 +0030 
These values lead to the following valuations of the voltaic 
constant for electro-silver in contact with this magma :— 
Zine-silver . Siete Meee res Fo oan eh grat ROE) 
Zine-cadmium + Cadmium-silver ‘ iB es = ae 
Means i cviiai 2606 
Since Thomsen finds Ag», 1,=27600, the value of Hy 
is *726 volt, corresponding with 32940 gramme-degrees : 
hence H— Eg = —:020. 
On comparing this value with those found for silver bromide 
suspended in zinc-bromide solution and for silver chloride 
suspended in zinc-chloride solution, all of the same strength 
*25 Zn X, 100 H,O, viz. —-099 and —:112 volt respectively, it 
is noticeable that the direction of variation is the same as that 
observed in the analogous cases for cadmium, the bromide 
value being intermediate between the chloride and iodide 
values, the last being algebraically the greatest. 
