18 Messrs. Wright and Thompson on the Determination of 
WE Hogrome. E.M.F. of cad- /Voltaic constant 
Value of m. ne ee. cel], | mium-copper for electro- 
cell. cadmium. 
Pi 362 752 +°362 
“25 "362 “752 +°362 
23) 361 "753 +°361 
1-0 "360 "754 +°360 
2-0 309 JOO - +°359 
3°0 308 +°358 
4-0 8575 +°3575 
5:0 307 +°357 
Unlike that of copper therefore, the voltaic constant for 
electro-cadmium immersed in m CdSO, 100H,O, referred to 
amalgamated zinc in zinc-sulphate solution of the same 
strength as zero, varies with the value of m, decreasing as m 
increases. 
From these values the corresponding thermovoltaie con- 
stants are thus calculable. According to Julius Thomsen the 
heats of formation of zine and cadmium sulphates in solution 
of strength °25 MSO, 100H,0 differ by 16210 :— 
Zn, O, SO3 aq. = 106090 
Cd, O, SO; aq.= 89880* 
—— —_ ___. — 
16210 
The heats of dilution to this strength of more-concentrated 
solutions are not equal in the two cases: calling them respec- 
tivelv h, and hg, the heat of displacement per gramme-molecule 
of cadmium from m CdSO,100H,0 by zine is calculable 
by the formula 
H=16210—(h,—A,), 
when m, h,, and hz have the following values :— 
mm. hy. hy | 16210—(2,—,). | Co™mesponding 
25 0 0 16210 357 
5 0 25 16235 358 
10 0 75 16285 359 
20 50 | 275 16435 362 
3-0 100 | 450 16560 365 
4-0 175 | 625 16660 367 
5-0 375 | 825 16660 367 
mel 
j 
Hence the values of the thermovoltaic constants for electro- 
* Thermochenusche Untersuchungen, vol. iii. p. 285. In the calcula- 
tions given in Part VI. the value 89500 is assigned, taken from Thomsen’s 
earlier publications (Jowrn. prakt. Chem. xi. p. 401). 
