Chemical Affinity in terms of Electromotive Force. 203 
and electro-cadmium with bromide solutions of the respective 
metals of strength :25MBr,100H,O (the ferrous-bromide 
solutions being prepared by agitating together pure spongy 
iron in excess and weighed quantities of bromine and water 
in a stoppered bottle, filtering and using immediately). The 
following numbers were obtained :— 
Zinc-Iron. Tron-Cadmium. 
i 44] — 086 
(oli ohiat)) cr 40] —°126 
LG CG are “417 —'103 
Probable error ......... +:005 +:005 
As with the cadmium-iron sulphate and chloride cells, the 
current passed in the second case in the direction opposite to 
that predicable from the relative heats of formations of iron- 
and cadmium-bromide solutions ; while the H.M.F’. actually 
set up in the first case notably eaceeded that similarly calculable 
from the heats of formation of zinc- and iron-bromide solutions. 
These numbers lead to the following valuations for the voltaic 
constant for bright iron in solution of strength -25 FeBr, 
100 H,O :— 
op CSC ed a? ae 
A ren penny 6 
Pisce Pay abi 
Zinc-cadmium — Cadmium-iron e0s = 418 
Mean ie" S408 
Julius Thomsen finds Fe, Br.aqg.=78070; whence Hy= 
"284 volt, corresponding with 12890 gramme-degrees. Hence 
EK —Ey = +°134 3 
i. e. the thermovoltaic constant is a notable positive quantity, 
less, however, in magnitude than the corresponding value for 
ferrous-chloride solution of the same strength, viz. + ‘204. 
VI. Aluminium. 
202. A number of cells were set up with bright aluminium 
plates opposed to amalgamated zine and electro-cadmium, 
immersed in their respective bromide solutions of strength 
°25MBr, 100 H,O. The readings exhibited the same fluc- 
tuations as were observed with the sulphate and chloride 
cells previously examined (§ 178 and § 186). The following 
readings were obtained, the current passing in the opposite 
direction to that calculable from the relative heats of forma~ 
P2 
