24 Messrs. Wright and Thompson on the Determination of 
These values, together with those above cited for zine- 
cadmium, zinc-lead, and zine-copper cells, give the following 
valuations for the voltaic constant for polished metallic iron in 
ferrous-sulphate solution :— 
: Zine-Cadmium—| Zinc-Lead— Zine-Copper— 
HUN MON Unilin. | beabiber Copper-Iron. Mem. 
°360 512 1-114 
399 \ 395 } -400 | 399 | 398 
+:°035 —1i2 (alee 
°362 De 1114 
Ol} -445 } 435 eA ag \429 | -432 
+:073 —'120 — ‘685 
It results from these figures that the thermovoltaic constant 
for polished iron in ferrous-sulphate solutions must always 
have a notable positive value. Admitting, what is doubtless 
the case, that the heats of dilution of ferrous- and zinc-sulphate 
solutions from the molecular strengths 1:0 MSO, 100 H,O to 
0-25 MSO, 100 H,O are not widely different, so that their 
difference is negligible, the heat of displacement of iron from 
1:0 FeSO, 100 HO by zine is 106090—93200=12890 
gramme-degrees per gramme-molecule, corresponding with 
284 volt. Since iron acquires the higher potential in zinc- 
iron-sulphate cells, the value of 4,—, must be positive and 
='398 —°284—+:114; whence the thermovoltaic constant 
for iron must be +114 for 1:0 FeSO, 100 H,O; and must con- 
sequently have higher values still for weaker solutions, approxi- 
mating to °432—'284= +:°148 volt for 0-1 FeSO, 100 H,O. 
Hence wron is an example of a class of metals opposite in 
thermovoltaic power to silver and lead, and differmg from 
copper and cadmium in that, whilst with the last two metals 
the thermovoltaic constant has never a value largely above or 
below zero, with silver and lead it has considerable negative 
values, and with iron it has a notable positive value. 
One result of this is that a zinc-iron-sulphate cell is analo- 
gous to a copper-lead-sulphate cell, in that it affords a consi- 
derably higher effective H.M.F. than corresponds with the net 
chemical action, viz. from °398 to -4382 instead of +284. 
Another result is that, since the thermovoltaic constants 
for cadmium and iron (in solutions 1:0 MSO, 100 H,O) are 
respectively +°001 and +:°114, if iron were to displace cad- 
mium from solution of this strength, there would be evolved 
93200 —(89880—75)=3395 gramme-degrees per gramme- 
molecule (75 being the heat of dilution from 1:0 CdSO, 
100 H,O to *25 CdSO, 100 H,O), corresponding with a value 
