22 Messrs. Wright and Thompson on the Determination of 
Lead sulphate suspen- a ee See 
ded in zine sulphate. Bhaegenic 
| m h. a sulphate. 
\. 
| KE. | B-E_. E. iE 
| H IL. 
| O-1 Q |) 712 "536 — 176 "550 — 162 
0-5 0 | 712 520 —°192 537 — 175 
10 Os ail2 512 —°200 "529 — 183 
2-0 SO: gia 502 — 209 5185 — 1925 
30 100 | -710 ‘491 —219 ‘514 — 196 
4:0 175 | “708 “487 —*221 “509 — 199 
50 375 | “704 487 — 217 "505 — 199 
5°75 487 ‘501 
Hence electro-lead in connexion with its sulphate is 
analogous to silver, the thermovoltaic constant possessing a 
considerable negative value ; so that zinc-lead-sulphate and 
cadmium-lead-sulphate celis have H.M.F.’s notably below the 
values corresponding with the net chemical actions taking 
place therein. On the other hand, since the thermoyoltaic 
constant for copper is a small positive value (+:°009 for all 
solution-strengths), and that for lead is in all cases a con- 
siderably larger negative value, whilst copper is the metal 
acquiring the higher potential in a lead-copper-sulphate cell, 
it follows that the value of £,—f, in such a cell is always a 
considerable + quantity, varying from +°009—(—:162) to 
+:009—(—°221); that is from +°171 to +°230, according 
as the lead sulphate is suspended in zinc- or cadmium-sulphate 
solution, and according as the molecular strength of the 
solutions is low or high. In other words, a lead-copper-sul- 
phate cell gives an effective maximum H.M.F. of from ‘171 
to °230 volt above that corresponding with the net chemical 
action taking place there. ‘This chemical action represents 
a heat-evolution of 73800—55960=17840 gramme-degrees 
per gramme-molecule, corresponding with -398 volt only. The 
actually observed values consequently correspond to an incre- 
ment of about 50 per cent. in the H.M.F. of the cell above 
that due to chemical action. | 
On the other hand, since the thermovoltaic constant for 
silver in contact with saturated silver-sulphate solution is a 
negative quantity numerically greater than that for lead, 
whilst in lead-silver-sulphate cells silver is the metal acquiring 
the higher potential, the value of k;—, in such cells must be 
negative. Thus a cell set up with electro-silver immersed in 
saturated silver-sulphate solution opposed to electro-lead 
immersed in a magma of lead sulphate suspended in -042 
ZnSO, 100 H,O, must yield a value for kj—k, close to 
—351—(—'176)=—'175 ; that is, the H.M.F. of such a 
