( 616 ) 
the heat of dilution required, ZnSO,. 16,81 H,O — ZnSO, . 
400 H,O = 33.8 + 390 + 10 = 433.8 calories. 
The heat of solution of ZnSO,.7H,O — ZnSO,. 400 H,O 
— — 4260 calories THOMSEN, l.c. p. 275 
Equation (C') now gives: 
W — 18430 + 0.713 X 433.8 + 1.713 X 4260 — + 26037 calories. 
The total heat-effect in the cell now becomes: 
E, = (280090 + 26037) — 175000 = SAS calories 
while the calculation based on the electrical measurements gave us 
$1490 which is a very satisfactory agreement. 
We can, of course, express this result in a different manner and 
calculate the temperature-coefficient at 18° C. from the thermochemical 
data and compare it with that found experimentally. 
The calculation gives: 
5 8006 
aap (ees — — 0.001207 Volt, 
\ dt 291 X 22782 ? ‘ 
Ies 
whilst 0.004235 Volt. was found experimentally. 
lod 
7. Thus far we have only considered cells such as are used in 
practice, that is to say cells which contain a large excess of 
undissolved Zn SO, . 7 H,O. 
If the cell has a higher temperature than 39°, which is the tran- 
sition point of the salt with 7 mols. of water of crystallisation, or 
when it has come down to a lower temperature after the complete 
change of the solid salt, the cell will contain undissolved ZnSO,4.6H,0 *). 
Similar considerations then lead to the following equation for the 
change which occurs during the passage of the current: 
6 a 
Some ZnSO, a H,0 = a 
Zn SO, = Zn SO,. 6 H,0 . . (D) 
For this cell we can also calculate Z, from JAEGER's electrical 
measurements and compare it with the value found by the calori- 
metric method. 
1) See CALLENDAR and Barnzs, Proc. Roy. Soc. 62, 117. Jarcer, Wiep. Ann. 63, 
354. ConeEn, Zeitschrift fiir phys. Chemie 25, 300 (1898). Barnes, Journal of physical 
Chemistry 4, 1 (1900). 
