( 612 ) 
sented by the difference of the heats of formation of Zn SO, and 
He, SOx. 
It must be at once pointed out that under the ordinary conditions 
in which the cell is used, namely below 39°, the ZnSO, generated 
by the current will take up 7 H,O, which action is accompanied 
by evolution of heat. 
One would thus be inclined to replace the equation A by: 
Zn —— He, SO, ~- aq. = 2 Hg > Zn SO, 7 H,0 e ° e (A') 
But even this representation of the matter is incomplete and gives 
totally wrong results if a calculation is based on it. I, here, refer 
to the article of Mac-Inrosu in the Journal of Physical Chemistry, 
Vol. 2. p. 185 (1898), in which this erroneous view forms the 
starting point. 
He investigated a cell of the CrLARK-type in which the Zn and 
Zn SO,.7H,0 had been replaced by Cu and CuS0,.5H,0O. The 
difference of the heats of formation of CuSO, .5 H,O and Hes SO, - 
in accordance with the equation : 
Cu + Hg; SO, + ay. =: 2 Hy + CuS0,.5 0,0. . . (B) 
is regarded as the heat evolved by the reaction taking place in 
the cell. 
From the measured E.M.F., 0,3613 volts, of this cell at T= 290°, 
dl 
and the temperature-coefficient CF ae 0,0006 volts at that tempe- 
¢ 
rature, he calculates (taking the heat of formation of CuSO,.5 H,O 
as 201000 calories) the heat of formation of HggSO, to be 175000 
calories. With the aid of this figure and the heat of formation of 
Zn 50, .7 H2O (253000 calories) he calculates, starting from the 
equation 4’ the K.M.F’. of the Cuarx-cell at 17°,0 to be 1.427 volts, 
whilst Kanre has found it to be 1.4304 at 17°,0. 
We will see later on that the difference of 2,6 millivolts is due 
to the wrong principle on which the calculation is based. 
4. I will now in the first place calculate Z, for the CLARK-cell with 
the aid of the data given by the electrical measurements of KAHLP, 
JAEGER, WACHSMUTH, CALLENDAR and Barnes. The calculations 
will all be worked out for 7= 291°, as the calorimetric determi- 
nations of THOMSEN, which we shall use later on are made at that 
temperature. 
