( 611 ) 
then apply those considerations to the Cnark-cell for which the 
necessary date are in existence. It will be seen that there is a very 
satisfactory agreement between theory and practice. 
2. By the investigations of Jann!), the accuracy of the celebrated 
equation of Gists and v. HermHoLtz, which gives the relation 
between the electrical energy, the chemical energy and the tempe- 
rature-coefficient of a reversible galvanic cell in qualitative as well 
as in a quantative direction, has been confirmed. We will take 
that equation as starting point, 
gen ae SEN 
neg dT 
in which Z represents the E.M. F. of the reversible cell at the 
temperature 7, ZE, the chemical energy of the chemical, or rather 
chemico-physical process which takes place in the cell (at 7°) when 
ne, Coulombs pass through it; » is the valency of the moving ion 
dE 
and aa the temperature-coefficient of the K.M.F. at 7e. 
3. Both the E.M.F. and the temperature coefficient of the CLARK- 
and WesroN-cell have been determined by the above mentione' 
with such a high degree of accuracy that they provide an excellent 
material from which Z may be calculated by means of equation (1). 
We will give those calculations provisionally for the CLARK-cell 
only, as some data for the Wesron-cell are still wanting and will 
have to be determined experimentally. In order that Ee may also 
be calculated from the caloric figures, so as to compare the value 
thus found with that found by the electric method, one must first 
understand what happens in the cell when & Coulombs pass through 
it. The mechanism of the change has so far, as for instance by 
Nernst in his “Theoretische Chemie” (2° Aufl. 1898), S. 657-658, 
been represented by the equation : 
Zn -+ He, SO, a 2 Hg + Zn S04 . ekke . e (A) 
If this representation were correct, then when 2 X 95640 Cou- 
lombs have passed through the cell and consequently 1 gram-atom 
of zine has passed into solution, the heat-effect “, would be repre- 
1) Wrep. Ann. 28, S. 21 en 491 (1886); ibid 63, S. 44 (1897). 
