( 335 ) 



Electrode, rever- i Saturated solution of the i Electrode, rever- 

 sible with respect [ salt S in presence of the sible with respect 

 to the anion. ; metastahle solid phase of the ' to the cathion. 

 ! salt. 



are coupled up in opposition to each other, a transition element of 

 the third kind ^j is obtained. 



If the salt (S' is zinc sulphate, the combination in question may 

 be composed of two Clark-ccHs; in the one Zn SO4.. 7 H3O, in the 

 other Zn SO4 . 6 H2 forms the solid phase provided that the tem- 

 perature lies between the crj'ohydratic temperature of ZnS04.6H20 

 and the transitionpoint (39°). 



2. Tlie electromotive force E of this transition element is the 

 measure of the maximum work which the reaction occurring in the 

 element at the temperature T can perform. 



In a later communication I shall show how E may be calculated 

 by thermodynamics. 



Experimentally, E may be directly determined or it may be cal- 

 culated from the measurements of Jaeger^), who has measured the 

 E. M. F. of CLARK-cells at different temperatures, Zn SO4 . 7 Hj 

 (the stable phase) or Zn SO^ . 6 H3 (the metastable phase) being- 

 present as the solid phase between 0'' and 39°. 



In this way the following numbers are obtained : 



EJI F. of the trausition elemeut. 



3. The velocity with which the reaction which occurs in the 



') See CoHE^f, Zeitsclirift fiir pbys. Chemie, Bd. 25 (1898). S. 300. 

 ') Wiedemann's Annaleu, lid. 63 (1897), 354. 



