720 HARMED 



ABT. M 



to do with proving that the data which existed at the time of 

 writing, and which were obtained chiefly by Favre, substantiated 

 the existence of heat changes during cell action. Since a great 

 many accurate observations obtained in recent years completely 

 confirm the contentions of Gibbs, and since the illustrations 

 employed by him are far less accurate, it seems unnecessary to 

 discuss this matter further. 



III. The Extension of the Theory of Galvanic Cells Not 



Explicitly Developed, but Contained Implicitly 



in the Thermodynamics of Gibbs 



Equation (17) [700] has proved to be of the greatest impor- 

 tance to chemistry, and since the f function is peculiar to Gibbs 

 it is to this extent unique in the history of the subject. This 

 equation states that the reversible electrical work obtainable 

 from a cell at constant temperature and pressure is equal to the 

 decrease — d'f, in thermodynamic potential, corresponding to 

 the cell processes. Since it is far more convenient to measure 

 a cell at constant pressure and temperature than at constant 

 volume and temperature, d^ is more easily obtainable than d\j/. 



If then a reversible cell can be constructed in such a way 

 that the net effect of all the changes in the cell during the flow 

 of current corresponds to a chemical reaction, the change in 

 thermodynamic potential may be computed. This affords a 

 very powerful experimental method for investigating the 

 increase or decrease of thermodynamic potential correspond- 

 ing to reactions which occur between solids, between solids and 

 liquids, or between solids, liquids, and gases. In fact, in recent 

 years cells have been constructed by means of which the changes 

 in thermodynamic potential of all types of chemical reactions 

 have been studied.* 



Early in the "Equihbrium of Heterogeneous Substances," 

 Gibbs has shown that the differential of the thermodynamic 



* Recent surveys and discussion of this subject may be found in 

 Taylor, Treatise on Physical Chemistry, 2nd Ed., Vol. I, pp. 731-745, 

 D. Van Nostrand Company, New York (1924). See also International 

 Critical Tables, Vol. VI, pp. 312-340, McGraw-Hill Book Co. (1930). 



