710 HARMED 



ART. M 



treatment by the methods of thermodynamics, but only those 

 cells which fulfil the following conditions: 



(1) No changes must take place without the passage of the 



current. 



(2) Every change which takes place during the passage of 



the current may be reversed by reversing the direction 

 of the current. 



These conditions define the "perfect electrochemical appa- 

 ratus," or the reversible galvanic cell. 



The first condition excludes cells containing metal to fiquid 

 surfaces which react chemically, such as Volta's in which 

 alternate copper and zinc plates were separated by a fibrous 

 material moistened with sulphuric acid. The second condition 

 makes possible the measurement of the reversible electrical work 

 of the cell, and, concomitantly, the change in thermodynamic 

 potential, f, or the change of work content, i/', which accom- 

 panies the physical or chemical changes occurring in the cell. 



Since this second condition is necessary for every direct 

 measurement of changes in f or \p, its more careful considera- 

 tion, particularly in reference to cell measurement, will help to 

 clarify further discussion. A reversible process is one in which 

 every successive state is a state of equilibrium. The maximum 

 or reversible work is that obtainable from this ideal reversible 

 process. Thus, the evaporation of a liquid against an external 

 pressure just equal to its vapor pressure is a reversible process, 

 and the work done by the vapor is the reversible work. 



Let us consider a cell which has proved of considerable im- 

 portance in recent physical chemistry, and which has the char- 

 acteristics necessary for the present discussion, namely, 



Pt 1 H2 (1 atm.) 1 HCl (m) | AgCl 1 Ag. 



This consists of a hydrogen electrode, at one atmosphere pres- 

 sure, in contact with a hydrochloric acid solution at a concentra- 

 tion m, which is also in contact with a silver-silver chloride 

 electrode. All these substances will remain unchanged after 

 the solution has become saturated with the slightly soluble 

 silver chloride. If we connect the terminals, this cell will dis- 

 charge, positive current will flow from the hydrogen electrode 



