ELECTROCHEMICAL THERMODYNAMICS 715 



geneous Substances." The directness and simplicity of his 

 method are strikingly manifest. 



Let us consider for the moment equation (15), which, allow- 

 ing for an irreversible process, is 



(7" - V')de ^ -de + tdr, - pdv. (15a) 



If the cell is maintained at constant volume, the last term 

 vanishes, and if no heat is absorbed or evolved by the cell, the 

 term tdr] vanishes, and the electrical work is equal to or less 

 than the diminution of energy. Owing to the lack of very 

 accurate experimental results as well as a confusion regarding 

 the fundamental concepts involved, and to the fact that, in 

 some cases of familiar cells, the term td-q is small compared to de, 

 many investigators of the last century were of the opinion that 

 the electrical work is entirely accounted for by the diminu- 

 tion of energy. Since cells are measured at constant tem- 

 perature and not at constant entropy, there is no reason why 

 the term td-n should vanish. Gibbs, therefore, takes great 

 care in the subsequent discussion (Gibbs, I, 340-347) to place 

 this matter in the correct light. 



We shall postpone the consideration of this matter and 

 consider the alternative deduction of the general law (equation 

 [6]) given in the second letter to the Secretary of the Electrolysis 

 Committee of the British Association for the Advancement of 

 Science (Gibbs, I, 408-112). Gibbs wrote this letter in order 

 to explain more fully his position, and its contents constitute 

 the only other explicit statement of his thermodynamics of 

 the galvanic cell. 



Consider a reversible cycle in which a cell discharges at a 

 constant temperature t', producing electrical work, mechani- 

 cal work and possibly heat effects. Chemical changes will take 

 place. Then, by reversible processes which do not involve the 

 passage of electricity, bring the system back to its original state 

 by supplying or withdrawing the necessary work and heat. 



Let W and Q equal the work done and the heat absorbed by 

 the system during the discharge of the cell, and [W] and [Q] 

 equal the corresponding work and heat changes during the 

 reversible processes employed to bring the cell back to its 



