ELECTROCHEMICAL THERMODYNAMICS 719 



action at constant temperature. He asks us to consider a 

 change in which two molecules, A and B, combine to form a 

 third, AB, with the evolution of heat Q. Now imagine them 

 to react in a galvanic cell at a temperature t', and then complete 

 a cycle by bringing the system back to its initial state by a series 

 of reversible processes which involve the supplying of heat, but 

 which for the sake of simplicity involve no work. This cycle 

 can be represented by 



A+B-^AB-^W + Q (t = t') 



A+B^AB + [Q] {t = t") 



in which the intrinsic energy changes are Ae = [Q] at t", and 

 — Ae = W -\- QbXI', respectively. According to equation (19), 

 we have the well known relation 



Q [Q] 



p + ^ = 0, (27) 



where t" is defined by equation (23), and equals the highest tem- 

 perature at which all the heat may be obtained. Obviously, if 

 [Q] exists and possesses a finite value at a finite temperature, Q 

 must exist at a temperature, t'. Since a change in a finite quan- 

 tity of substance will be accompanied by a finite change in internal 

 energy, [Q], the only condition which will cause Q to vanish will 

 be that under which all the heat may be obtained at an infinite 

 temperature. Gibbs does not deny this possibility, but simply 

 states that this certainly does not represent the usual case. 



t' 

 Further, the magnitude of Q is given by -r, [Q], and the work 



t" - t' 

 performed by the cell, W, is given by — -f, — [Q]. These con- 



siderations form the basis of the discussion on pp. 342-344 of 

 the "Equilibrium of Heterogeneous Substances," and in the 

 first letter (Gibbs, I, 406) to the Secretary of the British Asso- 

 ciation for the Advancement of Science. 



The remainder of the discussion of this subject on pp. 344-348 

 of the "Equilibrium of Heterogeneous Substances" has simply 



