340 Dr. J. Hopkinson on the Seat of the 



II. The thermodynamics of the voltaic circuit may be dealt 

 with on either method of treatment ; in the equations already 

 used, instead of speaking only of the heat disappearing from 

 any region, we have to consider the heat disappearing when 

 the unit electricity passes plus the energy liberated by the 

 chemical changes which occur. Consider a thermoelectric 

 combination in which there is chemical action at the junctions 

 when a current passes. 



If Gt be the function of the temperature which represents 

 the energy of the chemical reaction which occurs when unit 

 of electricity passes from X to Y across the junction, we have 



F(f 2 ) + G(* 2 )-F(f0-G(O+JVw^+j^(0^=/(^)-/(^), 



F'(t)+CY(t)+<l>(t)-1r(t)=f(i), 



W(t)/t-F(t)/t* + <j>(t)/t-f(t)/t=0; 



whence 



F(t)=tf(t)-tW(t), 1 



<Kt)-f(t)=Hf n (t)-G"(t)}J 



If now we proceed on the hypothesis of specific heat of 

 electricity, we are able to make the differences of potentials 

 at the junctions accord with the indications of electrostatic 

 experiments. We are, then, by no means bound in a voltaic 

 cell to suppose that there is a great difference of potential 

 between the electrolyte and the metal because there is a 

 reaction there, for we may suppose the energy then libe- 

 rated is taken up by the change that occurs in the specific heat 

 of electricity. 



III. Adopting the second method of expressing the facts, 

 we may consider further the location of the difference of 

 potential in a voltaic cell. In the case of a Daniell's cell 

 consisting of Cu | CuS0 4 | ZnS0 4 | Zn, at which junction is 

 the great difference of potential ? Dr. Lodge places it at the 

 junctions of the metals and the electrolytes. For this there 

 is really some experimental reason, but without such reason 

 it is not apparent why there may not be a great difference of 

 potential between CuS0 4 and ZnS0 4 . In that case, in an 

 electrolytic cell with zinc or copper electrodes and ZnS0 4 or 

 CuS0 4 as electrolyte there would exist a small difference of 

 potential between the metal and the electrolyte. Take the latter 

 case, an electrolytic cell of CuS0 4 , and let us leave out of 

 account the irreversible phenomena of electrical resistance and 

 diffusion. First, let us assume, as is not the fact, that the only 

 change in the state of the electrolytic cell when a current has 



