342 Seat of the Electromotive Forces in a Voltaic Cell. 



happens is that a certain quantity of S0 4 enters the region 

 and an equal quantity departs from it, from which follows 

 that there is no potential difference, other than a Peltier 

 difference, at this junction. 



Neither of the extreme suppositions we have made as to 

 concentration or impoverishment of the solution is in fact 

 true, but they serve to show that the position of the steps 

 in potential depends entirely on the travelling of the ions. 

 The fact is, that in general both ions travel in proportions 

 dependent on the condition of the electrolytes ; it is probable 

 that the travelling of the S0 4 depends on some acidity of the 

 solution. Given the proportion in which the ions travel and 

 the energy of the reversible chemical reaction which occurs, 

 and we can calculate the differences of potential at the 

 junctions. 



In the preceding reasoning an assumption has been made, 

 but not stated. It has been assumed that the passage of a 

 current in an electrolyte is accompanied by a movement of 

 ions only, and not by a movement of molecules of the salt ; 

 that is, when unit of electricity passes through a solution of 

 CuS0 4 , #Cu travels in one direction and (1 — #?)S0 4 in the 

 opposite direction, but that CuS0 4 does not travel without 

 exchanges of Cu and of S0 4 between the molecules of CuS0 4 . 

 In the supposed case when there is no concentration around 

 the anode, my assumption is that Cuis dissolved off the anode, 

 and that an equal quantity of Cu leaves the region around the 

 anode as Cu by exchanges between the molecules of CuS0 4 . 

 But it is competent to some one else to assume that in this 

 case S0 4 as SO± enters the region by exchanges between the 

 molecules of CuS0 4 , and that at the same time a molecule of 

 CuS0 4 leaves the region without undergoing any change. 

 Such a one would truly say that there was no inconsistency 

 in his assumption; and that if it be admitted, it follows that the 

 difference of potential at the junction CuS0 4 | Cu is that repre- 

 sented by the energy of the reaction. I prefer the assumption 

 I have made, because it adds nothing to the ordinary chemical 

 theory of electrolysis; but it is easy to imagine that facts may be 

 discovered more easily expressed by supposing that an electric 

 current causes a migration of molecules of the salt, as well as 

 a migration of the components of the salt. 



