518 'T. W. Gibbs — Equilibrium, of Heterogeneous /Substances. 



ison of different calorimetrical measurements, involving very mnch 

 greater quantities of heat. 



In considering the numbers which have been cited, we should 

 remember that when hydrogen is evolved as gas the process is in 

 general very far from reversible. In a perfect electrochemical appara- 

 tus, the same changes in the cell would yield a much greater amount 

 of electromotive work, or absorb a much less amount. In either case, 

 the value oi /\ Q would be much greater than in the imperfect appara- 

 tus, the difference being measured perhaps by thousands of calories.* 



It often occurs in a galvanic or electrolytic cell that an ion which 

 is set free at one of the electrodes appears in part as gas, and is in 

 part absorbed by the electrolytic fluid, and in part absorbed by the 

 electrode. In such cases, a slight variation in the circumstances, 

 which would not sensibly affect the electromotive force, would cause 

 all of the ion to be disposed of in one of the three ways mentioned, if 

 the current were siiflficiently weak. This would make a considerable 



* Except in the case of the Grrove's cell, in which the reactions are quite complicated, 

 the absorption of heat is most marked in the electrolysis of hydrochloric acid. The 

 latter case is interesting, since the experiments confirm the presumption afforded by 

 the behavior of the substances in other circumstances. (See page 514.) In addition 

 to the circumstances mentioned above tending to diminish the observed absorption of 

 heat, the following, which are peculiar to this case, should be noticed. 



The electrolysis was performed in a cell with a porous partition, in order to prevent 

 the chlorine and hydrogen dissolved in the liquid from coming in contact with each 

 other. It had appeared in a previous series of experiments {Mem. Sarants Etrang.. 

 loc. cit., p. 131 ; or Oomptes Bendus, t. Ixvi, p. 1231,) that a very considerable amount of 

 heat might be produced by the chemical union of the gases in solution. In a cell 

 without partition, instead of an absorption, an evolution of heat took place, which 

 sometimes exceeded 5000 calories. If, therefore, the partition did not perfectly per- 

 form its office, this could only cause a diminution in the value of A Q. 



A. large part at least of the chlorine appears to have been absorbed by the electro- 

 lytic fluid. It is probable that a slight difference in the circumstances of the experi- 

 ment — a diminution of pressure, for example, — might have caused the greater part of 

 the chlorine to be evolved as gas, without essentially affecting the electromotive force. 

 The solution of chlorine in w^ater presents some anomalies, and m,ay be attended with 

 complex reactions, but it appears to be always attended with a very considerable evolu- 

 tion of heat. (See Berthelot, Comptes Bendus, t. Ixxvi, p. 1514.) If we regard the evolu- 

 tion of the chlorine in the form of gas as tlie normal process, we may suppose that the 

 absorption of heat in the cell was greatly diminished by the retention of the chlorine 

 in solution. 



Under certain circumstances, oxygen is evolved in the electrolysis of dilute hydro- 

 chloric acid. It does not appear that this took place to any considerable extent in the 

 experiments which we are considering. But so far as it may have occurred, we may 

 regard it as a case of the electrolysis of water. The significance of the fact of the 

 absorption of heat is not thereby affected. 



