EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 347 



apparatus, 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 of AQ would be much greater than in the 

 imperfect apparatus, 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 sufficiently weak. This would make a con- 

 siderable difference in the variation of energy in the cell, and the 

 electromotive force cannot certainly be calculated from the variation 

 of energy alone in all these cases. The correction due to the work 

 performed against the pressure of the atmosphere when the ion 

 is set free as gas will not help us in reconciling these differences. 

 It will appear on consideration that this correction will in general 

 increase the discordance in the values of the electromotive force. 

 Nor does it distinctly appear which of these cases is to be regarded 



* Except in the case of the Grove'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 343. ) 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 (M4m. Savants Etrang., 

 loc. cit., p. 131 ; or Comptes Rendus, 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 perform 

 its office, this could only cause a diminution in the value of AQ. 



A large part at least of the chlorine appears to have been absorbed by the electrolytic 

 fluid. It is probable that a slight difference in the circumstances of the experiment 

 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 water presents some anomalies, and may be attended with 

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

 evolution of heat. (See Berthelot, Comptes Eendiis, t. Ixxvi, p. 1514.) If we regard 

 the evolution of the chlorine in the form of gas as the 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. 



