514 J.W. Gibbs — Equilibrium of Heterogeneous Substances. 



force exactly nine-tenths as great as is obtained on the supposition 

 that all the 34000 calories are convertible into electromotive or 

 mechanical work. But, according to all indications, the estimate 

 2500° C. (for the temperature at which we may regard all the heat of 

 combustion as obtainable) is far too high,* and we nnist regard the 

 theoretical value of the electromotive force necessary to electrolyze 

 water as considerably less than nine-tenths of the value obtained on 

 the supposition that it is necessary for the electromotive agent to 

 supply all the energy necessary for the process. 



The case is essentially the same with respect to the electrolysis of 

 hydrochloric acid, which is probably a more tyi:)ical example of the 

 process than the electrolysis of water. The phenomenon of dissocia- 

 tion is equally marked, and occurs at a much lower temperature, more 

 than half of the gas being dissociated at 1400° C.f And the heat 

 which is obtained by the combination of hydrochloric acid gas with 

 water, especially with water which already contains a considerable 

 quantity of the acid, is probably only to be obtained at temperatures 

 comparatively low. This indicates that the theoretical value of the 

 electromotive force necessary to electrolyze this acid (/. e., the elec- 

 tromotive force which would be necessary in a reversible electro- 

 chemical apparatus), must be very much less than that which could 

 perform in electromotive work the equivalent of all the heat evolved 

 in the combination of hydrogen, chlorine and water to form the liquid 

 submitted to electrolysis. This presumption, based upon the phenom- 

 ena exhibited in the direct combination of the substances, is corrobo- 

 rated by the experiments of M. Favre, who has observed an absorp- 

 tion of heat in the cell in which this acid was electrolyzed.J The 



* Unless the received ideas concerning the behavior of gases at high temperatures 

 are quite erroneous, it is possible to indicate the general character of a process 

 (involving at most only such difficulties as are neglected in theoretical discussions) by 

 which water may be converted into separate masses of hydrogen and oxygen without 

 other expenditure than that of an amount of heat equal to the difference of energy of 

 the matter in the two states and supplied at a temperature far below 2500° C. The 

 essential parts of the process would be (1) vaporizing the water and heating it to a 

 temperature at which a considerable part will be dissociated, (2) the partial separation 

 of the hydrogen and oxygen by filtration, and (3) the cooling of both gaseous masses 

 until the vapor they contain is condensed. A little calculation will show that in a 

 continuous process all the heat obtained in the operation of cooling the products of 

 filtration could be utilized in heating fresh water. 



\ Sainte-Claire Deville, Oomptes Rendus, t. Ixiv, p. ()7. 



\ See Meinoires (Ivs Sarants Etrangers, Ser. 2, t. xxv, No. 1, p. 142; or Comj)tes Rendus, 

 t. Ixxiii, p. 973. The figures obtained by M. Favre will be given hereafter, in connec- 

 tion with others of the same nature. 



