EQUILIBEIUM OF HETEKOGENEOUS SUBSTANCES. 343 



2500 C., and no higher, we should estimate the electromotive work 

 performed in a perfect electro-chemical apparatus in which these 

 elements are combined or separated at ordinary temperatures and 

 under atmospheric pressure as representing nine-tenths of the 34000 

 calories, and the heat evolved or absorbed in the apparatus as 

 representing one -tenth of the 34000 calories. * This, of course, would 

 give an electromotive 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,t and 

 we must 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 typical 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.} 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 tempera- 

 tures comparatively low. This indicates that the theoretical value 

 of the electromotive force necessary to electrolyze this acid (i.e., 

 the electromotive 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 



* These numbers are not subject to correction for the pressure of the atmosphere, 

 since the 34000 calories relate to combustion under the same pressure. 



t 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, Comptes Rend.us, t. Ixiv, p. 67. 



