342 EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



such a direction of the electromotive force would involve the pos- 

 sibility of obtaining an indefinite amount of electromotive work, and 

 therefore of mechanical work, without other expenditure than that of 

 heat at the constant temperature of the cell. 



None of the cases which we have been considering involve com- 

 binations by definite proportions, and, except in the case of the cell 

 with electrodes of mercury and zinc, the electromotive forces are 

 quite small. It may perhaps be thought that with respect to those 

 cells in which combinations take place by definite proportions the 

 electromotive force may be calculated with substantial accuracy from 

 the diminution of the energy, without regarding, the variation of 

 entropy. But the phenomena of chemical combination do not in 

 general seem to indicate any possibility of obtaining from the combin- 

 ation of substances by any process whatever an amount of mechanical 

 work which is equivalent to the heat produced by the direct union of 

 the substances. 



A kilogramme of hydrogen, for example, combining by combustion 

 under the pressure of the atmosphere with eight kilogrammes of 

 oxygen to form liquid water, yields an amount of heat which may be 

 represented in round numbers by 34000 calories.* We may suppose 

 that the gases are taken at the temperature of C., and that the 

 water is reduced to the same temperature. But this heat cannot be 

 obtained at any temperature desired. A very high temperature has 

 the effect of preventing to a greater or less extent, the combination of 

 the elements. Thus, according to M. Sainte-Claire Deville,t the tem- 

 perature obtained by the combustion of hydrogen and oxygen cannot 

 much if at all exceed 2500 C., which implies that less than one-half 

 of the hydrogen and oxygen present combine at that temperature. 

 This relates to combustion under the pressure of the atmosphere. 

 According to the determinations of Professor BunsenJ in regard 

 to combustion in a confined space, only one-third of a mixture of 

 hydrogen and oxygen will form a chemical compound at the tem- 

 perature of 2850 C. and a pressure of ten atmospheres, and only a 

 little more than one-half when the temperature is reduced by the 

 addition of nitrogen to 2024 C., and the pressure to about three 

 atmospheres exclusive of the part due to the nitrogen. 



Now 10 calories at 2500 C. are to be regarded as reversibly con- 

 vertible into one calorie at 4 C. together with the mechanical work 

 representing the energy of 9 calories. If, therefore, all the 34000 

 calories obtainable from the union of hydrogen and oxygen under 

 atmospheric pressure could be obtained at the temperature of 



* See Riihlmann's Handbuch der mechanischen Warmetheorie, Bd. ii, p. 290. 

 tComptes Rendus, t. Ivi, p. 199; and t. Ixiv, 67. 

 \ Pogg. Ann., Bd. cxxxi (1867), p. 161. 



