J. W^. Gibbs — Eqidlibrmm of Heterogeneous Substances. 513 



the diminution of the energy, without regarding the variation of 

 entropy. But the phenomena of chemical combination do not in 

 general seomi to indic^ate any possibility of obtaining from the com- 

 bination of substances by any process whatever an amount of mechani- 

 cal Avork 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 repre- 

 sented in roimd number's by 34000 calories.* We may suppose that 

 the gases are taken at the temperature of 0° 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 eifect 

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

 elements. Thus, according to M. Sainte-Claire Deville,f the tempera- 

 ture 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 itnder 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 pressiire 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 cal- 

 ories obtainable from the miion of hydrogen and oxygen under atmos- 

 pheric pressure could be obtained at the temperature of 2500° C, and 

 no higher, we should estimate the electromotive work performed in a 

 perfect electro-chemical apparatus in which these elements are com- 

 bined 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 electi'omotive 



*See RuMmann's Handhuch der mechanischen Warmeiheorie, Bd. ii. p. 290. 

 •j- Comptes Reiidus, t. Ivi, p. 199; and t. Ixiv, 67. 

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



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

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



