378 KEYES 



ART. J 



the energy of interaction, the pressure or volume, and the tem- 

 perature. At least this is the goal which is of chief interest to 

 the chemist using thermodynamics as a means of correlating 

 equilibrium data, and some conceptions of a molecular nature 

 are required in practice notwithstanding the often repeated 

 statement that thermodynamics has no need of molecular 

 hypotheses. The latter dictum is really true only in a restricted 

 sense in the field of the applications of thermodynamics to the 

 extensive and varied phenomena of chemistry. 



The term phases of dissipated energy may be assumed equiva- 

 lent to what is now generally called the equilibrium state. It is 

 for this state alone that the energy is a minimum and the 

 entropy a maximum (see Gibbs, I, 56, "Criteria of Equilibrium 

 and Stability' ' ) . Of course equilibrium states are not always easy 

 to realize, but in every case of doubt as to the establishment of 

 equilibrium in the case of chemically interacting components 

 the usual test in practice is to vary the independent variables, 

 pressures or temperature or both, at the supposed state of 

 equilibrium and to observe the displacement, finally verifying 

 the possibility of reproducing the original condition of true 

 equilibrium at the point in question. 



Gibbs' treatment involves the masses of the components 

 instead of the mols now used. Equation [299] in the concrete 

 case of the formation of water from the elements would be 

 written, 



1 g. (H2O) = 8/9 g. (O2) + 1/9 g. (H2). (106) [299] 



But for the condition of equilibrium it has been proved that 



Zfii8mi ^ 0, 



and our knowledge of the principles of chemical combination 

 allows us to identify the variations 5wi, 8m2, ... as proportional 

 to the X coefficients as in (106) [299]. In equation [300], 8ms may 

 be replaced by —1 if water is assumed to disappear in the 

 reaction, whence 5w2 becomes 8/9 and 8mi 1/9, both reckoned 

 plus, i.e., 



^ Ml + I M2 = M3, (107) [301] 



