102 BUTLER 



ART. D 



to (82) between the components, >Si, S2,. . . Sn, r equations 

 similar to (84) must be satisfied in addition to the general con- 

 ditions (68), (69) and (72), provided that each of the compo- 

 nents Si, 82,- . . Sn is an actual component of some part of the 

 system. 



But it must be understood that a relation between the com- 

 ponents such as (82) implies not merely the chemical identity of 

 the substances represented, but also that the change of the 

 substances represented by the left hand member into the 

 substances represented by the right hand member can occur in 

 the system and is not prevented by passive resistances to 

 change. For example, in a system containing water and free 

 hydrogen and oxygen, at ordinary temperatures, the combina- 

 tion of hydrogen and oxygen to form water is prevented by 

 "passive resistances to change," so that we cannot write 



l®H + 8©o = 9 ©^4, 



as a relation between the components, for under these conditions 

 there can be no change in the amounts of water in the system in 

 any possible variation of its state. Water must therefore be 

 treated as an independent component and there will be no 

 necessary relation between the potential of water and the 

 potentials of hydrogen and oxygen. 



12. Effect of a Diaphragm {Equilibrium of Osmotic Forces) * 

 Consider the equilibrium between two homogeneous fluids, 

 separated by a diaphragm which is permeable to some of the 

 components and impermeable to others. Suppose that the two 

 fluids are enclosed in a rigid, heat-insulating envelop as before, 

 but that they are separated by a rigid, immovable diaphragm. 

 We shall distinguish quantities which refer to the two sides of 

 the diaphragm by single and double accents. 

 As before, the total entropy of the system is constant, i.e., 



dv' + 8v" = 0, (85) [72] 



and the total quantities in both fluids of those components. 



* Gibbs, I, 83-85. 



