74 BUTLER 



ART. D 



Systems are frequently met with which are not in equilib- 

 rium, yet which appear to remain unchanged for an unlimited 

 time. Thus, a mixture of hydrogen and oxygen appears to 

 remain unchanged, although it Ls not in a true state of equilib- 

 rium, for a small cause such as an electric spark may cause a 

 change out of all proportion to its magnitude. In such a case 

 the change of the system into a state of equilibrium is supposed 

 to be prevented by "passive forces or resistance to change," the 

 nature of which is not well understood. It is evident that only 

 those forces or resistances which are capable of preventing 

 change need be considered. Those like viscosity, which only 

 retard change, are not sufficient to make impossible a variation 

 which they influence. 



The existence of such passive resistances to change can easily 

 be recognised. Thus, it is possible that a system composed of 

 water, oxygen and hydrogen which is not in equilibrium with 

 regard to changes involving the formation of water, will remain 

 unchanged for an indefinite period. This equilibrium can be 

 distinguished from that caused by "the balance of the active 

 tendencies of the system," i.e., when the tendency of hydrogen 

 and oxygen to combine is balanced by the tendency of water 

 to dissociate, for whereas in the former case we may vary the 

 quantities of any of the substances, or the temperature or pres- 

 sure without producing any change in the quantity of water 

 present in the system ; in the latter case an infinitesimal change 

 in the state of the system will produce a change in the amount 

 combined. 



Thus if we regard variations involving the combination of 

 hydrogen and oxygen as prevented by the passive forces or 

 resistances, and therefore impossible, we may still apply the 

 conditions of equilibrium to discover the equilibrium state of 

 a system containing given amounts of hydrogen, oxygen and 

 water under these conditions. 



5. Sufficiency of the Criteria of Equilibrium* Three cases 

 are considered, corresponding to the three kinds of equilibrium. 



(a) "If the system is in a state in which its entropy is greater 



* Gibbs, I, p. 58, line 41-p. 61, line 11. 



