36 WILSON 



ART. C 



gives rise to states which for some variations behave as stable 

 states but for others give indications of not being entirely 

 stable.* 



excepting those passive forces which only retarded change seem clearly 

 to indicate that there was a state of no process whatsoever associated 

 with the passive resistances rather than one of very slow process. And 

 again in the discussion of Certain Points Relating to the Molecular Con- 

 stitution of Bodies (Gibbs, I, pp. 138- 144) he seems to be drawing a pos- 

 sible logical distinction between passive resistances which prevent 

 change and those which only slow it down, though they may slow it down 

 very greatly. He certainly does seem to postulate that there may be 

 real states of equilibrium which are not states of dissipated energy and 

 which do not even with infinite slowness go over into such states. Lewis 

 and Randall would appear to postulate that there are in reality no such 

 states, that only states of dissipated energy are states of equilibrium. 

 They may be entirely right without Gibbs being in any way wrong. It 

 is important to have the solutions for both ideal cases — that in which the 

 change is absolutely prevented and that in which it is completely con- 

 sumated. A case in practice may well be intermediate between the two 

 so that both solutions might be inapplicable. Gibbs speaks as though 

 hydrogen and oxygen placed together at room temperature would never 

 unite to form water vapor; while Lewis and Randall expect them to unite 

 (almost completely, though slowly) according to their equation (22), p. 

 485, viz., H2 + 5O2 = H20(^) ; A F°2is = —54507, and so, too, we may pre- 

 sume that if hydrogen were shut up by itself they would expect it to go 

 over into helium. There is, of course, no practical difference between 

 the two postulates when the reaction is slow enough, but it would seem 

 that Gibbs' form would be at least as convenient practically as that of 

 Lewis and Randall. 



* The logical difference between stability and slowness in attaining 

 the stable state must be kept in mind. Thus a liquid in the presence of 

 its vapor may be very slow in evaporating to the point where the vapor 

 is saturated and the equilibrium is established. Things do not dry im- 

 mediately simply because there is not equilibrium between their state 

 of wetness or dryness and the humidity in the atmosphere. In thermo- 

 dynamics time is disregarded, the processes are permitted to take place 

 infinitely slowly. Indeed finite velocities may introduce irreversibility. 

 For example in the simple Carnot cycle in the decompression stage 2 

 (Lecture II) it is specified that the decompression is isothermal, which 

 means that it is slow enough so that the medium remains at the tempera- 

 ture of the reservoir. If the medium were a perfect gas pv = at, the 

 work would he W = Spdv = at log (?;2/fi). But if the decompression 

 be fast enough the medium would expand practically adiabatically (and 



