t 469 ) 



have looked more closely into one of Prof. Boltzmann's equations, 

 viz. the equation 1. c. pag. 169 



17 b^\ i^" / n b^ \ i^ 



-which equation is to be applied, when two phases are in equilibrium. 

 It is well known from thermodynamics that when two phases of 

 a simple substance coexist, besides p and T, the thermodynamic 

 potential must also be equal. The preceding equation of Prof. 

 BoLTZMANN must therefore be the kinetic interpretation of the ther- 

 modynamic relation 



^fff = ,uf 



or 



f — Tr] 4- pv —t — Tl] -{-pv . 

 a 3 9 f I J ■ 



In a paper (Verslag der Vergadering 26 Januari '95, also published 

 Arch. Neerl., T. XXX), entitled "de kinetische beteekenis van de 

 éhermodynamische potentiaal" I have given the kinetic interpre- 

 tation of this thermodynamic relation. For a fuller discussion I 

 refer to this paper, and when I compare this result with the equa- 

 tion of Prof. BOLTZMANN for that equilibrium, it is evident that we 

 have a different conception of the problem. I do not mean to say, 

 that our results do not agree at a first approximation. But they 

 are not identical. 



In the first place there is the following difference. According to 

 Prof. BoLTZMANN the energy required to remove one molecule' from 

 the liquid phasis is simply the work required to overcome the coher 

 sion; according to me that work must be diminished by the quantity 

 which I have called the work of the thermic pressure. In the second 

 place Prof. Boltzmajin subtracts from the, specific, volume a quantity 

 twice as large as the quantity which I think must be subtracted 

 from it. 



In my opinion there is no doubt that also the work of the ther- 

 miö pressure must be taken into account, if we ascribe real dimen- 

 sions to the molecules. If a molecule leaves a phasis, not only the 

 increase of its potential energy is. to ,be, taken .into- account, but we 

 must also pay attention to the fact that the quantity from which 

 the molecule has escaped, has diminished, tha.t the surface of that 

 quantity has contracted, and that therefore a quantity of work has 



