﻿Molecular Thermodynamics. 227 



In reply, Planck * pointed out that the theoretical pos- 

 sibility of the ideal transition depended only on the fact thai 

 the numbers of the various molecular species were, together 

 with temperature and pressure, the independent variables 

 which determined the phase. 



Now the present author would suggest that Planck's reply 

 can be construed (and, to be unanswerable, must be construed) 

 as a rider to the definition of the terms " molecule " and 

 " chemical compound," for the purposes of molecular 

 thermodynamics, viz. : — " That the numbers of the various 

 molecular species can be considered, together ivith temperature 

 and pressure, as the independent variables determining the 

 phase/'' or, in other words, " that it shall be theoretically 

 sound to conceive any desired change of temperature and 

 pressure of the system as taking place without change in the 

 numbers present of the various molecular species f.." 



Oniy with this rider to our definition can it be laid down, 

 for instance, that the "mass-action" equilibrium law must 

 be obeyed in sufficiently dilute solution, for it is to be 

 observed that purely " general " thermodynamics has no 

 cognisance of molecules, but takes for its independent 

 variables, besides temperature and pressure, the masses of 

 the "components.'" [See next section of this paper.] 



The misconceptions which have so long stood in the way 

 of a satisfactory general theory of electrolytic or "con- 

 ducting " solutions seem sufficiently to illustrate the indis- 

 pensability of this postulate. 



A parallel illustrating its signih'cance may be drawn from 

 the dynamical theory of chemical combination and dis- 

 sociation. 



The classical dynamical conception of a binary molecule 

 (for example) was a pnir of simpler molecules (or atoms) 

 moving relatively to one another in closed orbits, and the 

 principle of the conservation of energy forbade the spon- 

 taneous dissociation of such a " molecule," requiring that its 

 disruption should depend on collision with another molecule. 



The well-known fact that dissociation is (at constant 

 temperature) independent of collision-frequency, showed the 

 inadequacy of this conception, and pointed to some property, 

 in the "forces" producing and maintaining a molecule, 

 altogether incompatible with the older or "continuous" 

 dynamics. 



* Ann. der Phys. x. p. 436 (1903).^ 



f Intermediate and tinal states being unstable, of course, in general. 



Q2 



