Molecular Thermodynamics. 611 



molecular species vastly preponderates in each phase *, the 

 case in fact o£ very dilute solutions in non-associated, unmixed 

 solvents f. 



In section I. above, the " molecular''' treatment is extended 

 to the perfectly general system o£ phases of any composition 

 or state of aggregation, subject only to the very broad 

 assumption (1). 



That the " molecular " treatment when possible is 

 essentially a step towards simplicity can scarcely be 

 doubted, but apart from this, it appears to the author 

 that the great importance and value of what is here called 

 " Molecular thermodynamics " lies in the fact, apparently 

 not recognized so far, that by expressing the Total-Energy 

 in terms of: the concentrations of the various molecular 

 species (instead of, as in general thermodynamics, in terms 

 of the concentrations of the components) we relegate all the 

 chemical energy entirely to the linear terms. 



The "general terms ." must be considered to represent only 

 potential J energy of physical intermolecular forces. This 

 suggests at onee the possible use of Dynamical theory, which 

 is, at least theoretically, applicable to the calculation of such 

 energy. In section V. a striking and important example 

 of the realization of this possibility will be given. 



As regards the general application of molecular thermo- 

 dynamics, the determination of Sty, 8Q, SY, for various 

 isothermal isopiestic changes and of the relations of these 

 quantities to the molecular concentrations in the various 

 phases, to the temperature, and to the pressure, enables us, 

 proceeding from the simpler to the more complex cases, to 

 draw conclusions as to the form of the general terms in the 

 above expressions for U, V, Q, and ty. It is clear that the 

 complete solution of the problem would give us one general 

 form for these "general terms " which would be applicable to 

 any phase of any molecular composition, at any temperature 

 and pressure. It would then be a simple matter to describe 

 completely any system of phases given the temperature, 

 pressure, and total masses of the components. 



We can proceed at first only with relatively simple special 

 cases. On the one hand, the approach to the limiting linear 



* This is not the case, for instance, in dilute aqueous or alcoholic 

 solutions. 



t The remarkable " gas-analogy " discovered by van't Hoff (and so 

 often mis-used by others) was thus for the first time elucidated, — at least 

 in the case of non-associated simple solvents. 



% Conceivably also, some special Kinetic Energies (such as kinetic 

 energy of rotation) which might depend upon specific intermolecular 

 effects to some extent. 



2U2 



