﻿Molecular Thermodynamics. G27 



Practical Significance and Application. 

 Now the equation 



J^+BSa/l-loJEflJi . . ■ (82) 



or less concisely, but more practically, 



dMo 

 g=^_Elogc s 



is equivalent to the Raoult-van't Hoff limiting laws of dilute 

 solution, the older criterion of "perfect" behaviour. 



In the circumstances considered in this paper (as stated at 

 the beginning) C and c s are "experimental " — that is, directly 

 determinable — quantities* and to the experimenter, as such, 

 J and G will appear together simply as the measurable 

 departure (J + G) from "perfect" behaviour according to 

 the older (yan'jb Hoff) criterion. 



In fact, in terms of the "activity coefficient" (7) of 

 G. N. Lewis, and the osmotic coefficient (1 — <p) of Bronsted, 

 expressions now much used in practice for the observed de- 

 partures from (van't Hotfs) " perfect " behaviour, we have 



log7= — t>(J s + Gt s ) 



1 : 



where <p is a mean quantity for the whole solution, 

 characteristic, therefore, of the given solutes, mixed in given 

 proportions. 



It may be convenient therefore to call (J + G) the 

 " apparent " general terms, G the " true " general terms, 

 and J the " pseudo-general" terms. It is, of course, only G 

 which represents real departure from perfect behaviour (as 

 defined by the linearity of the full " molecular " expression 

 for -v/r ; see second paper, Section II.). 



It is clear that any physical interpretation of the 

 " apparent " general terms based upon the ignoring of either 

 J or G, without due consideration and adequate grounds, 

 must be unsound, and that in general neither will be 

 negligible, so that a separation of J from G must be 

 attempted. 



This will only be possible by a critical application of the 



2S2 



