﻿238 Mr. Bernard Cavanagh on 



practical significance. (24) is a quantitative statement, and 

 the form to which it is reduced in (26) has practical meaning 

 and value, as will be shown by means of a simple illustration 

 in a short appendix to this section. From (26) it is clear that, 

 owing to the relative smallness of the changes in c 01 , c 02 , 



produced by the presence of solutes, the difference 



between the "general" terms in the original " molecular " 

 expression for -v/r and those in the " experimental " expression 

 we have obtained will generally be very small. But it may 

 have to be taken into account in making use of dynamical 

 theory (at high concentrations). 



Of course, until and except when the "general" terms 

 can be given more definite form, we cannot say anything 

 about the way in which they will depend on the constitution 

 of the solvent and its variation . For the present we have to 



suppose that c i c 02 are eliminated in terms of T, p, c 1} c 2 , 



s from Gr, which takes some form 



G = UXcp x f f x (c lC2 ) (27) 



Theoretically, and in the general case, the application of 



dynamical theory will precede this elimination of c 0] c Q2 , 



will deal in fact with the original general terms M G-', so 

 that, in greater or less degree, knowledge of the constitution 

 of the solvent and its variation will be necessary before such 

 theory can join issue with practice, but in some cases, as the 

 appendix will illustrate, this may not be necessary, even 

 though the constitution of the solvent does affect the 

 " general " terms. 



It should be noticed that the value and convenience of our 

 " experimental " expression for -\|r is by no means entirely 

 dependent on a deficiency of knowledge of the constitution 

 of the solvent, though the latter makes it practically 

 indispensable. 



In the " linear " terms the effect of the constitution of the 

 solvent is concentrated in the quantity m, the mean molecular 

 weight of the solvent. 



In the pure solvent this will have a limiting value m 

 dependent only on temperature and pressure, and we can 

 therefore regard the quantity ( — R log m Q ) as included in the 

 quantity <f> s , when our " experimental" expression for yjr will 

 finally take the form 



^ = M [<£ M + R f = dlog (1+SC)] 



*c=o 

 4-Sn s [^-R{logc 5 -log(| +m c|]+M G . . (28) 



