146 
MR. W. R BOUSFIELD: IONIC SIZE IN RELATION TO 
Moreover, the value of C is nearly the same for both KC1 and NaCl, though the 
ionic volumes are very different. 
(c) Effective Molecular Freezing-point Depression a Linear Function of Solution 
Volume. —-In Part IV. ( a ) it was shown that the solution volume was a linear function 
of the ionic volume, that is the sum of the cubes of the radions, and it has now been 
shown that the effective molecular freezing-point depression is also a linear function 
of the ionic volume. If this be so, we can dispense with our hypothesis and obtain 
the effective molecular freezing-point depression as a linear function of the solution 
volume. 
With the object of testing this relation, the density measurements of KC1 and 
NaCl at 0°C. were made. From these have been already derived formulae expressing 
the solution volume in terms of hydration (see Part II. (c)). If we denote by SV s 
the difference between the solution volume at a given concentration, and the solution 
volume at infinite dilution, the expressions we obtained in Part II. (c) may be written 
for KC1.SV s = 0-059 (1-y), 
for NaCl.SV s = 0*089 (1 — y), 
where y = (1+B/i _2/3 ) -3 , B having the value 6 for KC1 and 4 for NaCl. At infinite 
dilution for KC1, V s = 0-3068, and for NaCl, V s = 0-2125. 
By means of these formulae we can calculate the solution volumes corresponding 
to the concentrations at which Jahn’s freezing-point observations were made, and 
we find, as our theory led us to anticipate, that the effective molecular freezing-point 
depression is in fact a linear function of the solution volume calculated from the 
densities. 
