150 
MR. W. R. BOUSFIELD: IONIC SIZE IN RELATION TO 
Part VII.—Ionic Size in Relation to Rkfractive Power. 
The refractive power of a medium is known to be closely related to the state of 
aggregation of its components. According to the hypothesis which we are discussing, 
the combination of water with the ions and molecules of a solute changes the state 
of aggregation of the fraction of the water which is so combined, and also, to some 
extent, of the uncombined water. Hence we should expect to find a close relation 
between ionic volume and refractive power. 
If n = refractive index of a solution at 18° C., 
n 0 = refractive index of water at same temperature, 
m = concentration (gramme equivalents per litre of solution), 
the simplest measure of the refractive power is ( n — n 0 )/m , say An/m, which we may 
refer to shortly as the equivalent refractive power of the solute. 
A recent paper by Dinkhauser (‘ Sitz.-Ber., Wien. Akad. Wiss./ July, 1905, 
p. 1001) gives the values of this equivalent refractive power for solutions of KC1 and 
NaCl. The values for KC1 appear to be a little rough, but the values for NaCl seem 
to be particularly accurate. * 
In the following tables are given the concentrations, the values of log h and of the 
radions and ionic volumes calculated therefrom, together with the values of the 
equivalent refractive power given by Dinkhauser. In Diagrams XI. and XII. are 
set out the values of the equivalent refractive power as ordinates upon the values 
of the ionic volumes as abscissse. It will be seen that a linear relation appears, which 
may be expressed by the equations 
for KC1. An/m = 0-009145 + 0-0001845 I t ., 
for NaCl.Aw/m = 0-00855 +0-0001271 1,,. 
The values calculated from these expressions and the differences from observed 
values are also given in the tables. If I y be the value of I„ at infinite dilution (see 
Tables XIII. and XIV.), and we put Al„ = I v — I v , the expressions may be written 
for KC1. An/m = 0-01048-0-0001845 Al„, 
for NaCl. An/m = 0"01054 — 0"0001271 Al„. 
* For KC1, Dinkhauser gives the value 9‘80 for m = D234 (p. 1020) and the same value for m = 1 
(p. 1025). The whole series shows that the latter value must be inaccurate. For m = 0'812 he gives the 
value 10"06 (p. 1020) and for m = 0 - 5 the value 10 - 05. The former of these values is rejected. The 
values for concentrations less than one-fiftieth normal are also rejected, since the large factor by which the 
observed value of n must be multiplied to get the equivalent refractive power greatly magnifies any error. 
These discrepancies are partly due to the fact that Dinkhauser has combined his own results with those 
of other observers. 
