THE FLUIDITY OF SOLUTIONS 169
this deviation much over 1 per cent. Brown found that the
deviation is usually larger in mixtures which contain an ester as
one or both of the components. This, however, is not shown by
this table very well, but if the conclusion is correct, the deviation
would be explained by the chemical character of the components.
This brings us to the consideration of the non-ideal types of
mixtures.
The reader will perhaps ask whether the fluidities of ideal
mixtures would be additive if plotted against weight concentra-
tions. The curves for carbon tetrachloride and benzene have
been published,1 using both volume and weight concentrations.
Using volume concentrations the curves are slightly sagged as
already pointed out, but using weight concentrations they show
marked negative curvature particularly at the higher tempera-
tures. The very slight contraction of carbon tetrachloride and
benzene on mixing in no way accounts for this negative
curvature.
II. NEGATIVE CURVATURE AND DISSOCIATION BY DILUTION
We will now consider a pair of substances which expand on
mixing, using the data of Thorpe and Rodger for methyl iodide
and carbon disulphide. The curvature of the fluidity-volume con-
centration curves is negative and greatest at the lowest tempera-
tures. This is in accordance with the view that the components
are less associated at the higher temperatures and therefore can
show less dissociation on mixing.
The expansion on mixing amounts to as much as 0.2 per cent
of the volume, as may be seen by comparing the observed specific
volumes with those calculated by the admixture rule, Table
XLV. The fluidities are given in Table XLIV and it is seen that
Batschinski's Law applies to each mixture, but the values of the
limiting specific volumes o> cannot be calculated by the admixture
rule as in the normal mixture. The actual limiting volume is
some 2 per cent less than the calculated value, presumably due
to the dissociation. The values of k, which measure the slope
—^— of the fluidity-specific volume curves are verv much less
v — co J ^
1 Zeitschr. f. physik. Chem., 83, 657 (1913).