184 FLUIDITY AND PLASTICITY
ness of the positive curvature is then due to the small amount of
hydration which is well-nigh universal in aqueous solution.
The negative curvature, on the other hand, must be due to
dissociation either (1) of the salt or (2) of the associated water.
Since the negative curvature occurs in dilute solution, the
electrolytic dissociation is immediately suggested. If the fluidity
of the anhydrous salt in the form of an undercooled liquid is
negligibly small, it is hard to conceive of how the dissociation of
the salt into two, or at the most a few, ions would increase the
fluidity so remarkably, for it must be remembered that there
must be a substance present whose fluidity is higher than that of
water. Then, as already pointed out, there are substances
which give negative curvature which are very slightly dissociated
into ions, such as urea.
We are then compelled to seek further in our explanation
and admit that water itself is dissociated by the presence of the
salt or its ions. There is nothing inherently improbable in this
since water is highly associated (2.3 at 56°C). The association
is less at high temperatures and in concentrated solutions so
that under these conditions negative curvature would be less
apparent as we have already seen to be the case. It is often
assumed that electrolytic dissociation is brought about by union
of simple water molecules with the ions of the salt, but if the ions
have low fluidity, the fluidity of the solution will evidently not be
raised by uniting with even simple water molecules, hence
hydration will not explain the phenomenon. In other words, the
formation of larger molecules does not tend to raise the fluidity.
Wagner (1890) has measured the volume of water required to
make a liter of normal solution of the chlorides of various salts.
In the cases of silver and thallium the nitrates were used instead.
Salts like calcium chloride, which unite strongly with water to
form hydrates, produce a contraction on going into solution, so
that a comparatively large volume of water is required. But
rubidium and caesium chlorides expand on going into solution so
that the volume of water required is correspondingly small. The
difference between the volume of water required and 1 1. is the
volume of the salt together with the expansion. Calculating
the volume of the salt from its specific gravity the expansion is
obtained. The resulting numbers, plotted in curves IV and V in