THE FLUIDITY OF SOLUTIONS 161
attributed to breaking down of association or to dissociation
which also give rise to the increase in volume. Benzene and
ethyl acetate may be cited as an example of this type.
III. When two liquids are mixed, perhaps more often than
not, there is a decrease in the volume, particularly in aqueous
solutions. With this decrease in volume there goes a positive
heat effect and a decrease in the fluidity, so the fluidity-volume
concentration curves are convex downward, i.e., d*<p/dc2 is
positive. Since the fluidity changes some 2,000 times as rapidly
as the free volume, as shown by Batschinski's law, the effects
of the solvation which is presumed to be present in this case
manifest themselves far more prominently in the fluidity data
than in the data on volume. Examples of this type are very
common, making up the greater portion of aqueous mixtures,
such as ethyl alcohol and water, acetic acid and water and many
mixtures of organic liquids such as chloroform and ether. In
some cases there is incontestible proof that a chemical compound
is formed on mixing, as when aniline is mixed with acetic acid.
In other cases, the formation of hydrates or solvates is very
probable. Whether there is a sharp line to be drawn between the
forces of cohesion of a purely physical nature and the forces
which bring about actual chemical combination must be decided
by future experiment.
IV. When associated solvents break down, to later unite
with each other, we have a combination of the second and third
types. The resulting curve may then show positive curvature
over part of its course and negative curvature over a part,
there being a point of inflection. Examples of this type are
found in both aqueous and non-aqueous solutions, ammonium
nitrate solutions being a good example of the former.
Instances of this type are instructive, since they put us on our
guard against assuming that because a given mixture displays
strong positive curvature, dissociation is not a factor. A pair
of liquids may fall into type II and yet have a tendency to unite
together chemically, provided merely that the effect of dissocia-
tion predominates in all mixtures.
Again the opposing effects may be nearly equal at all concentra-
tions, as is true of ethyl alcohol and acetone, and formic acid and
water. These mixtures evidently fall under type IV and not