94

FLUIDITY AND PLASTICITY

when put in the form
? = A + BT . (35)
is but an algebraic expression of the law that the fluidity of a
liquid is a linear function of the temperature. The law is only
approximately true, but even with the alcohols where the curva-
ture is greatest, there is an approach to linearity at high tempera-
tures. Like the Law of Boyle, we may assume that this law holds
in ideal cases, and that the theory underlying it is valid.
EMULSIONS
The study of the viscosity of mixtures near their critical-
solution temperatures affords another very sharp and distinct
means for testing the theory which has been outlined. It has
been pointed out that the fluidities should be additive in the per-
fect mixture but the viscosities additive in the emulsion.
According to the second conclusion page, 89, there should be a
sudden drop in the fluidity at or near the critical solution tempera-
ture. We do not propose to discuss in detail here the viscosity
of colloids but it is appropriate here to seek an answer to the ques-
tion "Has such a drop in fluidity ever been observedT'
Ostwald and Stebutt (1897) observed an abnormally large vis-
cosity mixture of isobutyric acid and water in the neighborhood
of the critical-solution temperature. This was attributed by
them—not to the reason given above—but to the fact that at the
critical-solution temperature, the surface energy becomes ^oro.
Friedlander (1901) investigated the phenomena which are
peculiar to the critical-solution temperature in an intensive
manner. He found a very marked increase in the viscosity as
the solution was cooled to temperatures where the opalescence
became evident and the critical-solution temperature was
approached. He observed the opalescence with particular care.
His investigation, was extended to include phenol and water,
and the ternary mixture of benzene, acetic acid, and water.
Similar relations were found in all proving that the phenomena
are quite general. He concluded that the temperature coefficient
of viscosity is greatest where the opalescence and the tendency
to foam are greatest. He says,1 "Der Triibungsgrad und Tem-
peraturkoefficient der inneren Reibung zeigen eine starke Zu-
^riedlandler, 439,