96 FLUIDITY AND PLASTICITY
critical temperature. Rothmund therefore called to his aid
Donnan's hypothesis that when drops are very small their surface
tension is very different from that of the liquid in bulk and is a
function of the radius of curvature. Since at the critical tem-
perature the surface tension is normally zero, it was thought that
the small drops might thus exist in a state of stable equilibrium
in the neighborhood of the critical-solution temperature. As the
temperature is raised the opalescence would become less and less,
due to the solution of the drops. Rothmund found that the
addition of naphthalene to his solutions greatly increased the
opalescence, while the addition of grape sugar decreased it very
greatly, although the effect of these additions upon the viscosity
was negligible. He reasoned that the refractive index of butyric
acid is greater than that of water and sugar and electrolytes
raise the refractive index of water, hence they make the presence
of small drops less evident. Naphthalene does not dissolve in
water but does dissolve in butyric acid, raising its refractive
index, and therefore it makes the opalescence more apparent.
Von Smoluchowski (1908) regards Rothmund's hypothesis as
superfluous, believing that the kinetic theory is sufficient to
explain the opalescence. According to him, differences in
molecular motion, local differences in density, and therefore
differences in surface tension cause the critical-temperature to be
not entirely definite. Due to this indefiniteness in the critical
temperature, rough surfaces are formed, which must have a
thickness of less than a wave length of light, since, greater thick-
nesses would not reflect the light. The inequalities in the density
would reach their maximum at the critical temperature.
Bose and his co-workers (1907-1909) have also verified
these earlier observations that abnormally large viscosities are
obtained at the critical-solution temperature. Bose regards this
as due to the rolling of drops of liquid along the capillary. They
did a considerable amount of work to prove that "crystallin" or
"anistropie" liquids are similar to the emulsions here discussed.
Bose proved that these liquids have abnormal viscosities near the
clarifying point and they also possess marked opalescence.
Vorlander and Gahren had found that a crystallin liquid may
result from the mixing of two liquids neither of which is itself
"crystallin" in the pure condition. The mixture therefore