SUPERFICIAL FLUIDITY 2^)
extension with our more recent knowledge of the of
in mind, but whatever the surface of a given
be, we may profitably distinguish three typical U)
the superficial layer is a true solution but of
tration from the interior and is in contact with it or
some gas; (££) where the surface Is made up of a layer of
tie liquid; which may be so thin as to be imperceptible by ordi-
nary means; (C) where the surface is formed either by a continuous
solid or by solid particles in more or less intimate
each other. It is evident that in the last two we are
not with the superficial fluidity of the liquid but of a
ous mixture of liquid-liquid or solid-liquid respectively.
Soap solutions perhaps afford the best examples of the
case and if such solutions have extraordinarily high
viscosity, it serves to explain the stability of the bubble.
The liquid between the two Hghly viscous surfaces can
downward very" slowly in so narrow a space.
Oil films on water give frequent examples of the sort,
and the use of oil "to calm troubled waters" is a practical
cation, of superficial viscosity in the damping of vibrations. The
slinple harmonic motion of the wave causes the to
in. vertical circles, so that an oil film is alternately
compressed. The water underneath not being subjected to
same tendency is pulled aloag by the oil film and in
flow energy is of course dissipated. A method for the
loent of viscosity by Watson (1902) depends upon the
of small -waves in a free surface, and apparently is
capable of being used to measure superficial viscosity, but
appears not tx> have been attempted.
The connection of superficial fluidity -with emulsions be
mentioned at this point although we cannot stop to it.
We can merely refer the reader to the fascinating studies of Pla-
teau, Quincke, and Lord Rayleigh upon the nature of
ting films. The recent paper by Irving Langmuir (1919) on the
theory of flotation is very suggestive.
Maay of the examples which we would naturally cite as
pies of the second ease given above may really be of the
third instead. It is certain that in most emulsions a
stance is necessary to stabilize it and it may give rigidity.