SUPERFICIAL FLUIDITY 257
radius whereas the volume of flow varies as the of the
radius, Eq. (6), we may expect to find the effects
fluidity shown to the best advantage in very fine tubes.
There "are a variety of causes which may cause the near
the bouo-clary- to have a different fluidity. The most
cause results from the selective adhesion of the of
the fluid for the solid. If one of the components of the L*
more strongly attracted than another, separation becomes
ble, and the magnitude of the fluidity of the mixture as
will theoretically he affected The adhesion between and
liquid or liquid and liquid Is doubtless Just as specific a
as is the better known cohesion or surface tension of and
we are coming to understand the nature of adhesion tetter
through, the efforts of Langmuir (1919) and Harkins (1920). We
have seen that it is possible to greatly affect both the
and the mobility of plastic substances by the addition of
amounts of acid or alkali. Just what happens in such
might be subject to dispute, but it is certain that
of substances adsorbed on to the surface of a solid may
change the character of the solid which is in contact with the
liquid. Thus Henry Green (1920) has observed the
of small amounts of gum arabic to a suspension may
deereas© the yield value and increase the mobility, in of
the high viscosity of gum arabic solutions. This is
as being due to the decrease in adhesion between the
pended particles. The well-known work of Schroeder (11)03)
upon thte effects of electrolytes on the viscosity of gelatine of
Handowsky (1910) upon serum albumin should also
to.
We Imve already proved on page 86 that if any
in the fluid near the boundary becoming different from the re-
mainder of the liquid, the resulting fluidity will be
This theorem is therefore useful in explaining superficial fluidity.
We will now prove that the components of a mixture under
conditions will undergo partial separation. The conditions
will be unade more general by using the non-homogeneous mixture
considered on page 86. Considering the mixture as up
of the two components A and B7 arranged in alternate plane
layers, the total quantity of A flowing in a unit of time, regardless
IT