54 FLUIDITY AND PLASTICITY
part, on a smooth curve indicating that the phenomenon under
consideration is not one of mere clogging, as by accidental dust
particles in ordinary measurements. The effect is pronounced
a tube of about 0.8 cm radius with a 90 per cent colopho-
nium mixture, but the effect is not noticeable in a tube of 0.1
cm radius with an 80 per cent mixture. It is therefore no
wonder if this effect is not noticeable in ordinary liquids which
are millions of times yet more fluid.
The fact which seems to have been overlooked by Glaser
and is of prime importance in explaining the phenomenon, is
that the shearing stress and the mean velocity of efflux is very
much less in the smaller tubes. Obtaining the critical value of
the radius for each mixture by the graphical method, we have
calculated the mean velocity by means of Eq. (6). It is of the
same ordqr of magnitude in all three cases being around 0.000,01
cm per second. It seems probable that had these experiments
been repeated at a very greatly different® pressure, it would have
been discovered that the viscosity is dependent upon the shear-
ing force rather than upon the radius of the tube, and the con-
clusion that the viscosity is independent of the velocity would
have been amended. It is highly desirable that experiments
be made to establish this point.
It is important to observe that each mixture used by Glaser
gave a zero fluidity when the radius of the tube fell below a
certain well-defined limit. Bingham and Durham (1911)
have studied various suspensions of clay, graphite et cetera in
different liquids over a range of temperatures, using a single
capillary and a nearly constant pressure. As shown in Fig. 19,
the fluidity-concentration curves are all linear and at all concen-
trations and temperatures they point to a well-defined mixture
with zero fluidity, at no great concentration. This mixture
apparently sharply demarcates viscous from plastic flow, for
be it noted that the mixture having "zero fluidity" was not a
hard solid jjiass, but rather of the nature of a thin rnud. In
the mixture of "zero fluidity" it appears that with the given
instrument all of the pressure is required for some other purpose
than to produce viscous flow. The amount of pressure used up
in this way Is zero for the suspending medium, alone but increases
in a linear manner with the concentration or* solid. If this view is