THE PLASTICITY OF SOLIDS

233

sodium hydroxide containing 1.795 g per liter. The pressure
seems to have been measured as centimeters of water head, and
the volume of flow in milliliters per 600 sec. He measured the
flow of 16 mixtures and pure water, designated by the numbers on
the curves in Fig. 81. The amounts of sodium hydroxide solu-
tion added are noted in the second column of Table LXI.

The curves are nearly linear except when the volume of flow
is small. The curvature is probably due to seepage. ' The hori-
zontal distance of the different curves from the curve No. 10
is evidently a relative measure of the friction constant. The
values of the friction constant / as obtained graphically are given
in the table. We have found that it is possible to calculate this
relative friction constant /' by means of the formula

/' = 154 - 14. Ic

(94)

where c represents the number of milliters of sodium hydroxide
added. It appears, therefore, from a comparison of the values of
/ and /' that Simonis' experiments confirm our conclusion that
the friction is a linear function of the concentration. We note
that the friction constant continually decreases as water is added
until 11 ml have been added after which further additions are
without effect upon the rate of flow. On adding 11 ml, the
material reaches the concentration of zero fluidity or zero friction,
and the curve 10 should pass through the origin. That the curves
10 to 17 all coincide with curve 10 accords with what we should
expect of liquids flowing through an orifice.

The fact that all of the curves are sensibly parallel constitutes
the remarkable difference between flow through a capillary and
flow through an orifice. It does not signify that the plastic
mixtures all have the same mobility any more than it signifies
that all of the liquid mixtures have the same fluidity. It means
simply that the rate of flow through an orifice is independent
of the fluidity or mobility. If in the equations for the flow of a
viscous or a plastic substance through a capillary we make the
length of the capillary zero, we obtain the identical equation

i iPg,
\mp

(95)

where k is a constant. This is the characteristic and familiar
equation for the flow of liquids through an orifice in which the