THE PLASTICITY OF SOLIDS 235

dity or mobility does not appear. The table shows that the
her rates of flow may be calculated quite accurately by means
.he formula

j- = 23.4(P -/') -f- 168.7 (96)

HISTORICAL
. large amount of work has been devoted to the flow of solids.
:hods of measuring plasticity, consistency, and hardness
e aimed to give a single numerical value to a property which
mnd to be complex. Plasticity itself has hardly been meas-
1, but rather some property instead which is supposed to be
bed to it, such as the amount of water required to bring a
to a given consistency, the tensile strength on drying, the
>rptive capacity for certain dyes such as malachite green, the
unt of shrinkage on drying, etc. It is no doubt true that
e properties are dependent in large measure upon the fineness
~ain which also essentially affects the plasticity, but a knowl-
j of these properties leaves the subject of plastic flow in a
ilous state.
"any investigators have investigated the so-called "viscosity
>lids,37 assuming that solids obey the ordinary laws of viscous
, and Tarnraann has identified fluidity with plasticity. Heyd-
.er (1897) has measured the viscosity of menthol in both the
d and the solid condition. Weinberg (1913) Dudetzkii
i) and Pochettino (1914) have measured the viscosity of
i or asphalt. Segel (1903) worked with sealing-wax and
is (1893) with marine glue. Barus made the important
rvation that if the rod of material coming out of the capillary
in his measurements was cut off neatly with a knife, the
ders thus formed were in a strained condition. They
taneously change their shape, the advancing end becoming
wed in and the following end being bulged out. This proof
rain is very similar to that observed by Trouton.
esca (1868) did valuable work in forcing metals through
;es and proving that they may be made to flow in a linear
ler rmich as liquids do. It gives good reason for the pre-
>tion that it is practicable to measure the friction and mobil-