228

FLUIDITY AND PLASTICITY

TABLE LIX.—EXPERIMENTS ON THE SAGGING OF A ROD OF PITCH AT 15°,
DEMONSTRATING THAT THE TIME T REQUIRED TO SAG A GIVEN DIS-
TANCE VARIES INVERSELY AS THE FOURTH POWER OF THE DISTANCE
L BETWEEN THE SUPPORTS (AFTER TROUTON)

L
T
TL4

33
14.6
1.7X107

30
18.5
1.5

27
30.4
1.6

24
47.0
1.6

That the different methods agree with each other is shown
in Table LX.

TABLE LX.—A COMPARISON OF THE COEFFICIENTS OF PLASTIC TRACTION
AND MOBILITY AS DETERMINED BY VARIOUS METHODS (AFTER TROUTON)

Substance
X
Method
u
Method
A*A

Pitch I
2 3X10"*11
Traction
7 1 X10~ l
Torsion
3 07

Pitch II .......
2 8X10"11
Traction . .
1 OX10~ °
Torsion .
3 60

Pitch II .......................
3.0X10-11
Sagging .....

3 30

Pitch and tar I .
7 8X10~9
Traction . .
2 4X10" °
Torsion
3 07

Pitch and tar II .........
1.5X10~10
Traction. . .
4.5X10" °
Torsion .
3 04

Shoemaker's wax ................ Pitch and tar 3:1 III. . .
1.9X10-7 1 3X10~5
Traction .... Sagging
5.0X10-3 8X10"8
Torsion . . Efflux
2.95 3 25

Pitch and tar 3:1 IV .............
1.1 X10'6
Descending

column .....
3.6X10-8
Efflux. . . .
2.91

THE THEORY OF PLASTIC FLOW
A plastic solid is made up of particles which touch each other
at certain points. The spaces between the particles may he
empty or it may be filled with gas, liquid, or amorphous solid.
Flow necessitates the sliding of these particles the one over the
other according to the ordinary laws of friction, so long as the
particles are large enough so that their Brownian movement is
negligible. It is by no means necessary that the particles be
touching at the maximum number of points, corresponding to
"close-packing." As a matter of fact, close-packing of the
particles prevents flow from taking place. It is merely necessary
that the particles touching each other form arches capable of
carrying the load, as already indicated on page 201. It is evident