56

FLUIDITY AND PLASTICITY

Further work is therefore demanded in order that we may clearly
define and separate the coefficients of plasticity and fluidity
which are here measured together.

Surface Tension and Capillarity.—Several investigators have
attempted to measure viscosity by means of a capillary opening
directly into the air. Poiseuille (1846) found that whether drops
were allowed to form on the end of the capillary or the end of the
capillary was kept in contact with the wall of the receiving
vessel, he was unable to obtain consistent results. The effect
of surface tension varies with the rate of flow, with the tempera-
ture, and it also depends upon the shape and position of the end
of the capillary, so that as a whole the effects are quite indeter-
minate. That the effects are large and variable, may be inferred
from the measurements of Ronceray (1911) with a capillary,
I = 10.5 cm, R = 0.0275, immersed under water or opening
into the air, given in Table XIX.

TABLE XIX.—EFFECT OF SURFACE TENSION ON THE FLOW OF WATER

(RONCEBAY)

P centimeters, water
Time of flow of 10 ml in air at 17°
Time of flow immersed
Difference

10
1,132.0
1,089.44
42.6

20
559.5
550.4
9.1

30
373.0
368.5
4.5

40
280.6
277.4
3.2

50
224.9
222.7
2.2

60
187.9
186.1
1.8

70
161.8
159.5
2.3

Poiseuille recorded similar results. The irregularity is com-
pletely removed by having the end of the capillary immersed.
Nevertheless in an apparatus like that used by Poiseuille there
may still be a correction for capillary attraction within the bulb
which is considerable (cf. p. 66).
Summary.—From the foregoing, it appears that under proper
conditions, the only correction that it is necessary to make
to the simple Law of Poiseuille is that for the kinetic energy
of the fluid as it leaves the capillary :Q_7u . Other sources