12

FLUIDITY AND PLASTICITY

of the 6.4482 cc of water at 10°C contained in the bulb at a
constant pressure of 775 mm. of mercury are given in column 3.'
Assuming that the time of flow is directly proportional to the
length of the tube, Poiseuille used the time of one experiment
to calculate the one immediately succeeding, and thus are ob-
tained the values given in column 4. It is evident that the last
two lengths are too short, but the others fairly substantiate .the
law. The agreement is still better when corrections are made
for the varying diameters of the tube. This correction is espe-
cially important since, as will be shown, the efflux rate varies
as the fourth power of the diameter. From results like those
exhibited in Table III Poiseuille concluded that the quantity of
liquid passing through a tube of very small diameter at a given
temperature and pressure varies inversely as the length) and we have
that V = K"pjl where I represents the length. But the last
two observations show that this law has its limitations.

Law of Diameters.—To discover the relation between the
diameter of the capillary and the rate of flow, Poiseuille calculated
the quantity of water which would flow through 25 mm of the
different tubes at 10°C under a pressure of 775 mm of mercury
in 500 seconds, obtaining the values given in Table IV.

TABLE IV

Designation of tube
Mean diameter of tube in centimeters
Volume efflux in 500 sec. from observations
Volume calculated
Per cent, difference

M
0.0013949
0.0014648
0.001465
-f<3.02

E
0.0029380
0.0288260
0.028808
-0.07

D
0.0043738
0.1415002
0.141630
+0.10

C
0.0085492
2.0673912
2.066930
-0.02

B
0.0113400
6.3982933
6.389240
-0.14

A
0.0141600
15.5328451
15.547100
+0.10

F
0.0652170
6,995.8702463

The volumes calculated in the fourth column are obtained by
comparing each tube with the one following on the assumption
that the quantity traversing the tube is proportional to the fourth
power of the diameter, thus 0.0029384: 0.00139494 = 0.028826 :x,
or x = 0.001465. The agreement is very satisfactory, hence the