SUPERFICIAL FLUIDITY 259

simply by comparing the times required by gravity to empty two
pipettes filled with a heavy oil, each of the pipettes being similar
in every respect except that one is moistened with water previous
to filling with oil.

In an experiment by the author at 25°C and a pressure
of 60 g per square centimeter a given volume of water required
33 sec. and the same volume of cottonseed oil required 1,640 sec.
A mixture was then used containing one-third oil and two-thirds
water by volume. Had the heavier water flowed completely
through the capillary ahead of the oil, the time of flow should
evidently have been 22 + 547 = 569 sec.; yet only 391 sec. were
actually required which is less than the time theoretically required
by the oil alone. The difference of 178 sec. is due to the water
forming a lubricating film for the oil as the water drained out
through the capillary t

Rate of Absorption. — It is appropriate here to show how the
rate of absorption of a fluid into a porous material depends upon
the fluidity of the medium. From Poiseuille's Law, Eq. (8), it

follows that the rate ~r at which a liquid enters a long capillary

tube under the driving force P will be

dl PrV

dt SI
If the capillary is very small, the surface tension 7 exerts a force

— which must be added to the external pressure and this force

arising from the surface tension may be so great that the external
pressure is negligible in comparison, in which case

dl

and by integration
«• - f •*
The quantity of 0.5^ is called the coefficient of penetrance of
the fluid and it is a measure of the tendency of a liquid to pene-
trate a given material which it wets. (Cf. Washburn Physical
Chemistry, 2d ed., p. 62.)
The distance that a liquid will penetrate a given porous mate-
rial due to capillary action alone is often of practical importance.