Page 20 - AN INTRODUCTION TO SURFACE CHEMISTRY By ERIC KEIGHTLEY RIDEAL

DROP WEIGHT METHOD 15

a knowledge of the drop weight and the tip radius alone. The
following example given by Iredale may be cited.
The weight of a mercury drop detached from a capillary tube of
radius 00852 cm. was found to be 01794 gm. Since p = 1353
for mercury the radius of the mercury drop is 01469 em. Hence
r 00852
01400 05801.

From a graph constructed from the table above we find that the
tube radius required to give a drop of water of the same charac-
teristie ratio ', would be 0·1239 em. Hence
r
00852
K =01239'
also ,
= 7280 and p, = 0998 whence
Ko, .« 4 6
'Te"_ 6 dynes per cm.
P,o
Many modifications of the drop weight method have been utilised
in practice.
Instead of measuring the weight directly we may calculate it
from the volume and the density: the drop volume method has
been applied by Harkins chiefly to the measurement of the tension
between two liquid phases, and it probably falls little short in
accuracy from the previous method. More frequently it has been
modified, especially for biochemical purposes, as a drop number
method: that is, a known volume of liquid is allowed to flow out
of a tube, and the number of drops formed is compared with that
formed by a standard fluid. 'This method is necessarily very rough,

Bubble pressure.
By reversing the position of liquid and gas assumed in the pre-
ceding section we obtain the bubble pressure method. The theory
corresponds closely with that of the drop weight and has been de-
veloped by Cantor, Feustel and Schr~dinger (An, d. Physik, xLVI.
413, 1915).
The equation derived is
·-»-3;-3