of Gibhss Theory of Surface-Concentration. 
507 
Fisr. 1. 
Method of measuring the Interfacial Tension. 
The Drop-Pipette method was employed, the apparatus 
being of the form shown in fig. 1. This 
was first filled with oil (by suction at C) 
up to the mark E. The tap B was then 
closed during a determination. The aper- 
ture F is carefully wiped before using, 
and the pipette placed in a fixed position 
relatively to the solution in all deter- 
minations. The bulb D is of about 45 c.c. 
capacity between E and E t . The distance 
E F is about 8 cms. As soon as the tall 
jar containing the solution is placed in 
position, tap A is turned full on, the 
constriction above only allowing very 
slow entrance of air ; and hence the drops 
of oil rising through the denser solu- 
tion are formed with exceeding slowness 
— at 12 to 15 sees, interval. This, as 
Lord Rayleigh * has pointed out, is of 
great importance in order to obtain 
fairly accurate determinations; the approx- 
imateness of the method being due to 
the application of a statical theory to 
what is really a dynamic phenomenon. 
The method consisted simply in count- 
ing the number of drops formed while the 
oil fell from E to E x . 
Theory of the Drop-Pipette Method. 
It may be used in the first instance 
fy simply to determine the relative tensions 
thus : — 
of solutions of different concentrations. 
Let 
V" = total volume of oil used, 
n = total number of drops formed, 
V 
then the volume of each drop = — ; 
x n 
and if p = the densitv, 
Yp 
the weight of a drop= — -. 
The tension a is taken to be proportional to the weight of 
a drop, 
n 
* Lord Rayleigh, Phil. Mag. [5] vol. xlviii. p. 321 (1899). 
