ON THE THICKNESS AND SURFACE TENSION OF LIQUID FILMS. 
651 
t 
P 
0 
m. 
19 
21-58 
21-43 
0-15 
21 
21-74 
21-52 
0-22 
The result of this experiment was satisfactory. The mean difference of the 
diameters, having been increased from 0'05 mm. to 1'30 mm., was reduced to O' 18 mm. 
two minutes after communication between the films was re-established. There appears 
to have been a slight alteration of the zero reading, but the new position of equilibrium 
was rapidly taken up. When the large rings were used the movements of the films 
were slower, but, as all the principal experiments described in this paper were made 
with the small rings, the above observation is the only one which need be given in 
full to illustrate the point. 
Experiment II. 
The following is a typical example of a large number of experiments which proved 
that in general the thicker film behaved as though it had the greater surface tension. 
The left film was flooded at intervals of ten minutes, while the right was allowed to 
thin. The diameters were measured every ten minutes. When the first traces of 
black appeared in the right film it was flooded, and the left was allowed to thin till 
traces of black appeared. The interval between the two extreme measurements, 
which are alone given below, was l h 20 ra . During this time the temperature of the 
film box was steady at 19 0, 2 C. 
The glass rings were used. 2Y=32'5, 2X=2"5Y. 
C„ 
C; 
£=p —\ 
{R (2, 3), R (2, 9)} 
Colourless 
Colourless 
{R (2, 1), P (2, 5)} 
3-18 
-2-25 
We have already remarked that the alternate flooding of the two films doubles the 
sensitiveness of the apparatus as a means of detecting small changes in surface 
tension. If, therefore, the values given on p. 645 may be applied to such considerable 
differences of the diameters, we have 
cZT/T=0'543/2x 7-03=3-8 per cent. 
This calculation is given chiefly as an illustration of the sensitiveness of our method. 
A number of possible objections have to be considered before it can be accepted as a 
trustworthy determination of an actual difference of surface tension between the 
films. 
4 o 2 
