ON THE THICKNESS AND SERFAGE TENSION OF LIQUID FILMS. 
661 
The films were then cut off* from communication, and the current was passed up the 
right film for 90 s : the black disappeared and the lower part became colourless. The 
stop-cock was opened again eight minutes after the passage of the current, and a 
measurement taken when it had been open two minutes. 
C,.= {B (3, 0), colourless}, C/={14 - 0h, R (2, 0)}. 
§=- 1-18. 
Shortly afterwards the thin film broke. 
The foregoing observations proved that the disturbance produced by thickening one 
of the films died out some minutes after the disturbing cause was removed. If, then, 
the surface tension assumed its final value while a film was still moderately thick it 
might be possible, by balancing films of very different thicknesses, to obtain some 
evidence as to whether any further difference of tension due only to the difference in 
thickness could be detected. We, therefore, gave one of the films a start by thicken¬ 
ing the others some time after their first formation, and then compared them when 
another considerable interval of time had elapsed. 
This method is attended with several difficulties. In the first place, the operation 
takes a long time, during which slow changes in the zero error take place. Thus, if 
the two films are first compared when both have been recently flooded, and afterwards 
when one is black and the other coloured, a slight difference in the position of equili¬ 
brium may be observed, due to changes in the liquid rings, &c. On some occasions, 
when one of the films has been entirely black, the lower liquid ring has disappeared. 
This phenomenon is accompanied by a slight shift in the position of equilibrium. 
Again, if the start is too short the films are too similar to make the comparison of 
much use ; if it is too long the thin film is apt to break before the thick one is ready 
for observation. These difficulties are increased by the fact that it is not safe to 
leave the films in communication while one of them is being thickened. A black film 
is almost certain to break if subject to a disturbance which may alter its diameter by 
one or two millimetres. It is, therefore, necessary to separate the films while the 
thickening takes place, and not to put them again into communication until after the 
interval which experience has proved to be sufficient to prevent any great change in 
diameter. We have attempted as far as possible to overcome these difficulties, and 
have carried out our observations in two different ways. 
First Method of Experiment. 
The fact that the method of measuring three ordinates confirmed that based upon 
the measurement of the principal diameters only, in indicating a small difference of 
surface tension between the films, even when both had been recently flooded, proves 
either that such differences really exist or that some slight outstanding cause of error 
affects both methods equally. In either case it seemed best to argue in each experi- 
