OX THE THICKNESS AND SURFACE TENSION OF LIQUID FILMS. 
659 
Beriihrung mit Luft uberhaupt nicht existirt.”* It seems probable that a liquid 
drawn out into a film would be especially liable to such changes, and that the 
constant renewal of the surface of the thick film would tend to maintain its purity, 
and therewith its surface tension. 
Adopting, then, the conclusion that the rapid fall in the surface tension of a newly 
formed film is due, not to its thinning, but to a disturbing cause, it remains to be seen 
whether this can be reduced or eliminated so as to permit a comparison of films 
of very different thicknesses. We have found that this is possible, but, though the 
methods which we have adopted are such as would naturally suggest themselves, the 
determination of the best conditions of experiment has cost a large expenditure 
of time and trouble. 
In the first place, we have employed the method of measuring the principal 
ordinates only, and using the sensitiveness to calculate the difference of tension 
(if any) between the films. In what follows these differences will be very small, and 
we have shown that, in this case, both methods lead to very similar mean results. 
The method of measuring three ordinates is also too laborious to be used if another 
will serve equally well. It necessitates continual measurements being made, and, 
though very useful for the elucidation of the particular point to which it was directed, 
is very troublesome and fatiguing, as twenty-three readings are necessary to obtain a 
single comparison. Again, we were not satisfied with the flooded film as a standard 
with which to compare that which was allowed to thin. The difference of the surface 
tensions, and, therefore, of the forms of the films, was too great for accuracy. It was 
thought possible that the comparatively small disturbance produced by retarding the 
thinning by the electric current might make a film, up which a current was either 
constantly or at intervals passed, a better standard than one which was flooded 
between consecutive measurements. 
The following experiment, which was made to see if a film could be kept thick and 
in an approximately constant state by short intermittent applications of the current, 
will perhaps serve to show the difficulty of eliminating the disturbing cause by any 
such means. 
Experiment VII. 
Two cylinders were formed, flooded, and put into communication. 
Five minutes afterwards S= — 0'20 mm. Both films were colourless for the most 
part, but displayed thin rings of colour at the top. 
The current was then sent up the right cylinder for 10 s . The coloured rings dis¬ 
appeared. 
At 9 m from the beginning of the experiment, and about 3 m after the passage of the 
current, the right film was colourless. 
* ‘Wiedemann, Annalen,’ vol. 11, 1880, p. 650. 
4 p 2 
