ON THE THICKNESS AND SURFACE TENSION OF LIQUID FILMS. 
633 
cylindrical. The other, though of the same mean curvature, has been of various 
forms as convenience suggested. 
The general idea of the apparatus is represented, in the accompanying diagram. 
Bubbles are blown on the inverted cups A. and B, until they adhere to the rings 
C and D. They can be cut off from or put into communication with each other or 
with the external air by means of stopcocks. By withdrawing or forcing in air they 
can be made to assume any required form. 
This arrangement, like Ludtge’s, possesses the great advantage that the indications 
sought for are not complicated by changes in the temperature or in the pressure of 
the atmosphere. These may cause the diameter of the films to increase or contract, 
but will affect them equally. A difference in surface tension would be indicated by a 
difference in their magnitudes. 
The two simplest surfaces which can be formed with such an apparatus are the 
cylinder and the sphere. The fact that if they change their shape they become 
unduloids or nodoids has, of course, made the mathematical theory of the experiment 
more complicated. The justification of the particular method adopted is, however, to 
be found in the fact that liquid cylinders offer such much greater facilities for 
obtaining a completely black film than any other geometrical form, that every other 
consideration has been made to give way to this, which, for reasons to be detailed 
hereafter, we conceive to be of prime importance. 
In the following pages we shall deduce from the assumption that films of different 
thicknesses have different surface tensions the conclusions to which it leads, and shall 
then submit to the test of experiment the question as to whether these conclusions are 
in accord with the observed facts. 
On the Sensitiveness of the Methods of measuring Changes in the Surface Tension 
of a Film. 
All the methods which we have described measure the change in the surface tension 
of' the film indirectly by a change in length. In Plateau’s experiment the move¬ 
ment of the liquid in the manometer, in the others the alterations in the sagittee or 
MDCCCLXXXVI. 4 M 
