‘ 
4 
J 
SURFACE-FILMS. 9 
viscosity developed at such interfaces on adding a solution of 
a suitable solid to one or other of the pure liquids, and also 
to show that there was a lessening of the surface-tension of 
the interfaces. It was obvious that such _ interface- 
concentrations and solid membranes must profoundly 
influence the durability of emulsions made of the two 
liquids, and it was found that they were in many cases 
capable of rendering the emulsions permanent, and also 
were invariably present when emulsions with large droplets 
were permanent. 
In bubble-films it was clear that each of the two free 
> and 
surfaces would acquire its excess of ‘* surface coating,’ 
therefore that such films must have something like a 
sandwich structure. 
CONTRACTILE SURFACE-IILMS. 
The explanation of the surface phenomena you have seen 
being thus bound up with “‘ surface tension,’’ let us consider 
briefly what is meant by the term. Everyone knows that a 
small mass of liquid in air or other gas tends to assume a 
spherical shape. ‘This tendency might conceivably be 
explained in more than one way, but there is good evidence 
that actually it is due to the very thin stratum of liquid 
nearest the free (i.e., gas or air) surface, and only this 
stratum, being in a stretched condition, always trying to 
contract, i1.e., always in a state of tension. A striking way 
of observing this is to see what happens when the end of a 
glass rod moistened with soap solution, or other. watery 
solution with a much lower surface tension than water, is 
brought into contact with the surface of clean water 
previously lightly dusted with sulphur; instantly the surface 
stratum of the water contracts away from the glass rod, 
incidentally carrying the sulphur away with it to the side of 
the vessel,—there is a tug-of-war in which the soap- 
