ON THE THICKNESS AND SUBFACE TENSION OF LIQUID FILMS. 
679 
Our observations also justify the statement that— 
This conclusion is based upon a method of experiment by which a change of one-ludf 
per cent, in the value of the tension must have been detected, had it existed. 
The further question remains, can any inference be drawn from these observations 
as to the magnitude of the radius of molecular action ? 
On this point we venture to think that, if the magnitude of that length is to be 
settled merely by reference to the equality of the surface tensions of thick and thin 
films, we have found evidence in favour of its being less than 6X 10 -G mm. (half the 
thickness of an ordinary black film), far stronger than that on which the frequently 
quoted statement of Plateau that it is less than 50 X 10 -6 mm. is based. 
Our observations have been more numerous and more sensitive than his, and on the 
other hand we have used films much thinner than those employed by him, or by Van 
dee, Mensbeugghe in his confirmatory observations. 
We must admit, however, that we are unwilling to draw any such conclusion until 
an explanation of the discontinuity in the thickness at the edge of the black is forth¬ 
coming, which is not incompatible with it. The regular recurrence of that pheno¬ 
menon suggests that it must in some way be connected with the extreme tenuity of 
the film on one side of it. We thought it well that Plateau’s method should be 
pushed (as we venture to think we have pushed it) beyond the point where he and 
Van dee Mensbeugghe left it, but the fact that it has led to a negative result does 
not diminish the significance of the sharp edge of the black. 
As far as we are aware, no suggestion has yet been made as to the cause of the 
discontinuity. We have necessarily had our attention directed to it, and, though our 
views are at present little more than speculations, yet it may perhaps conduce to a 
useful discussion if we suggest a cause which might, we think, furnish a satisfactory 
solution of the difficulty. 
Suggestion as to the Cause of the Discontinuity at the Edge of the 
Black Part of a Soap Film. 
It may be convenient if we collect together the various facts which constitute all 
that is certainly known as to the behaviour and constitution of a black film. 
1. " The black part of a soap film in equilibrium, and not subjected to the action of 
an external force, is always separated from the rest of the film by a clearly defined 
boundary. 
A gradual transition from the black to the thicker parts of the film is only observed 
either in a film which thins rapidly and breaks soon (when it is a very transient 
phenomenon) or when an electric current is passed through the film. 
2. The discontinuity in the thickness indicated at the boundary varies very much 
* ‘ Roy. Soc. Proc.,’ vol. 26, p. 334. 
