[ G45 j 
XX. The Limiting Thickness of Liquid Films. 
By A. W. Rein old, M. A., Professor of Physics in the Roycd Naval College, Greenwich, 
and A. W. Rucker, M.A ., Professor of Physics in the Yorkshire College, Leeds. 
Communicated by R. B. Clifton, M.A., F.P.S., Professor of Experimented 
Philosophy in the University of Oxford. 
Received March 6,—Read April 19, 1883. 
[Plate 47.] 
The experiments described in this Paper are an extension of our previous investi¬ 
gations on the properties of liquid films. The interest and the difficulty of such 
inquiries increase as the thickness of the films diminishes, and culminate when they are 
sufficiently thin to show the black of the first order of Newton’s rings. We can in 
that case only infer from the colour that the thickness is less than a certain possible 
maximum. Our knowledge as to the real value of this maximum is, we venture to 
think, very uncertain, but it furnished, we believe, previous to our own investigations, 
the only clue to the thickness of a black liquid film. 
In a Paper on the “Thickness of Soap Films” (Proc. Roy. Soc., 1877, No. 182, 
p. 345), we were however able to show, for the particular liquid and apparatus used :— 
i. That the variations in thickness of the black portions of the films were but a 
small fraction of that thickness. 
n. That the thickness was independent of the breadth of the black ring, 
m. That it was also independent of the thickness of that portion of the film which 
appeared to the naked eye to be in immediate contact with it. 
We also proved, on the assumption that the specific resistance of the liquid in the 
film was identical with that of the same liquid in mass, that the average thickness of 
the black films observed must have been 12 X 10 -6 millims. 
In a more recent Paper (Phil. Trans., 1881, p. 447) we have shown that this 
assumption is correct for thicknesses greater than 374 X 10 -6 millims., below which the 
number of our observations was insufficient to enable us to arrive at a reliable conclu¬ 
sion. It was also shown that very slight changes in the temperature or hygrometric 
state of the air produce great variations in the composition of films formed of a mixture 
of soap solution and glycerine. 
MDCCCLXXXIII, 4 O 
