6S4 
ON THE THICKNESS AND SURFACE TENSION OF LIQUID FILMS. 
we are not aware of any other suggested explanation. That which we offer is based 
upon a view of the nature of the inter-molecular forces in a liquid which is frequently 
neglected, but which was recognised by Maxwell as tenable. The first hypothesis 
offers, at all events, a plausible explanation of the discontinuity, of the persistence of 
the film in spite of the sudden decrease in thickness, and of the constant thickness of 
the black portion of the film. 
Postscript. 
(Added June 18, 1886.) 
Sir William Thomson has informed us in the course of conversation that he 
suggested a minimum of surface tension as the cause of the sharp edge of the black 
in a lecture delivered at the Royal Institution on Friday, January 29, 1886. The 
discourse has not yet been published, but we are, by his kindness, enabled to give the 
passage in question from a proof :— 
“ Well-known phenomena of bubbles, and of watery films wetting solids, make it 
quite certain that the molecular attraction does not become sensible until the distance 
is much less than 250 micro-millimetres. From the consideration of such phenomena, 
Quincke (‘ Poggendorff, Annalen,’ 1 869) came to the conclusion that the molecular 
attraction does become sensible at distances of about 50 micro-millimetres. His 
conclusion is strikingly confirmed by the very important discovery of Reinold and 
Rucker that the black film, always formed before an undisturbed soap bubble breaks, 
has a uniform or nearly uniform thickness of about 11 or 12 micro-millimetres. The 
abrupt commencement and the permanent stability of the black film demonstrate a 
proposition of fundamental importance in the molecular theory. The tension of the 
film, which is sensibly constant when the thickness exceeds 50 micro-millimetres, 
diminishes to a minimum, and begins to increase again when the thickness is 
diminished to 10 micro-millimetres. It seems not possible to explain this fact by 
any imaginable law of force between the different portions of the film supposed 
homogeneous, and we are forced to the conclusion that it depends upon molecular 
heterogeneousness.” 
It is, perhaps, unnecessary to say that we were unaware of Sir W. Thomson’s views 
till within the last few days, or that we are glad to find that our opinion has the 
support of so high an authority. 
