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XVI. On the Relation between the Thickness ancl the Surface Tension of Liquid Films. 
By A. W. Reinold, M.A., F.R.S., Professor of Physics in the Royal Naval College, 
Greenwich, and A. W. Rucker, M.A., F.R.S. 
Received May 15,—Read May 27, 1886. 
[Plate 33.] 
Introduction. 
Laplace’s theory of capillarity is based upon the assumption that the forces in play 
between the neighbouring molecules of a liquid are insensible at sensible distances. 
Each molecule is thus frequently considered as the centre of a sphere which bounds 
the “ insensible ” region within which the molecular forces exerted by the central 
molecule on others are appreciable. The radius of this sphere is called “ the radius of 
molecular action. ’ It is evident from its definition that it is not a determinate 
physical constant. The point at which a force ceases to be “ sensible ” depends upon 
the delicacy of the methods which are employed to detect its effects, and thus also 
upon the nature of the particular effect which is studied. Many physicists have, 
however, rightly considered that an approximate measurement of the distance at 
which the mutual actions of neighbouring molecules become negligible was well worth 
obtaining. They have, therefore, by different methods sought indications of the 
magnitude of this distance, and, if successful, have added their result to the other 
recorded measurements of the radius of molecular action. The nomenclature thus 
adopted is somewhat vague, but we are not aware that any general misconception 
exists as to the indefinite character of the magnitude, to which, nevertheless, a 
definite name is given. 
The theory that molecular forces are only sensible at very small distances leads us 
to regard a liquid as bounded by a very thin external layer, the properties of which 
are different from those of the mass in the interior. To this the name of the surface 
layer may be given. The definition of its thickness is subject to the same difficulties 
as the definition of the magnitude of the radius of molecular action. The two, 
however defined, are closely related and are generally assumed to be equal. 
Plateau, arguing that the surface tension would decrease if the thickness of a 
soap film became less than twice that of the surface layer, first investigated the 
question as to whether the pressure exerted on the enclosed air depended on the 
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