XVI. On the relation between the thickness and the surface tension of liquid films

Abstract 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.