SURFACE-FILMS. ll 
apart, a distance reckoned from the centre of the molecules 
and known as the range of intermolecular forces (frequently 
represented by the symbol z), (8) they can rarely be equal 
along all radii between the centres of two molecules, in which 
case rotation of the molecules round their centres must bring 
about alterations in the intensity of the intermolecular 
forces operative between them. It is safe also to assume 
that (4) they increase greatly in intensity as two molecules 
are brought nearer together than the distance at which they 
first begin to be operative, and that the repulsions increase 
at a much greater rate than do the attractions, average 
equilibrium being attained when the average intermolecular 
distance is such that both are equal, (5) the range of the 
attractions extends over several molecular diameters. 
Applying these assumptions to the case of pure liquid 
in contact with gas, and dealing with the system for 
convenience as if its molecules were in static equilibrium, it 
is clear that every molecule remote from the surface has its 
entire field of force occupied by similar molecules, but that 
every molecule in the surface stratum down to a depth z has 
some gas molecules within range in place of liquid, and as 
each of these ‘ occupies’ a very much larger average volume 
than does a liquid molecule, those within range must be com- 
paratively few; hence the intermolecular forces operative 
on the surface-stratum must come predominantly from the 
liquid, i.e., from the ‘‘cis’’ side of the surface. It will 
be clear also that one result of this specific direction of the 
predominant intermolecular forces must be that equilibrium 
could only be attained by correspondingly specific molecular 
spacing and strains different from those obtaining remote 
from the surface, and that special intramolecular strains 
between the atoms composing the molecules are also likely 
to be produced. What this spacing is and how it gives rise 
to a contractile state along the surface it is impossible to 
