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SURFACE-FILMS. 17 
attraction expect a priori that a solid which is moistened by 
the liquid so that an acute angle of contact is formed would 
be one which attracts the liquid more powerfully than does 
a solid which is not moistened by the liquid; and further, since 
increase of solid-liquid contact must necessarily be accom- 
panied by decrease of liquid-liquid contact, that moistening 
of the solid could only take place when the inter-molecular 
attraction K,, exceeds K,,. 
Consider the phenomena now from another point of 
view, that of equilibrium of the forces* in the region where 
the liquid-air surface-film intersects the solid. These forces 
are (1) the attraction of the solid for the liquid along a 
line at right angles to the solid surface,} (2) the contractile 
pull of the free liquid surface, (5) the force acting along the 
liquid at the solid-hquid interface. If the last mentioned is 
a contractility, equilibrium is only possible if it pulls down- 
wards and the free liquid surface pulls upwards, 7.¢., if the 
liquid dips towards the solid and forms an obtuse angle of 
contact. If, on the other hand, it is an expansile thrust 
upwards, equilibrium is only possible if the free liquid 
surface pulls downwards, 7.¢., if the liquid rises towards the 
solid and forms an acute angle of contact. 
An acute angle of contact must therefore, I hold, be 
regarded as positive proof that the liquid stratum in contact 
with the solid is exerting an expansile sideways thrust, 7.e., 
has a negative tension. Carefully considered, it is also 
impossible to see how, without such sideways thrust, the 
liquid could ever creep up. the walls of the solid at all, 
quite apart from maintaining itself there when it had crept up. 
We have then an association between— 
(1) a negative tension or expansility ; 
(2) an acute angle of contact; 
(3) molecular attractions across the surface greater 
than those of the liquid for itself. 
* Tensions of gas condensed at free surfaces are assumed to be negligible. 
+ This opposes the component of the tension of the air-liquid surface 
normal to the solid. 
