62 SECTIONAL ADDRESSES. 
and relatively small compared with molecular dimensions. The chemical 
constitution of the molecule is now regarded as determining the varying 
nature of the field of force surrounding it, so that parts of the molecule 
possessing high ‘ residual chemical affinity ’ give rise to specially power- 
ful regions of force. In this way the older ‘physical’ theories of 
cohesion according to central forces with uniform orientation have been 
to some extent replaced, or at all events supplemented, by ‘ chemical’ 
theories according to which the attractive force-fields are highly 
localised round active chemical groups and atoms, are relatively minute 
in range, and can be saturated or ‘ neutralised’ by the atoms or groups 
of neighbouring or juxtaposed molecules. 
Dr. W. B. Hardy has been the chief pioneer in the development 
of these newer theories, having been led thereto by his researches on 
surface tension, surface films, composite liquid surfaces and static fric- 
tion and lubrication. As the matter is one of great importance, I shall 
take the liberty of giving two quotations from Hardy’s scientific papers. 
‘ The corpuscular theory of matter traces all material forces to the 
attraction or repulsion of foci of strain of two opposite types. All 
systems of these foci which have been considered would possess an 
unsymmetrical stray field—equipotential surfaces would not be disposed 
about the system in concentric shells. If the stray field of a molecule, 
that is of a complex of these atomic systems, be unsymmetrical, the 
surface layer of fluids and solids, which are close-packed states of matter, 
must differ from the interior mass in the orientation of the axes of the 
fields with respect to the normal to the surface, and so form a skin 
on the surface of a pure substance having all the molecules oriented 
in the same way instead of purely in random ways. The result would 
be the polarisation of the surface, and the surface of two different fluids 
would attract or repel one another according to the sign of their surfaces.’ 
(Hardy, 1912.) 
These ideas are even more clearly expressed in the following passage. 
‘Tf the field of force about a molecule be not symmetrical, that is to say, 
if the equipotential surfaces do not form spheres about the centre of 
mass, the arrangement of the molecules of a pure fluid must be different 
at the surface from the purely random distribution which obtains on the 
average in the interior. The inwardly directed attractive force along 
the normal to the surface will orientate the molecules there. The 
surface film must therefore have a characteristic molecular architecture, 
and the condition of minimal potential involves two terms—one relating 
to the variation in density, the other to the orientation of the fields of 
force.’ (Hardy, 1913.) 
Hardy thus bases the notion of molecular orientation at the surface 
on the existence of unsymmetrical fields of force surrounding the mole- 
cule; in other words, the parts of the molecule possessing the most 
powerful stray fields will be attracted inwards towards the bulk and 
thus cause a definite orientation of the whole molecule at the surface. 
If , be the surface tension of a liquid A, y, that of another prac- 
tically immiscible liquid B, and y,, the interfacial tension at the 
interface A/B, then the quantity W =7~,+7,—yYa, represents the de- 
crease of free surface energy, and therefore the maximum work gained, 
