B.—CHEMISTRY. 63 
when a surface of A is allowed to approach normally and touch a surface 
of B at constant temperature. Comparing different liquids A with water 
as a constant liquid B, Hardy has shown that the quantity W is ex- 
tremely dependent on the chemical constitution of A, and is especially 
high when A contains the atomic groups characteristic of alcohols, acids, 
and esters. Thus, for such saturated substances as octane, cyclo- 
hexane, CS, and CCl,, the values of W at ordinary room temperature 
lie between 21 and 24. Compare with these values the following :— 
(a) Introduction of a hydroxyl group :— 
Octyl Aleohol . ; ‘ ; 
Cyclohexanol . : j 2661.4 
(b) Introduction of a carboxyl group :— 
n—Caprylic acid : 3 - 46.4 
Oleic acid . : : - 44.7 
The natural inference from results such as these is that the 
cohesional forces are essentially chemical in origin and that they depend 
in large measure on the presence of ‘ active’ atoms or groups of atoms, 
namely those possessing strong fields of ‘residual chemical affinity’; 
in other words, powerful and highly localised stray fields of electrical 
or electromagnetic force (or of both types). The existence of such atoms 
or atomic groups is strong presumptive evidence of the unsymmetrical 
fields of force postulated by Hardy and therefore of the molecular orienta- 
tion at surfaces. 
The conclusions drawn by Hardy have been amply confirmed by 
W. D. Harkins, and his collaborators, who in a long series of accurate 
measurements of surface and interfacial tensions have found that in 
the case of very many organic liquids the ‘ adhesional work’ towards 
water is greatly increased by the presence of oxygen atoms (as in 
alcohols, acids, and aldehydes). They find that the very symmetrical 
halogen derivatives CCl: and C2H«Brz (which possess specially high 
values for their own cohesional work) give markedly low values for 
their adhesional work towards water, and that in the case of unsymme- 
trical molecules, the adhesional work towards water is determined by 
the presence of certain active atoms or atomic groups. 
In his work on static friction and lubrication, Hardy has found 
that the influence of chemical constitution and the effects of active 
atomic groups are very pronounced. This, comparing aliphatic or open 
chain compounds, the co-efficient of static friction falls (and the lubri- 
cating power increases) as we pass through the series hydrocarbon— 
alecohol—acid. The corresponding ester is in this case a much worse 
lubricant than the related acid or alcohol. These results suggest, as 
Hardy has indicated, that friction is caused by the molecular cohesion 
of surfaces, and that in the action of such lubricants the molecules are 
oriented with their long axes normal to the surface, whereby the active 
atomic groups play an important part in ‘ taking up’ or saturating a 
_ portion of the stray force-fields of the molecules of the solid surfaces, 
_ and in orienting and anchoring the lubricant molecules to these surfaces. 
Many facts lend strong support to Hardy’s views. Thus it is true, 
T believe, that the addition of aliphatic esters improves the lubricating 
1923 _ 
