64 SECTIONAL ADDRESSES, 
value of hydrocarbon oils, whilst H. Wells and W. Southcomb have 
demonstrated the marked improvement due to a small addition of a 
fatty acid. In this connection it is interesting to note that W. E. Garner 
and §. S. Bhatnagar have recently shown in my laboratory that the 
interfacial tension between mercury and B.P. paraffin oil is markedly 
lowered by small additions of oleic acid. The oleic acid molecules are 
therefore absorbed or concentrated at the mercury-oil interface, an 
action which may well be due in part to the fixation and orientation of 
these molecules at the metal-oil interface. 
This question of the orientation of molecules at the surfaces of 
liquids has been greatly extended in recent years by a detailed study of 
the extremely thin and invisible films formed by the primary spreading 
of oily substances on the surface of water. In a continuation and 
development of the work of Miss Pockels, the late Lord Rayleigh showed 
many years ago that when olive oil forms one of these invisible films 
on water, there is no fall in surface tension until the surface concen- 
tration reaches a certain very small value. He made the highly interest- 
ing and important suggestion that this concentration marks the point 
where there is formed a continuous layer just one molecule thick. In 
the case of olive oil, he found this critical thickness to be 10-7 cm., 
and concluded that this number represented the order of magnitude of 
the diameter of a molecule of the oil. Increase in surface concentration 
beyond this point causes the surface tension to fall, until a second point 
is reached, after which no further fall in surface tension occurs. Lord 
Rayleigh assumed that at the second point a layer two molecules thick 
is formed. This pioneer work of Lord Rayleigh was repeated and 
extended by H. Devaux and A. Marcelin, who showed the correctness of 
his first suggestion, namely that the primary film consists of a 
unimolecular layer. It appears, however, that the fall in surface 
tension which he ascribed to the building up of a bimolecular layer, must 
be ascribed to the closer packing of the molecules of the unimolecular 
layer, and that the second point occurs when these molecules are packed 
as tightly as possible. 
Instead of varying the surface concentration by adding more and 
more of the oily substance to a definite surface, we may attain the same 
end by means of a moving boundary and a variable surface, and study 
the relation between the force of surface-compression (difference between 
the surface tension of pure water and that of the contaminated surface) 
and the surface concentration. This method was greatly developed by 
Devaux. Although these researches had firmly established the theory 
of the formation of a unimolecular surface layer and therefore of the 
existence of a new ‘two dimensional’ phase of matter, we owe it to 
I. Langmuir to have made a very important advance by connecting this 
conception with the ideas of chemically active groups and molecular 
orientation. Influenced, no doubt, by the ideas of Hardy, Langmuir 
reasoned that the formation of these primary unimolecular films must 
be due to the presence of active groups in the molecules, which are 
attracted inwards towards the water and thus cause the long open chain 
molecules of the fatty acids to be oriented on the water surface with 
their long hydrocarbon axes vertical and side by side. 
