66 SECTIONAL ADDRESSES, 
of the theory of unimolecular films of juxtaposed and oriented molecules. 
If we calculate the average distance between two carbon atoms for the 
four acids, we obtain the following results :— 
Distance_(cms.) x 108 
Myristic Acid : é : ceo ALG 
Pentadecylic Acid . S 0 35 AE 
Stearic Acid : : ‘ See alits: 
Behenic Acid ° f : Age 
As pointed out before, these values do not deviate much from the 
value for the distance between the carbon atom centres in the diamond 
(1.52x10-* cm.). Too much stress cannot, however, be laid on this 
point, since in calculating the lengths of the oriented carbon chains an 
assumption has to be made regarding the density of the film, because 
only its area and mass are given directly by experiment. 
Concerning this point some very interesting results have been 
recently obtained in Sir William Bra ge’s laboratory by Dr. A. Miiller. 
In these experiments layers of crystallised fatty acids on glass plates 
have been examined by an X-ray photographic method. From these 
results it appears that the unit cell is a long prism, the cross section of 
which remains constant for the substances investigated, whilst the 
length of the prism increases linearly with the number of carbon atoms 
in the molecule. The increase in length of the unit prism per carbon 
atom in the molecule is found to be 2.0x10-*cm. Since it appears 
likely that there are two molecules arranged along the long axis of each 
unit cell (prism), it would follow that the increase in the length of the 
molecule per carbon atom added is 1.0x10-* cm. Comparing this result 
with the value for the distance between the carbon centres in the 
diamond lattice, it would appear that the carbon atoms in the long 
hydrocarbon chains of the higher saturated fatty acids are arranged in 
a zig-zag, or more probably in a spiral or helix. If this be the case, 
the closer packing or compression of the juxtaposed molecules in the 
unimolecular films, as revealed in the investigations of Devaux, Lang- 
muir, and Adam, may be to some extent explained by the straightening 
out of these zig-zags, or perhaps by the ‘ elastic compression’ of the 
helices. 
As pointed out by Langmuir, the question of the formation of uni- 
molecular surface films can be attacked in a different manner. It is 
known that gases or vapours can be condensed or adsorbed by solid and 
liquid surfaces. The question then arises, does the formation of 
primary unimolecular films ever occur in such cases? It will be recol- 
lected that Hardy made the suggestion that the formation of the 
primary unimolecular film in the spreading of oily substances on water 
might be due to adsorption from the vapour. In order to examine this 
question, 1} Mr. T. Iredale has recently measured in my laboratory the 
fall in the surface tension of mercury caused by exposing a fresh mercury 
surface to vapours of increasing partial pressure. The excess surface 
coricentration q of the adsorbed vapour can then be calculated by means 
of Gibbs’ formula 
