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B.—CHEMISTRY. 69 
probably communicated to them by the thermal agitation of the water 
molecules to which they are attached. 
It is, indeed, highly probable that the molecules which are con- 
centrated in the surface from the state of solution in the liquid phase 
are not in quite the same situation as the molecules of practically 
insoluble substances which are placed on the surface. In the former 
case the molecules are still ‘dissolved,’ so that they will be more 
subject to thermal agitation and less able to form a juxtaposed uni- 
molecular layer. They may also be ‘hydrated.’ The difference 
between the two cases is rendered very evident from the fact that in 
the production of surface layers from dissolved molecules of the fatty 
acids (and other ‘ surface active’ substances) there is a very marked 
fall of surface tension, whilst the uncompressed unimolecular surface 
films placed on the surface from outside do not affect the surface ten- 
sion of the water. Thus the molecules of the surface-active substance 
in the former case are in much closer relation to the solvent molecules, 
and are in kinetic equilibrium with the molecules of both solvent and 
‘solute in the bulk of the liquid. Nevertheless, the agreement as 
regards order of magnitude in the values of the surface area per mole- 
cule in the two types of case is certainly very suggestive and significant. 
Moreover, the experiments of Mr. Iredale show that molecules which 
are adsorbed on the surface from the vapour phase lower the surface 
tension, and are therefore from this point of view comparable with 
molecules concentrated in the surface from the bulk of the liquid phase. 
The question as to whether the simplified form of Gibbs’ equation 
yields a sufficiently accurate value for the excess surface concentration 
can scarcely be decided without more experimental data. In the 
experiments made by Dr. Barker and myself, the values calculated 
from the surface tension-concentration curve were 1.3x10~’ and 
0.6x10-7 grm. per sq. cm., according as the value of the van’t Hoff 
factor i for the very dilute solutions of nonylic acid was taken as 1 or 2 
‘respectively ; whilst the corresponding directly determined value was 
about 1.0x10-7 grm. per sq. cm. ‘This discrepancy was probably well 
within the experimental error of our measurements. 
Let me now direct your attention to another very interesting 
phenomenon relating to the surfaces of liquids and solutions—namely, 
the existence of an electrical potential gradient or potential difference 
in the surface layer. ‘These interfacial potential differences are of great 
importance, and play a fundamental réle in determining the stability or 
instability of many colloidal states of matter. The liquid-gas interface 
offers the simplest case of such interfaces, and so the investigation of 
the potential differences which may exist at this interface is a matter 
of fundamental interest. In 1896 F. B. Kenrick developed, on the 
basis of earlier experiments of Bichat and Blondlot, an electrometric 
condenser method for the comparative determination of the gas-liquid 
P.D.'s. The results which he obtained show that substances (such as 
the aliphatic alcohols and acids) which concentrate at the surface pro- 
duce a very great change in the surface P.D., whilst highly dissociated 
univalent inorganic salts, such as KCl, do not. The results obtained 
by Kenrick have been much extended by an investigation carried out 
