18 REPORTS ON THE STATE OF SCIENCE.—1918. 
The connection between low interfacial tension and emulsification 
was first insisted on by Quincke, but the principal quantitative 
investigations are due to Donnan. They comprise experiments 
carried out with the drop pipette on the emulsification of glycerides 
in alkali solution, which show, in agreement with Quincke, that free 
fatty acid, 7.¢. the possibility of soap formation at the intérface, is a 
necessary condition if emulsification is to take place; determinations 
of the drop numbers for hydrocarbon oils discharged into solutions 
of salt of the fatty acids,and experiments in which the hydrocarbon 
oil was dispersed in these solutions by shaking under strictly defined 
conditions. The two last series give concordant results and show 
that the first salt of the fatty acid series to exert an appreciable 
emulsifying action is the one which shows the first marked reduction 
of surface tension, viz. that of lauric acid. Donnan also finds that 
there is an optimum concentration of soap, and explains it as due to 
the balance between the reduction of interfacial tension, which 
promotes, and the electrolyte effect, which counteracts, dispersion. 
A similar result, viz. that increase of soap concentration beyond 
a certain limit was detrimental to emulsification, had been obtained 
by 8. U. Pickering. According to him the optimum concentration 
depends both on the phase ratio and on the absolute volumes of the 
phases, so that no simple explanation appears to offer itself. The 
same author also gives a number of experiments in which the emul- 
sifying agent is not in solution—either true or colloidal—but a 
precipitate such as ‘basic iron, copper or nickel sulphate,’ 7c. the 
precipitates formed by adding lime water to the respective sulphates. 
If water containing one of these substances-is agitated with oil— 
more particularly mineral hydrocarbons of 250° to 359° boiling-point 
and about 0°85 sp. gr.—the oil is completely emulsified. After an 
. examination of various finely divided solids which show only a 
transient effect or none at all, Pickering comes to the conclusion that 
the chief factor in the formation of stable emulsions of oil as disperse 
phase in water is the existence of a layer of small solid,. non- 
crystalline particles which are more easily wetted by water than by 
oil (the italics are mine, EH. H.) at the interface. 
While Pickering thus concludes that a low interfacial tension is 
not the principal, or in fact a necessary, condition of emulsification, 
the discrepancy between the two views is probably only apparent, at 
any rate if the ‘solid’ state of the particles in the interfacial layer is 
not insisted upon. If low tension is not the direct agent it is never- 
theless active indirectly in bringing about—in such solutions as alone 
come into question—the formation of absorption films having some 
of the properties postulated. On the other hand, the substances used 
by Pickering would obviously not accumulate at the interface (more 
strictly on the water side of it) unless this arrangement led to a 
lowering of the interfacial tension. 
That a film or membrane covering the whole of the interface is 
a necessary condition of stability in emulsions is the conclusion 
pronounced by W. D. Bancroft after a most exhaustive review of the 
available material. The relative solubility of this film in the two 
phases, or the difference in interfacial tensions between them and 
the film, determines the nature of the emulsion, 7.e. which of the 
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