72 SECTIONAL ADDRESSES. 
and these quickly rise to the surface and burst. If now a very smail 
quantity of a substance which concentrates at the air-water interface 
be added, an almost milk-white ‘ air emulsion’ of small bubbles is 
produced, which rise to the surface and produce a relatively durable 
froth. These phenomena were discussed by the late Lord Rayleigh 
in a very interesting Royal Institution lecture on ‘Foam.’ It is clear 
that the diminution in interfacial tension facilitates the subdivision or 
dispersal of the air. The existence of the surface layer will also confer 
a certain amount of stability on the resultant froth, since it will give 
rise to forces which resist the thinning of a bubble wall. Any sudden 
increase in the surface will produce a momentary diminution in 
the concentration or ‘thickness’ of the surface layer, and hence 
a rise in surface tension, which will persist until the normal thick- 
ness or concentration is readjusted by diffusion of molecules from 
the inside volume—a process which in very dilute solution will occupy 
a perceptible time. That this explanation (due to the late Lord 
Rayleigh) is the correct one can be seen from the fact that very 
often stronger solutions of the same surface-active substance scarcely 
foam at all. In this case the readjustment of the equilibrium thickness 
or concentration of the surface layer occurs with such rapidity (owing 
to the greater concentration of the molecules in the inside volume) that 
practically no rise in surface tension, and hence no counteracting force, 
comes into play. These effects will be the more pronounced—other 
things being equal—the greater the mass and hence the smaller the 
motion of the solute units, as in the case of large molecules or colloidal 
micelles. It is probable, however, that the explanation of the stability 
of very durable forms, as, for example, those produced by the sea at 
the sea coast, by beer and stout, by aqueous solutions of soap or saponin, 
&e., is often more complex, and that we must seek it in the forma- 
tion of very viscous or semi-rigid or gel-like membranes at the interface. 
Moreover, small solid particles may contribute to the stabilisation of a 
froth, as in the case of the ‘mineralised froths’ of the ‘ore flotation 
process ; and the preferential aggregation of small particles in the inter- 
face between two phases has been demonstrated in the experiments of 
W. Reinders, F. B. Hofmann, and many others. 
Let us now inquire how far the phenomena which we have seen to 
be characteristic of a gas-liquid interface occur also at the interface 
between two immiscible or partially miscible liquids. Many years ago 
it was shown by Gad and by Quincke that a fatty oil (such as olive oil) 
is very readily dispersed in the form of an emulsion by a dilute solution 
of caustic soda. Some experiments which I once made showed that a 
neutral hydrocarbon oil could be similarly emulsified in a dilute aqueous 
solution of alkali if one of the higher fatty acids was dissolved in it, 
whilst the lower fatty acids do not produce a similar action. It was 
shown that the action runs parallel to the lowering of interfacial tension 
and must be ascribed to the formation of a soap, which lowers the inter- 
facial tension and concentrates at the interface. These phenomena 
have have been further investigated by S. A. Shorter and S. Ellings- 
worth, by H. Hartridge and R. A. Peters, and by others. 
If a substance which is dissolved in one liquid A, and which is 
