80 SECTIONAL ADDRESSES. 
in water, produced by mechanical agitation without the addition of any 
‘emulsifier.’ I pointed out that in the emulsification of oils in water 
by means of soap, the soap lowers the interfacial tension and concen- 
trates at the interface. When we wish to produce oil emulsions in the 
ordinary sense of the term we must use some such emulsifying agent, 
and for this purpose many substances are employed, such as soap, 
gum acacia, gelatine, casein, starch, &c., &c. All these substances 
concentrate or condense on the surfaces of the oil globules. If we may 
regard these surface films as very mobile from the molecular-kinetic 
point of view, it is clear that they will confer an increased degree of 
stability on the emulsion. For any sudden decrease of interface 
(caused, for example, by coalescence or partial coalescence of two 
adjacent globules) will produce a momentary increase in the surface 
concentration or thickness of the adsorption layer, and so a decrease in 
the interfacial tension, if the surface layer is not saturated. It may 
require a perceptible time for the molecular-kinetic motion (especially 
in the case of large molecules or hydrated micelles) to readjust the 
equilibrium between the surface layer and the bulk. 
It is probable, however, that the stability of the emulsion is in many 
cases due to the fact that the surface films possess a very viscous, 
quasi-rigid, or gel-like character, so that a more mechanical explanation 
is necessary. As S. U. Pickering showed, oils may be emulsified in 
water by the gels of certain basic salts; and A. U. M. Schlaepfer has 
shown that emulsions of water in kerosene oil may be obtained by means 
of finely divided ‘carbon.’ Nevertheless, even in cases where an 
emulsifier is used, we may hope to succeed in obtaining a more precise 
physical analysis of the system. It is interesting in this connection to 
note that Mr. W. Pohl has recently found in my laboratory that when 
a neutral hydrocarbon oil is emulsified in water by means of sodium 
oleate, the electrical potential difference at the oil-water interface is 
almost doubled, and that the effects of alkalies and salts on this potential 
difference are very similar to those found in the case where no emulsifier 
is employed. 
I cannot conclude this account of certain aspects of surface actions 
and properties without making a passing, though all too brief, refer- 
ence to the beautiful investigations of Sir George Beilby on the amor- 
phous layer. He has shown that when the surface of crystalline matter 
is subjected to shearing stress there is produced a surface layer of 
a vitreous or amorphous character—a ‘ flowed’ surface—in which the 
particular ordered arrangement of the molecules or atoms which is 
characteristic of the crystalline matter largely disappears. Working at 
University College, London, Dr. Travers and Mr. R. ©. Ray have 
recently obtained a very interesting confirmation of the Beilby Effect. 
The heats of solution (in kilogram calories per gram mol) of vitreous 
silica and silver sand (silica as crystalline quartz) in aqueous hydro- 
fluoric acid were found to be 37.24 and 30.29 respectively. After 
grinding for fifteen hours the corresponding values were 36.95 and 
32.46 respectively. If we assume that the internal energy of the amor- 
phous phase produced by grinding is the same as that of the vitreous 
silica (silica glass), we can calculate from these results that about 31 per 
