160 Dr. AY. Kauisden. ' Separation of Solids in the [June 8, 



formed are all concerned in conferring on different bubbles the 

 marked individuality which characterises them, and in making the 

 formation of large or persistent bubbles possible. Solid particles, 

 which diminish the surface energy of the free surface, may be regarded 

 as adding in three ways to the persistence of a bubble :- 



i. By serving appoints d'appui* 



11. By actual contact, friction, or cohesion of the particles, opposing 



local disturbances of the film. 



iii. By opposing such deformation of the surface as tends to expose 

 a new surface with higher surface-tension (i.e., like oil on water, 

 by the effect of the surface coating in diminishing the superficial 

 energy). 



12. It has been demonstrated that an actual solid membrane forms 

 around the globules of several persistent emulsions, and at the contact 

 interfaces of several pairs of liquids capable of forming persistent 

 emulsions (e.g., pure neutral olive oil and saponin solutions). 



The membrane manifests itself by producing the following 

 phenomena : 



i. Intense viscosity peculiar to the interface, absent at the interface 



of pure water and the other liquid, and developing only when 



an emulsifying substance is added to one or other of the 



liquids. 



ii. Persistently deformed sharply angular ;ind grotesque shapes of 



the emulsified globules, 

 iii. .Folds of semi-opaque membrane when the surface of separation 



is subjected to appropriate deformation. 



Such direct optical evidence of the presence of a constraining mem- 

 brane separating liquids which form persistent emulsions is exceptional 

 or, if it occur, is usually fugitive. (Cf. air-bubbles.) An intense 

 " special interface viscosity," pointing to the presence of solid or highly 

 viscous matter, has, however, been found with every pair of liquids 

 capable of forming persistent emulsions hitherto examined. 



13. The persistence of many emulsions is therefore determined 

 largely, among other factors, by the presence of solid or highly viscous 

 matter at the interfaces of the two liquids. Direct measurements of 

 the various surface tensions concerned are not available, but the close 

 resemblance of the phenomena to those occurring at a free surface 

 points to the view that accumulation of solid matter at the interfaces 

 of the above emulsion-pairs occurs because the "surface-energy" is 

 thereby diminished. 



14. Numerous precipitations of colloids from their solutions by 

 chloroform, ether, carbon bisulphide and amyl-alcohol, are attended by 



* Cf. Frankenheim, ' Die LeUre von der Cohasion ' : Breslau, 1835. 



