and the Action of the Bile in Digestion, 311 



The immobility or permanence of the common surface thus 

 modified is explained by the fact that each opening in the film of 

 extraneous liquid is immediately closed by the molecular forces ; 

 the surface-tension of the pure liquid in the opening is greater 

 than that of the rest of the film contaminated by contact with 

 the extraneous liquid. The extraneous film may also consist 

 of a solid body, provided that it be not completely solid, but to 

 some extent mobile. This follows from the above-described 

 (under 5) phenomena and properties of the common surfaces 

 of liquids and solid bodies (compare Wiedemann's Annalen, 

 ii. p. 145, 1877). 



In emulsions of the fixed oils in a solution of soda, the thin 

 film of soap, each aperture in which is closed again by molecular 

 force, prevents the oil-globules from coalescing. In emulsions 

 of gum as prepared by the druggists, each minute oil-globule 

 is separated from the aqueous fluid by a film of the solution of 

 gum ; for at their common surface with gum-solution the fixed 

 oils have a less surface-tension than at that with water; this 

 property I have proved by special measurements, as I shall have 

 occasion to explain at greater length in another place. The 

 longer the gum-solution remains attached to the surface of the 

 oil, the firmer it will adhere. 



When mercury is agitated with water and olive-oil, a viscous 

 mass of a grey colour is formed : this is an emulsion of mer- 

 cury, consisting of a number of very small globules of the 

 metal, each of which is coated with a film of oil kept adhering 

 to it by molecular force. A fissure in the oil-film of one mil- 

 limetre breadth will reunite with a force which, according 

 to my own measurements, amounts to 6'09 mgr., and is there- 

 fore comparatively great. In fact, an emulsion of this kind 

 will endure for months. The addition of an acid breaks up 

 the oil-film, and consequently destroys the emulsion. So-called 

 grey ointment is an emulsion of mercury in a highly viscous 

 fat ; its permanence increases with keeping, in consequence of 

 the mercury forming with the rancid fat a compound (mer- 

 curial soap ?) which appears to diffuse itself over the common 

 surface of the two substances. 



Froth which may be seen in a solution of soap or in beer, is 

 an emulsion of air in an aqueous fluid. Its permanence is 

 greater in proportion as the original surface-tension of the 

 pure liquid is decreased by an extraneous film on the free sur- 

 face that is bounded by air. Homogeneous fluids containing 

 no heterogeneous substance do not form a froth ; it can be ob- 

 tained from fatty but not from pure water. The force with 

 which a rent of the breadth of a millimetre in the film of ex- 

 traneous fluid on water tends to reunite, in the case of soap- 



