308 Dr. Gr. Quincke on the Formation of Emulsions, 



5. Solid and Liquid Films at the common Surface of two Fluids. 

 Castor-oil. 



For the reason last stated, castor-oil will not freely form an 

 emulsion. 



When a drop of a fixed oil is allowed to fall at as small a 

 distance as possible from the end of a glass rod onto a layer 

 of a dilute solution of soda which is only a few millimetres in 

 thickness, and is contained in a watch-glass of from 50 to 100 

 millimetres diameter, the oil will generally, as explained more 

 fully above (in 3), disperse over the surface of the liquid; for 

 the surface-tension of the more perfect fluid is greater than that 

 of the oil. Shortly afterwards soap is formed and dissolved in 

 the aqueous liquid, the tension of the free surface of the 

 aqueous liquid becomes considerably less, and the oil contracts 

 again into a lenticular-shaped drop. On its lower surface the 

 drop of oil is coated with a film of soap, which may consist of 

 a thin membrane either of liquid solution of soap or of solid 

 soap. The latter is well seen when a number of solid particles 

 of soap lying close to each other form a whitish matted film, 

 as in the case of cod-liver oil in a from \ to 2-per-cent. solu- 

 tion of soda. 



If the soapy membrane be liquid, the oil-drop assumes a 

 spherical form, which it will retain even when the liquid in 

 the watch-glass has a rocking or rolling motion communicated 

 to it. If, on the other hand, the membrane be solid, the drop 

 under the rocking motion takes a cylindrical shape, which dis- 

 appears more or less slowly in proportion as the membrane is 

 thicker or thinner. This was the case with ofrve, almond, 

 and cod-liver oils in a 0*06-per-cent. solution of soda, and of 

 almond-oil in a 0*25-per-cent. solution. A similar appearance 

 is observed in the case of a drop of mercury in a clean watch- 

 glass, which, when coated with a thin layer of oil, retains its 

 spherical form, notwithstanding its being rolled about on the 

 glass. If, however, the mercury contain a small quantity of a 

 solid metal, as, for example, lead, which forms in contact with 

 the air a film of solid oxide, the drop will become cylindrical 

 under the influence of a rocking motion. 



With castor-oil in dilute solutions of soda, I have observed 

 after the lapse of some hours a trace of a whitish precipitate 

 at the lower surface of the oil-drop; but the latter is always in 

 the shape of a sphere. When shaken, the drop becomes larger 

 for a few minutes, probably because the agitation causes a little 

 soap to be thrown from the interior of the drop to its surface, 

 where it is dispersed. But so soon as this soap is dissolved in 

 the aqueous fluid, and the common surface of the two liquids 



