310 Dr. GL Quincke on the Formation of Emulsions, 



at the common surface of the oil and aqueous liquid renders 

 the fluid column in the capillary tube less mobile, and there- 

 fore prevents the oil from rising so high. But by the addi- 

 tion of the solution of bile the soap is rendered soluble, the 

 friction in the interior of the tube is diminished, and conse- 

 quently the height to which the oil rises is indirectly increased. 

 It has been often stated (compare Gad, Du Bois's Arch. 1878, 

 p. 202) that from experiments of this kind we obtain an idea 

 of what takes place on a very much reduced scale with the 

 diffusion of fluids through the capillaries of the animal mem- 

 brane. But it must be remembered that the ascending action 

 in capillary tubes depends on the nature of two surfaces — one 

 the common surface of water or aqueous fluid and air, the other 

 that of the same substance and oil. Now the first of these, 

 on account of its greater surface-tension, has a much more 

 decided influence on the capillary height than the second. 

 And it also happens that the height varies with the curva- 

 ture of the common surface of the oil and the fluid ; in glass 

 tubes the convex side of the common surface is generally 

 turned towards the aqueous liquid. But this curvature will 

 vary considerably in the same tube, and depends materially on 

 the edge-angle of the common surface with the wall of the 

 tube ; and the edge-angle has very different values in the case 

 of tubes of glass and of those of the animal membranes. Now, 

 under normal circumstances the surfaces of the fluids of the 

 animal body are in contact neither with glass nor with air ; 

 consequently no opinion can be formed of the operations of 

 diffusion in the animal body by the measurement of capil- 

 lary heights, which depend essentially on the common surfaces 

 of fluids with glass and air. A closer insight into these phe- 

 nomena of diffusion can only be obtained by an investigation 

 of the physical and chemical actions at the common surface 

 of one kind of fluid with another, or with the animal mem- 

 brane itself. 



7. Permanence of Emulsions. Froth. 



J. Plateau {Mem. de Brux. xxxvii. p. 3, 1868) first showed 

 that many liquids appear to possess greater viscidity on their 

 outer than on their inner surface. More recently I myself 

 (Poggendorff 's Annalen, cxxxix. p. 71, 1870), as well as Maran- 

 goni {Cimento [2], v.-vi. p. 239, 1872), found that a free liquid 

 surface in contact with the air becomes less mobile as soon as 

 a thin film of some extraneous fluid is diffused over it. The 

 same thing occurs when an extraneous liquid (solution of soap) 

 is diffused over the common surface of the other liquids (oil 

 and water). 



