and the Action of the Bile in Digestion. 305 



In a similar way a fluid like soap-solution will spread itself 

 over the surface of a flat oil-bubble in water. At the common 

 surface of soap-solution and olive-oil the tension is =0*36 

 mgr. ; but at the common surface of soap-solution and water 

 the tension is = ; and as the original surface-tension of olive- 

 oil and water is 2*3 mgr., this tension has been reduced to 

 the extent of 84 per cent, by the dispersion of the solution of 

 soap. It will be found that the oil-bubble itself has been made 

 considerably flatter and broader by the dispersion. 



The alteration in form of an air- or oil-bubble increases with 

 the thickness of the film of the applied fluid, and attains its 

 maximum value when that thickness is more than '0001 millim., 

 or one-fifth of the length of a mean light-wave in air. The 

 thickness of such a film therefore cannot be recognized by even 

 our best microscopes; for they only detect a distance equal to 

 half a wave-length*. As exceedingly dilute solutions of soap 

 (1 per cent, and less) are able to produce this effect, we ob- 

 serve that an excessively small quantity of solid soap, which 

 in any other way we could scarcely detect, is sufficient to 

 cause this phenomenon of dispersion. And this dispersion 

 takes place with great rapidity ; a drop of olive-oil will spread 

 itself over the surface of still water in a single second, and 

 cover a space several metres in diameter. 



Similar to the action of soap-solution is that of diluted 

 ox-gall, or of a fluid obtained from the action of a dilute solu- 

 tion of soda on the free fatty acid contained in oil. In a rectan- 

 gular glass trough a horizontal plane glass plate was suspended, 

 the latter being pierced in the centre with a hole of 2 millims. 

 diameter. The trough was then filled with water up to the 

 lower surface of the glass plate; and through the hole, by means 

 of a pipette, a globule of oil was introduced underneath the glass 

 plate. With a second pipette a few drops of a *25-per-cent. 

 solution of soda were introduced; and this immediately sank 

 in the oil and retreated to the lowest side of the globule. The 

 soda formed with the free fatty acid a soap, which then en- 

 veloped the drops of soda solution lying in the oil with a more 

 or less thick whitish membrane. If this membrane be then 

 broken up by agitation, or by its coming into contact with and 

 dissolving in the adjacent water, the solution of soap or a mixed 

 solution of soap and soda spreads itself over the surface of the 

 globule of oil, and the latter becomes flatter and broader. 



* The proof of these propositions will be found in my treatise " On the 

 Distance at which the Molecular Force of Capillarity can act," Poggen- 

 dorft's Annalen, cxxxvii. p. 402, 1869 ; also " On the Edge-angle and 

 Dispersion of Fluids on Solid Bodies," Wiedemann's Annalen, ii. p. 177, 

 1877 (Phil. Mag. [5] vol. v. pp. 321, 415). 



