and the Action of the Bile in Digestion. 307 



surface of still water were observed and described by me nine 

 years ago (Poggendorff's Annalen, cxxxix. p. 76, 1870). 



The periodic dispersions of soap solution over the common 

 surface of the oil and an aqueous fluid, which are not simulta- 

 neous at all points of the oil-surface, will be found to be con- 

 nected with vortex-movements in the interior of both fluids, and 

 to draw the oil more especially towards the dispersion-centre. 

 This is the cause of the Amoeba-like movements at the edge of 

 the main mass, while the detached particles of oil form the emul- 

 sion. In fact, under the microscope, not only the aqueous 

 fluid can be seen, but also, and more conspicuously, the oil in 

 the vicinity of the common surface, in a state of gyration. 



When the oil possesses a tolerable degree of toughness, and 

 the dispersive force at the common surface of the two fluids is 

 moderately great, the vortex-movements and the detached 

 oil particles will be very numerous, and may be produced, as 

 indeed follows from the dispersion theory (see above, 3, p. 305), 

 by means of films of diluted soap solution not exceeding in 

 thickness a few millionths of a millimetre. A very minute 

 quantity of soapy matter is therefore sufficient to produce the 

 appearances in question. The free fatty acid necessary for 

 the formation of this soap is nearly always present in oil, and 

 reaches the surface of the latter by means of diffusion. It can 

 be produced in the fluid contents of the intestine by means of 

 the pancreatic juice, and in the open air by the action of car- 

 bonic acid on the neutral alkaline oleates*. If the soap be 

 formed too rapidly, the common surface of the oil and the 

 aqueous fluid is coated with a film of solid soap. Hence the 

 oil-surface becomes immobile, and the dispersion and its con- 

 sequences are impeded or are altogether wanting — -just as, in 

 Leidenfrost's experiment, when water is brought into contact 

 with red-hot metal the formation of steam is impeded or does 

 not occur at all. 



In the same way the consequences of the dispersion, the 

 formation of emulsion and the movements of the particles of 

 oil, will be prevented, if only a very small quantity of soap be 

 formed, or if the soap be dissolved too rapidly by the surround- 

 ing fluid. Every, even the minutest, particle of soap will 

 then immediately be diffused in a solution, and the dispersive 

 energy will not be sufficient to produce the vortex-movements 

 in the oil, and to detach the particles of that substance. If a 

 mill-brook be allowed to trickle in a small stream over the 

 water-wheel, the latter will not be set in motion; but by perio- 

 dically damming it back a small body of water can be made 

 with advantage to produce mechanical work. 



* Compare Heintz ; Zoochemie, p. 439. 



