446 CHEMISTR V OF THE DIGESTIVE PROCESSES. 
different fluids depends (a) on the degree of alkalinity of the fluid, (b) on 
their chemical composition, in so far as this influences the solubility of 
the soaps formed. 
3. The maximum of quantity and quality of emulsion formed coin- 
cides with those conditions under which no formation of a membrane 
can be demonstrated. 1 
There has been much discussion as to the factors at work in the formation 
and conservation of emulsions. Briicke Avas of the opinion that it was the 
dissolved soap which conferred on the solution the power of holding the 
globules apart, after they had been mechanically formed in it. Gad supposed 
that the breaking up of the globules into smaller ones was due to a want of 
correspondence of the rate of solution and diffusion of the soap formed into 
the outer solution with the rate of diffusion of fatty acids towards the sur- 
face of the globule. In case fatty acid diffuses from the inner part of the fat 
globule towards the common surface of oil globule and solution more quickly 
than it can be dissolved by the solution, a film or membrane of soap will form 
around the globule. This film will not form at all parts of the globule equally, 
and this will give rise to amcebadike movements (due to differences in surface 
tension).- Gad also supposes that the ultimate microscopic globules are sur- 
rounded by soap films which keep them from coalescing. 
Quincke attributes the formation of the emulsion to the differences in sur- 
face tension produced by the formation of a soap solution round the globule ; 
and he also assumes the existence of films of soap (solid or in solution) around 
the ultimate oil globules in the emulsion, which have the property of keeping 
the globules from coalescing. 
There is no doubt that soap formation is an accompaniment to the forma- 
tion of an emulsion of rancid oil in an alkaline solution, and it is easy to see 
how the formation of such a soap film, at accidentally varying rapidity, at 
different points on the surface of a globule of oil, will cause variations in 
surface tension at these points, and so cause the oil globule following the soap 
film which covers it, to be drawn out into various shapes and split up. Such 
surface tension phenomena may be observed when two liquids which mix, 
such as alcohol and water, are brought together. More mixing takes place at 
one point than another ; as a consequence, the mutual surface tension is less at 
one point than another, and those rapid, streaming movements are produced 
which may always be observed when alcohol and water are mixed. .Similarly, 
when a small piece of a substance like camphor is placed on water, rapid 
shooting movements take place, due to an accidentally unequal solution of the 
substance at different points in the circumference, and consequently varying 
surface tension, as a result of which the piece of camphor is rapidly moved 
about from place to place. In an exactly similar manner a globule of rancid 
oil will be pulled about, altered in shape, and broken up in an alkaline solu- 
tion, from accidental variations in the strength of soap solution at different 
points on its surface, causing variations in surface tension and corresponding 
movements. 
It is much more difficult to see how any permanent film of insoluble soap 
can be formed round the ultimate globules, or even a film of soap solution of 
different concentration from the rest of the menstruum in which the globules 
float. From Gad's conclusions, it should be observed that in the cases where 
emulsion takes place best and most quickly, no such soap film can be observed, 
so that this soap film cannot be experimentally demonstrated ; it is merely a 
theoretical thing, devised from the supposed necessity of having something to 
keep the globules from coalescing. A proteid membrane surrounding the fat 
globules in milk was supposed to have a similar office, but microscopically or 
1 Vide infra. - The words in parenthesis are added. 
