DIGESTIO.\ A. XI) ABSORPTION OF FA TS. 4 4 7 
otherwise no such membrane can be demonstrated, and its existence is very 
doubtful. 1 
A cloud is an emulsion, an emulsion of water particles in air, and no one 
has ever supposed that the water particles are surrounded by membranes -winch 
keep them apart. The prevention of coalescence is the result of the action of 
several factors, of winch our knowledge is not yet perfect. 1. One such 
factor is the magnitude of the suspended drops ; the bigger the drops the more 
rapidly they will come together, and fall (or rise) out of solution.- The more 
mechanical agitation an emulsion is given, the longer it will persist under 
otherwise unfavourable circumstances. 2. Another factor is the viscosity of 
the menstruum : the greater this is the more slowdy will the finely-divided 
globules be able to move through the fluid, under the influence of differences 
in specific gravity or mutual attraction, so as to pass out of solution or 
coalesce. 3. Another factor is the comparative specific gravities of the fat 
and menstruum. 4. Still another is the mutual surface tension between 
globule and menstruum ; the greater this is, the greater will be the tend- 
ency to diminution of surface, and hence to coalescence. On the other 
hand, if the mutual surface tension were zero, the two fluids would mix in all 
proportions. 
It has been objected, by those who believe in the existence of a film around 
the fat globules, to the contention that the altered nature of the menstruum is 
sufficient to account for the permanency of the emulsions obtained with fats 
and alkaline solutions, that a permanent emulsion cannot be obtained by 
shaking up neutral fat with a soap solution. But the conditions in the two 
cases are essentially different. Neutral fats and fatty acids mix together in a 
rancid fat or oil in all proportions. When such a mixture is submitted to the 
action of alkali, the soap formation takes place where the fatty acids are, that 
is, intimately mixed with the neutral fat. So that soap is formed everywhere 
at the surface of the mass, and, dissolving, carries away (in the surface tension 
diffusion streams above described) the intimately admixed fat from the main 
mass in a very finely subdivided condition. If the proper conditions exist in 
the solution, these minute fat particles will not coalesce again. Such a result 
is brought about by the viscosity and reduction in surface tension which the 
solution acquires by means of the dissolved soap. On the other hand, when 
neutral fat is shaken up with soap solution, no such disintegrating agency 
comes into action, and the only thing to replace it is the mechanical subdivision 
due to shaking. As v. Frey points out, the smaller the diameter of the fat 
globules, the greater is the mechanical force necessary to subdivide them ; and 
it is probable that by no amount of agitation can so fine a subdivision be 
reached as is naturally attained by the formation of the soap amongst the fat. 
By very prolonged and vigorous agitation, v. Frey has obtained "mechanical 
emulsions " of very considerable stability, even with neutral fats and water. 
The very fine subdivision of the fat, and the increased viscosity of the men- 
struum occasioned by the dissolved soap, are hence quite sufficient to explain 
the permanency of emulsions of rancid oils and fats in alkaline solution. 
Formation of emulsions in the intestine. — The formation of an 
emulsion of fats in the intestine was already known to Eberle 3 in 1834-, 
but was first brought into prominence by the classic researches of 
Claude Bernard. 4 Bernard was unacquainted with our modern theories 
of the formation of emulsion, and did not associate this process with 
1 See v. Frey, Arch. f. Anat. u. Physiol., Leipzig, 1881, S. 382 ; Soxhlet. Landwirthsch. 
Versuchsstat., 1876, Bd. xix. 
- See v. Frey. loc. •■',<. 3 " Physiologie d. Yerdauung, " Wiirzburg, 1834. 
4 Gompt. read. Acad,, d. sc, Paris, 1849, tome xxviii. p. 249 ; Arch. g6n. de me'd., Paris, 
1349, Ser. 4, tome xix. p. 60 ; "Memoire sur le pancreas," Paris, 1856. 
