DIGESTION AND ABSORPTION OF FA TS. 447 



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 which 

 keep them apart. The prevention of coalescence is the result of the action of 

 several factors, of which 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. 2 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 slowly 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. 

 Versnchsslat., 1876, Bd. xix. 



2 See v. Frey, loc. cit. 3 "Physiologic d. Verdauung," Wiirzburg, 1834. 



4 Compt. rend. Acad. d. sc., Paris, 1849, tome xxviii. p. 249 ; Arch. gtn. de med., Paris, 

 1849, Se"r. 4, tome xix. p. 60 ; "Me"moire sur le pancreas," Paris, 1856. 



