444 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 



these subjects by different experimenters, various theories have been 

 propounded as to the form in which fats leave the intestine. These 

 theories may be divided into two classes (a) Those in which it is held 

 that the fats are absorbed in participate form, as emulsified fats or 

 fatty acids ; (b) those in which it is held that the fats are absorbed in 

 solution as fatty acids or as soaps. 



Emulsification. All fat or oil which has not been specially 

 neutralised contains a slight amount of free fatty acid. On long 

 standing in contact with air, the amount of this fatty acid is increased, 

 probably by bacterial action; when this proceeds beyond a certain 

 limit, the fat is said to become rancid. 



If such a rancid oil, or fat melted by gently warming, be briskly 

 shaken up with a solution of an alkaline carbonate (e.g. a 0'25 per cent, 

 solution of sodium carbonate), it becomes suspended permanently in the 

 alkaline solution in the form of very minute particles or globules, 

 and so forms what is known as a permanent emulsion. But if the 

 rancid oil be previously carefully neutralised (e.g. by mechanically 

 shaking for some hours with a saturated solution of barium hydrate 

 at 95 C., and then pipetting off), 1 no amount of shaking with a solution 

 of an alkaline carbonate afterwards will cause it to yield a permanent 

 emulsion; the fluid on standing will quickly settle into two distinct 

 layers. Neither can a lasting emulsion be obtained by shaking up a 

 rancid oil or fat with distilled or acid water ; some free fatty acid and 

 some alkali must be simultaneously present. In other words, the 

 necessary conditions for the formation of a soap must be satisfied. 2 



Emulsifying action of alkaline salts and bile. Attention was first 

 drawn to the action of alkaline salts in promoting emulsion by Marcet 3 

 in 1857 ; this author investigated the effect of both disodic phosphate 

 and of bile on fatty acids and on neutral fats ; his results have not 

 obtained, even in English text-books, the attention they deserve, and 

 seem in part to have become forgotten. The results with bile and 

 fatty acids have an important bearing on more recent researches, to be 

 subsequently described, and for this reason are here quoted at length. 



Disodic phosphate, " when mixed with pure stearic and margaric acids 

 prepared from sheep's fat, and heated, produced a perfect emulsion, resembling 

 milk ; on cooling, a substance solidified, consisting of fatty acids with more or 

 less soda, soap, and a small quantity of phosphate of soda; therefore the 

 formation of the emulsion had been attended with that of a small proportion 

 of soap. When neutral fats were heated, suspended in a solution of phosphate 

 of soda, no emulsion occurred ; the fats fused, and, on cooling, solidified under 

 the form of a hard cake ; the warm mixture, although strongly shaken, was 

 not converted into an emulsion, but the minutely divided globules of fat rose 

 to the surface, uniting with each other, and solidified on cooling; the fluid 

 remained perfectly clear. 



"The next subject for inquiry was to determine whether bile exerts 

 a similar action on fatty acids and neutral fats. On heating and agitating 

 gently a mixture of fresh sheep's bile and fatty acid (margaric, stearic, and 



1 Rachford, Journ. PhysioL, Cambridge and London, 1891, vol. xii. p. 73. 



2 Only formation of "artificial emulsions," if the expression may be used, from rancid 

 oils is referred to here ; it will be seen later that a pancreatic emulsion can be formed and 

 persist in presence of an acid reaction due to fatty acids. 



3 Compt. rend. Soc. de liol., Paris, 1857, p. 191 : Proc. Roy. Soc. London, 1858, vol. ix. 

 p. 306 ; Med. Times and Gaz., London, 1858, N. S., vol. xvii. p. 209. The extracts are 

 taken from the last quoted Journal. 



