444 CHEMISTRY 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 particulate 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. 
Smulsification. — 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 025 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 
lavers. Neither can a lasting emulsion be obtained by shaking up a 
ramid 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 sua]) 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 Racbford, Journ. Physio?., 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 bere ; it will be seen later that a pancreatic emulsion can be formed and 
persist in presence of an acid reaction due to fatty i 
l 'ompt. f'ii'/. Soc. de biol., Paris. 1 V .J7, p. 191 : Proc. Eoy. Soc. London, 1858, vol. ix. 
p. 306: Med. Times and Gaz., London, 1S">8, N. S., vol. xvii. p. 200. The extracts are 
taken from the last quoted Journal. 
