33 



each of the three add about i cc. of fresh melted butter. 

 Shake well and place all three in a warm bath. Note in 

 which tube the emulsion continues longer. 



144. Free fatty acids have the power of decomposing the 

 bile salts with liberation of their acids. The emulsifying 

 power of bile is slight ; but in the presence of fatty acids it 

 forms soaps, which have a much greater emulsifying power. 

 Animal membranes moistened with bile permit the passage 

 of fatty oils, while if they are moistened with water only 

 the oil cannot pass through. This is important in connec- 

 tion with certain digestive phenomena. 



XIII. 



MILK. 



145 Newly drawn milk is an opaque fluid of a white 

 color. Its color and opacity are due to its being an emul- 

 sion, i. e., consisting of little globules of fat suspended in a 

 solution of albumin, sugar and salts. Each globule of fat 

 is covered by a thin coating of casein. When the milk is 

 allowed to stand, the fat globules, being lighter than the 

 fluid in which they swim, rise in great part to the top and 

 form cream, and part of the fluid often acquires a bluish 

 tinge. It is said that a similar separation also takes place 

 in the milk gland itself, so that the milk last drawn is rich- 

 est in cream. The globules of fat are prevented from 

 uniting by the thin albuminous coating which surrounds 

 each, but when this is broken by agitation, they coalesce, 

 forming butter. Changes also occur in the milk, sugar, 

 casein and fats, more or less quickly, according to the 

 higher or lower temperature to which the milk is exposed. 

 The milk sugar becomes converted, apparently through the 

 agency of a ferment, into lactic acid. This gives the milk 

 an acid reaction, and precipitates the casein, causing the 



