49 



170. Add a few drops of oleic acid to 5 cc. of bile in a test 

 tube, shake well and at once place a drop of the mixture on a 

 slide and examine, under the microscope, the numerous fatty 

 g-lobules. Place the test tube with the bile in a warm bath for 

 an hour or so, shakingf occasionally and then examine a drop 

 with the microscope ; comparatively few fatty globules will 

 be seen. The oleic acid has combined with the base of the 

 bile-salts to form a soap. 



171. Prepare three test tubes as follows : (1) In one test 

 tube put 5 cc. of bile and a drop of oleic acid. (2) In another 

 S cc. of water. (3) In another S cc. of bile. To each of the 

 three add about 1 cc, of fresh melted butter or lard. Shake 

 well and place all three in a warm bath. Note in which tube 

 the emulsion continues longest. 



172. 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 connection with 

 certain digestive phenomena. 



XIII 

 MILK 



173. Newly drawn milk is an opaque fluid of a white 

 color. Its color and opacity are due to its being an emulsion, 

 /. e. , consisting of little globules of fat suspended in a solution 

 of albumin, sugar and salts. 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 richest in cream. The globules 

 of fat are prevented from uniting by the thin albuminous coat- 

 ing (the presence of this coating is denied by some) which 

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



