230 PHYSIOLOGICAL CHEMISTRY 



its sharp, disagreeable odor. Heat a small amount of lard or 

 olive oil with boric acid or potassium bisulphate in a dry test 

 tube. Continue the heating until the material has become prac- 

 tically dry and note the penetrating odor of acrolein. Kepeat 

 the test using a few drops of glycerine in place of the lard. The 

 characteristic acrolein odor again will be observed. Fats give the 

 test because of the glycerine which they contain. Fatty acids 

 and pure soaps do not give the test, a result which might be 

 expected from the fact that they contain no glycerine. 



II. Phosphatides 



The phosphatides contain glycerine, phosphoric acid, choline 

 and fatty acid radicles. Chemically they are closely allied to the 

 fats (see lecture notes) . A lecithin will be studied as an example 

 of this group. 



Lecithins occur probably in all animal cells and are especially 

 abundant in the brain, which contains also other members of the 

 group of phosphatids and from which these substances may be 

 prepared. For laboratory purposes, lecithin may be obtained 

 from egg yolk. 



a. Lecithin. 



i. Preparation. The yolk of one egg is allowed to stand with 

 30 c.c. ether over night. To 30 c.c. of ether extract of egg yolk add 

 50 c.c. of alcohol. If a precipitate forms (another extractive sub- 

 stance if present in large amounts may be thrown down by alco- 

 hol) filter. Evaporate on the water bath, dissolve the residue in 

 15 c.c. ether and filter. Add 40 c.c. acetone to the filtrate. The 

 lecithin is precipitated. Filter it off. The filtrate contains chol- 

 esterol. Use the lecithin for the following tests : 



ii. Add a small amount to 1 c.c. of water. Note opalescence. 

 The mixture may be filtered unchanged, although the lecithin 

 does not form a true solution. 



iii. On a small portion make the acrolein test. Since lecithin 

 contains glycerol, a positive test should result. 



iv. Fuse about % of the lecithin with fusion mixture, dissolve 



