182 PRACTICAL PHYSIOLOGY 



if saponification be complete, a clear solution of soap will be obtained 

 and no globules of oil will separate out on the surface of the liquid. 



EXPERIMENT II. Separation of fatty acid from soap. Place about 

 40 c.c. of 20% sulphuric acid in a small beaker, and heat it to near 

 boiling point ; drop into this the contents of the flask, (which should 

 have been previously allowed to cool) stirring with a glass rod between 

 each addition. The acid displaces the alkali from its combination 

 with the fatty acid, and the latter separates out as an oily layer on the 

 surface of the water. 



EXPERIMENT III. Reactions of fatty acids. (1) Eemove some of 

 the fatty acid with a clean glass rod, and place it on a piece of glazed 

 (ordinary) paper, and a greasy stain will result. 



(2) Allow the contents of the beaker to cool, when the fatty acid 

 will solidify into a skin, which can be easily removed with a pen-knife 

 and transferred to distilled water in a test tube. Shake up the fatty 

 acid in the water so as to wash it free of sulphuric acid, pour the con- 

 tents into a flat dish, again remove the fatty acid to more distilled 

 water, and repeat until the wash-water no longer reacts acid. Now 

 divide the fatty acid into two portions, a and b. Place a in a dry test 

 tube, and dissolve in ether. The resulting solution reacts acid to 

 phenolphthaleine, an indicator which reacts red with alkali, and is 

 bleached by acid, being especially sensitive to fatty acids. To apply 

 this test, two or three drops of the phenolphthaleine are dropped into a 

 test tube containing a very dilute solution of alkali (one drop of 20% 

 KHO in 5 c.c. water), the resulting red solution being then dropped 

 into the solution of fatty acid, when the red colour at once disappears. 



Add to b some half saturated solution of sodium carbonate, and 

 warm; the fatty acid dissolves, carbonic acid gas is liberated arid a 

 solution of soap obtained. 



Besides these reactions of the fatty acid produced from it, neutral 

 fat gives an important reaction, depending on the glycerine which it 

 contains. 



EXPERIMENT IV. Place a small piece of fat in a thoroughly dried 

 test-tube, add to it a crystal of acid potassium sulphate, and heat. A 

 pungent vapour of acrolein 1 is given off, which blackens a piece of 

 filter-paper which has been dipped in ammoniacal silver nitrate solu- 

 tion, thus demonstrating that the vapour acts as a powerful reducing 

 agent. 



Emulsification. When oil is mixed with water it floats to the 

 surface, but when a soap is present in solution in the water the oil 



1 Acrolein is the aldehyde of allyl alcohol and has the formula CH 2 = CH - CHO. 



