222 PRACTICAL PHYSIOLOGY 



EXPERIMENT I. Shake up some oleic acid or its alcoholic solu- 

 tion with dilute bromine water. The colour of the bromine dis- 

 appears. Repeat with an alcoholic solution of stearic acid, when 

 the colour of the bromine persists. 



Under suitable conditions unsaturated fatty acids and fats will also combine 

 with iodine. The proportion of iodine with which a given mixed fat will 

 combine therefore represents the amount of unsaturated acid present. This 

 is called the Iodine Number of the mixed fat. (See p. 300.) Common fats 

 are made up almost entirely of varying proportions of stearin and palmitin, 

 which are solid at ordinary temperatures, and olein, which is liquid. The 

 more olein a fat contains, therefore, the lower will be its melting point and 

 the higher its iodine number. 



All the fatty acids possess one property in common, viz. that they 

 form salts. These salts are called soaps. By boiling neutral fat 

 with caustic alkali, it is split up (by a process of hydrolysis) into 

 its constituents, the glycerol being set free and the fatty acid 

 uniting with the alkali to form a soap. This process is called 

 saponification . 



EXPERIMENT II. Saponification of Neutral Fat. Place about 

 50 c.c. of strong caustic soda in a dish, and add about 10 grammes 

 of fat. Heat to near the boiling-point and stir the mixture fre- 

 quently. When all the fat has disappeared allow the mixture to 

 cool. The soap forms a jelly or cake, and can be washed in cold 

 water to remove any excess of caustic soda. A hard soap is formed 

 if caustic soda is used ; but with caustic potash a soft soap is 

 obtained. 



EXPERIMENT III. Separation of Fatty Acid from Soap. Place 

 about 40 c.c. of 20 per cent, sulphuric acid in a small beaker, 

 and heat it nearly to boiling-point ; drop into this pieces of the 

 washed soap, 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 on the surface of the water as 

 an oily layer. 



EXPERIMENT IV. Reactions of Fatty Acids. Remove some of 

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

 ordinary paper ; a greasy stain will result. 



In order to purify the fatty acid allow the contents of the beaker to cool, 

 when the fatty acid will solidify and can be easily removed with a penknife, 

 and transferred to distilled water in a small beaker. This removes a great 

 part of the adherent sulphuric acid. But to free it completely it is necessary 

 to dissolve the fatty acid in alcohol, and pour the resulting solution into excess 

 of cold distilled water. The fatty acid which separates is filtered off and 

 washed with distilled water. Use the purified fatty acids for the following 

 reactions : 



A. Demonstrate that fatty acid is acid in reaction. For this purpose 

 place some alcohol in a test tube, add a few drops of an alcoholic solution of 

 phenolphthalein (an indicator which turns red with alkali, but is colourless 



with acids), and then a few drops of weak caustic soda. Warm the 



resulting red solution on the water-bath, and drop into it small pieces of fatty 

 acid. The red colour will disappear. Repeat the experiment with a piece 

 of neutral fat ; the result is negative. 



