314 PRACTICAL PHYSIOLOGY 



7i=15. Thus the formula for stearic acid is CH 3 . (CH 2 ) 16 . COOH. 

 The first member of this series is acetic acid, CH 3 . COOH. 



The unsaturated acids contain relatively less hydrogen in the hydro- 

 carbon chain attached to the carboxylic group. This is due to the fact 

 that there are one or more double bonds (unsaturated) between the carbon 

 atoms of the chain. Thus oleic acid, the commonly occurring unsatur- 

 ated acid of fats, has the formula : CH 3 . (CH 2 ) 7 . CH = CH(CH 2 ) 7 . COOH, 

 and belongs to the series C n H 2n _!. COOH. Other unsaturated acids, 

 containing two, or even more, double bonds occur in the fat of the 

 liver, heart and kidney, and in drying oils, such as linseed oil. The 

 unsaturated nature of these acids is shown by their combining directly 

 with chlorine or bromine, thus becoming saturated. 



EXPERIMENT I. Shake up some oleic acid or its alcoholic solution 

 with dilute bromine water. The colour of the bromine disappears. 

 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. 322). Common fats are made up almost entirely of varying pro- 

 portions of stearin, palmitin (which are solid at ordinary temperatures) 

 and oleiri 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 con- 

 stituents, the glycerine being set free and the fatty acid uniting with 

 the alkali to form a soap. This process is called saponificatim. 



EXPERIMENT II. Saponificatwn 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 frequently. 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. 



