74 PLANT COMPOUNDS 



66. Soap. — The saponifying property of fixed oils is 

 made use of in the manufacture of soap. If potassium 

 hydroxide is used a soft soap results, if sodium hydroxide, 

 a hard soap. Both kinds are soluble in water, but the potash 

 salt of the fatty acids is a soft substance, whereas the soda 

 salt is a hard substance. The oil or fat is boiled with the 

 alkali until saponification is complete. If a hard product 

 is being made, the soap is separated from the resulting 

 glycerine and the excess of alkali by the addition of common 

 salt. The soap is insoluble in this solution, and can be 

 separated, further treated, and made into cakes. 



67. Glycerine. — From the residue after the removal of 

 soap, glycerine may be obtained by special processes in- 

 cluding purification by distillation under reduced pressure. 

 Glycerine is a thick, oily liquid, hygroscopic, miscible with 

 water in all proportions, and has a very sweet taste. Its 

 formula is C3H 5 (OH) 3 , graphically: 



H 



I 



H— C— O— H 

 H— C— O— H 



H— C— O— H 



I 

 H 



It is used in the manufacture of nitroglycerine and dyna- 

 mite; as a solvent in confectionery on account of its hygro- 

 scopic qualities which keep candy soft; in printing inks, etc. 



68. Classification of the Fatty Acids. — The drying property 

 of oils depends on the existence of double bonds in the fatty 

 acid radicle ; or to express it in another way, on the existence 

 of unsaturated carbon atoms which readily take up oxygen. 

 On this basis the fatty acids of the fixed oils may be classified 

 as saturated or unsaturated compounds as follows: 



(a) Saturated Fatty Acids. — CnH 2 n+iCOOH. Stearic 

 acid, C17H35COOH, melting point 69° C. 



CH 3 .(CH 2 )i6.COOH 

 Palmitic acid, C15H31COOH, melting point 62° C. 



CH 3 .(CH 2 )i4.COOH 



