FATS AND RELATED SUBSTANCES. 53 



plaster. Here lead oxide and water are equivalent to lead 

 hydroxide. Much of the glycerol of commerce is produced 

 by such decompositions. When sodium hydroxide is used 

 with the common fats ordinary hard soap results. 



An analogous change occurs when fats are subjected to the 

 action of water at a high temperature or superheated steam. 

 We have here hydrolysis purely, although the term saponifica- 

 tion by steam is sometimes applied. The same reaction is 

 brought about by certain enzymes at the ordinary tempera- 

 ture, for example by the enzyme known as lipase in the pan- 

 creatic secretion. This will be discussed fully later as it is 

 important in the digestion of fats. Sometimes the reaction 

 is complete as shown by this equation : 



CsHsCdsHssOOs + 3H 2 O = C 8 H B (OH), + adsHaeOa 



But products of partial hydrolysis, monostearin and distearin, 

 for example, may be left, as : 



f CH0 3 f OH 



C 3 H 5 \ G.HO a + H 2 = C 3 H 5 \ CHO,-f HCuHO, 

 ( GsHssO, I CHO a 



ILLUSTRATIVE TESTS. 



Some of the saponifications are illustrated by these experi- 

 ments : 



Ex. Boil about 25 gm. of tallow with a solution of 10 gm. of potas- 

 sium hydroxide in 100 cc. of water. Stir the mixture thoroughly until 

 it becomes homogeneous; that is, until no oil globules are seen floating 

 on top of the aqueous liquid, which may require half an hour. Add water 

 from time to time, to make up for that lost by evaporation. The result- 

 ing mass is a mixture of glycerol, excess of alkali and soft soap. To this 

 now add a solution of 15 gm. of common salt in 75 cc. of water and heat 

 again, which brings about a conversion of the soft soap into the hard or 

 sodium soap. On cooling this separates and floats on the excess of spent 

 lye and salt solution. 



Ex. The presence of fatty acids in the above soap can be shown by 

 adding to a portion of it enough hydrochloric acid to decompose the soap. 

 Use about half the product of the experiment, dilute with water, and add 

 the acid in slight excess, about 10 cc. of the strong commercial acid. 

 Warm on the water-bath, which will cause the liberated fatty acids to col- 

 lect on the surface as a liquid layer as soon as the temperature becomes 



