X. THE CHEMICAL CONSTITUENTS OF PLANTS 197 



oils. The former are used in the arts in the manufacture of oil-cloth, 

 linoleum and paints, the latter for lubrication. 



Glyceryl, C 8 H 5 '" = CH 2 .CJTCH 2 is not known in the free state, 

 but its hydroxide, C 3 H 5 (OH) 3 , is the well-known substance glycerine, or, 

 as it is systematically called, glycerol. 



Glycerol, C 3 H 5 (OH) 3 , is a colourless, viscid liquid of sweetish taste. 

 It has a specific gravity of 1-27 and mixes with water in all proportions. 

 It is hygroscopic, and articles smeared with glycerine are kept moist, 

 both because of its non-volatility and also because it absorbs water from 

 the air. 



Glycerol is made from oils or fats by a process which shows clearly 

 the constitution of these substances. 



The oil or fat is boiled with caustic soda solution, when decomposi- 

 tion occurs, the sodium of the alkali taking the place of the glyceryl 

 group, the latter uniting with the OH of the sodium hydrate. A 

 sodium salt of the fatty acid and glycerol are thus produced, and being 

 soluble, remain mixed in solution. In order to separate them the 

 sodium salts are precipitated by the addition of common salt, in a 

 solution of which they are insoluble. The sodium salts rise to the sur- 

 face and, on cooling, form a solid cake, constituting soap. 



From the solution below, the glycerine can be recovered by evapora- 

 tion and distillation in vacuo. Taking olive oil, which consists mainly 

 of glyceryl oleate, C 3 H 5 (C 18 H 33 O 2 ) 3 , as example, the reaction may be thus 

 expressed : 



C 3 H 5 (C 18 H 33 2 ) 3 + 3NaOH ., 3NaC 18 H 33 2 + C 3 H,(OH) 3 



Glyceryi oleate. Sodium hydrate. Sodium oleate. Glycerol. 



Glycerine is also prepared by the hydrolysis of fats by means of 

 superheated steam. Again taking olive oil as example, the reaction 

 would be 



CflA.ftA), + 3H 2 = 3HC 18 H 33 2 + C 3 H-(OH) 3 



Glyceryl oleate. Water. Oleic acid. Glycerol. 



Although in the above example olive oil has been assumed to be 

 glyceryl oleate, it must be clearly understood that this is not true and 

 that all the oils are mixtures of the glyceryl salts of several fatty acids. 

 Thus in linseed oil, glyceryl compounds of linolenic and isolinolenic, 

 linoleic, oleic, stearic and palmitic acids have been found. It is not 

 definitely known whether, in the oils, these substances are mixed or 

 whether they are chemically combined, e.g., whether, say, the first three 

 compounds are present as such, i.e., as 



C 3 H 5 (C 18 H 29 2 ) 3 , C 3 H 5 (C 18 H 31 ,0 2 ) 3 , and C 3 H 5 (C 18 H 33 2 ) 3 , 



In plants, the oils are chiefly found in the seed or fruit, though 

 small quantities are contained in the straw and roots. Some seeds 

 are valued chiefly for the oil which they yield, e.g., rape, cotton-seed 



