ADVANCED PHYSIOLOGICAL CHEMISTRY 435 



To Separate the Solid (i.e. palmitic and stearic) from the Fluid 

 Fatty Acids (i.e. oleic). Melt the fatty acids in a beaker, and add to 

 the resulting fluid about four times its bulk of 70 per cent, alcohol. 

 Place the beaker on the boiling water bath for a few minutes, and then 

 filter quickly through a folded filter. Allow the filtrate to cool, when 

 the solid acids will separate out as a crystalline mass, whereas the oleic 

 acid will remain in solution. The two can then be separated by filtra- 

 tion. The further separation of stearic from palmitic acid is a laborious 

 process, and consists of the addition of an alcoholic solution of lead 

 acetate in small quantities at a time to a solution of the acids in alcohol. 

 Each addition produces a precipitate which is filtered off and treated 

 with dilute hydrochloric acid and ether. The acid decomposes the lead 

 salt and the liberated fatty acid goes into solution in the ether. This 

 process is called fractional precipitation, and the higher the melting 

 point of the acid the more easily is it precipitated by the lead acetate. 



To Estimate the Melting Point of a Fatty Acid or other Body. 

 The substance is placed in a capillary tube of such a width that a pin 

 can easily be pushed into it. This tube is tied on to the bulb of a 

 thermometer so that the substance is opposite the centre of the bulb. 

 The thermometer is then placed in a test-tube, which is suspended in a 

 combustion flask containing concentrated sulphuric acid, which is very 

 gradually heated over a thick asbestos plate. The substance is very 

 carefully observed as the temperature rises, and the exact temperature 

 at which it begins to melt is noted. 



Lecithin. Preparation. The principle of the method depends on the 

 fact that lecithin is not so soluble in cold ether as the other fatty bodies 

 are, but it is easily soluble in alcohol at 60 C. The yolks of two eggs 

 are shaken with cold ether until the ether is no longer pigmented. The 

 residue is then extracted with methylated spirit at a temperature of 

 about 60 C. The alcoholic extract is evaporated to dryness, and 

 the residue frequently washed with cold ether. The purified residue 

 is dissolved in as little absolute alcohol as possible, filtered, and 

 allowed to evaporate slowly, when the lecithin remains as a gummy 

 mass. (The lecithin can be obtained as small round crystals by cooling 

 the alcoholic solution to 5 C.) 



Reactions and Tests for Lecithin. These depend entirely on its 

 chemical constitution, and consist in the recognition of the various 

 decomposition products. It will be remembered that lecithin consists 

 of a central molecule of glycerin, two of the hydroxyl radicles of which 

 are combined with a fatty acid, and the third one with phosphoric acid, 

 and that this latter is combined with a body called cholin. By 

 boiling with baryta water the fatty acid separates as an insoluble 



