142 PHYSIOLOGICAL AND PATHOLOGICAL CHEMISTRY. 



On cautious evaporation on the water-bath, the ethereal- 

 alcoholic solution yields fat, which slowly solidifies. 



Reactions of Fat. 



1. Rub a small portion of the fat on paper (not filter- 

 paper). The paper becomes transparent. 



2. Add to a few cubic centimeters of alcohol one to two 

 drops of very dilute sodium hydroxide solution (about tenth- 

 normal caustic soda solution containing 0.4 per cent of NaOH), 

 then add enough rosolic acid solution or phenolphthalein 

 solution to make the fluid intensely red. At the same time 

 dissolve a little fat (one drop or a piece the size of a pea) 

 in a few cubic centimeters of ether and pour the ethereal 

 solution of the fat into the solution of the indicator. The 

 solution does not change its red color, the fat reacts neutral. 



3. Grind in a mortar a small quantity (one drop or a piece 

 the size of a pea) with some powdered potassium bisulphate 

 (monopotassium sulphate) and heat the mixture in a dry 

 test-tube: penetrating odor (caution!) of acrolein (acrylic 

 aldehyde, CH 2 = CH CHO). A strip of filter-paper moist- 

 ened with an ammoniacal silver nitrate solution (see Acetone, 

 page 121) turns black immediately (reduction to silver) when 

 placed in the upper part of the tube. The fat is here decom- 

 posed and the glycerin converted into acrolein by the elimi- 

 nation of water. 



4. Warm a small portion of the fat in a test-tube with 

 sodium carbonate solution: the fat forms a temporary emul- 

 sion, but does not dissolve; saponification does not take place. 

 Sodium hydroxide solution also does not saponify fat at room 

 temperatures. 



II. DECOMPOSITION OF FAT, SAPONIFICATION. 



When heated with caustic potash or caustic soda (very 

 readily in alcoholic solution) the fats are decomposed with 



