24 THE PATS. 



effected by boiling the fat with a caustic alkali. Here, in- 

 stead of the free fatty acid being left, it unites with the 

 alkali to form a salt. These metallic salts of a fatty acid 

 are the soaps: 



C.H B (C 18 H M O a ) + 3KOH = C 3 H 6 (OH) 3 + 3C 17 H 85 C0 2 K. 



stearin + alkali = glycerin + soap 



The soaps of the alkalies are soluble in water, the 

 potassium compound being hygroscopic and forming soft 

 soap. The sodium compound forms a hard soap. The 

 compounds of the heavy metals with the fatty acids are in- 

 soluble, and can be formed by adding a solution of their 

 b-alts to a soap solution. The lead soap or lead plaster used 

 in medicine is made by heating lead oxid with one of the 

 fats. The soluble soaps can be thrown down from their 

 solutions by saturation with a neutral salt. If a strong 

 acid is added to a soap solution the soap is decomposed, 

 the metal uniting with the strong acid and the fatty acid 

 being set free as an insoluble substance: 



C 17 H 3B C0 2 K + HC1 = KC1 + C 17 H 35 C0 2 H. 



goap acid fatty acid (stearic) 



These fatty acids which enter into the most of the 

 animal and vegetable fats are one of the unsaturated series: 

 oleic acid (C 17 H 33 C0 2 H); and two of the saturated series: 

 stearic acid (C 17 H 35 C0 2 H) and palmitic acid (C 15 H 31 C0 2 H). 

 Besides these acids which constitute, by far, the larger 

 part of those present in fats there are found in some cases 

 certain of the lower members of the saturated series, such 

 as butyric (C 3 H 7 C0 2 H), capronic (C^^GO^), caprylic 

 (C 7 H 15 C0 2 H), and capric (C 9 H 19 C0 2 H), which occur in 

 butter. 



