30 TEXT-BOOK OF PHYSIOLOGY. 



The fats are compounds consisting of carbon, hydrogen, and 

 oxygen, of which the first is the chief ingredient, forming by weight 

 about 75 per cent., while the last is present only in small quantity. 

 The fat found in animals is a mixture, in varying proportions 

 in different animals, of three neutral fats stearin, palmitin, and 

 olein. Each fat is a derivative of glycerin and the particular acid 

 indicated by its name e. g., stearic acid, in the case of stearin, etc. 

 The reaction which/ takes place in the combination of glycerin and 

 the acid is expressed in the following equation: 



G 3 H 5 (HO) 3 + (HC 18 H 35 2 ) 3 - C 3 H 5 (C 18 H 35 2 ) 3 + 3 H 2 O. 



Glycerin. Stearic Acid. Stearin. Water. 



Hence, strictly speaking, the fats are compound ethers, in which 

 the hydrogen of the organic acid is replaced by the trivalent radicle, 

 tritenyl, C 3 H 5 . 



Stearin, C 3 H 5 (C 18 H 35 O 2 ) 3 , is the chief constituent of the more 

 solid fats. It is solid at ordinary temperatures, melting at 55 C., 

 then solidifying again as the temperature rises, until at 71 C. it melts 

 permanently. It crystallizes in square tables. 



Palmitin, C 3 H 5 (C 16 H 31 O 2 ) 3 , is a semifluid fat, solid at 45 C. 

 and melting at 62 C. It crystallizes in fine needles, and is soluble 

 in ether. 



Olein, C 3 H 5 (C 18 H 33 O 2 ) 3 , is a colorless, transparent fluid, liquid at 

 ordinary temperatures, only solidifying at o C. It possesses marked 

 solvent powers, and holds stearin and palmitin in solution at the 

 temperature of the body. 



Saponification. When subjected to the action of superheated 

 steam, a neutral fat is saponified i. e., decomposed into glycerin and 

 the particular acid indicated by the name of the fat used: e. g., stearic, 

 palmitic, or oleic. The reaction is expressed as follows: 



C 3 H 5 (C 18 H 33 2 ) 3 + 3 H 2 = C,H 8 (HO), + 3(C 1? H 34 O 2 ) 



Olein. Water. Glycerin. Oleic Acid. 



The fatty acids thus obtained are characterized by certain chemic 

 features, as follows: 



Stearic acid is a firm, white solid, fusible at 69 C. It is soluble 

 in ether and alcohol, but not in water. 



Palmitic acid occurs in the form of white, glistening scales or 

 needles, melting at 62 C. 



Oleic acid is a clear, colorless liquid, tasteless and odorless when 

 pure. It crystallizes in white needles at o C. 



If this saponification takes place in the presence of an alkali, 

 e. g., potassium hydroxid or sodium hydroxid, the acid produced 

 combines at once with the alkali to form a salt known as a soap, while 

 the glycerin remains in solution. The reaction is as follows: 

 3 KHO + (C 18 H 34 2 ) 3 = 3 (KC 18 H 33 2 ) + C 3 H 5 (HO) 3 



Potassium. Oleic Acid Potassium Oleate. Glycerin. 



