244 PHYSIOLOGICAL CHEMISTRY. 



easily. Fat produces spots on paper which do not disappear ; it 

 is not volatile ; it boils at about 300 C. with partial decomposi- 

 tion, and burns with a luminous and smoky flame. The fatty acids 

 have most of the above-mentioned properties in common with the 

 neutral fats, but differ from them in being soluble in alcohol-ether,, 

 in having an acid reaction, and by not giving the acrolein test. 

 The neutral fats generate a strong irritating vapor of acrolein, due 

 to the decomposition of glycerine, C 3 H 5 (OH) 3 2H 2 = C 3 H 4 0, 

 when heated alone, or more easily when heated with potassium 

 bisulphate or with other substances removing water. 



The neutral fats may be split by the addition of the constitu- 

 ents of water according to the following equation : C 3 H 5 (OR) 3 -}- 

 3H 2 = C 3 H 5 (OH) 3 + 3HOR. This splitting may be produced by 

 the pancreatic enzyme or by superheated steam. We most fre- 

 quently decompose the neutral fats by boiling them with caustic 

 alkali not too concentrated, or, still better (in zoochemical re- 

 searches), with an alcoholic potash solution. By this procedure, 

 which is called saponification, the alkali salts of the fatty acids 

 (soaps) are formed. If the saponification is made with lead oxide, 

 then lead-plaster, lead-salts of the fatty acids, is produced. 



Stearin, or TEISTEARIK, C 3 H 5 (C 18 H 3ft 2 ) 3 , occurs especially in the 

 solid varieties of tallow, but also in the vegetable fats. 



Stearic acid, C^H^Ca, is found in the free state in decomposed 

 pus, in the expectorations in gangrene of the lungs, and in cheesy 

 tuberculous masses. It occurs as lime-soap in excrements and adi- 

 pocere, and in this last product also as an ammonia soap. It per- 

 haps exists as sodium soap in the blood, transudations, and pus. 



Stearin is the hardest and most insoluble of the three ordinary 

 neutral fats. It is nearly insoluble in cold alcohol and soluble with 

 great difficulty in cold ether (225 parts). It separates from warm 

 alcohol on cooling as rectangular, less frequently as rhombical 

 plates. In regard to the melting-point, which may be changed by 

 alternately warming and cooling, opinions are somewhat divergent; 

 for stearin from the fatty tissues it is often stated as 63 C. 



Stearic acid crystallizes (on cooling from boiling alcohol) in 

 large, shining, long- rhombical scales or plates. It is less soluble 

 than the other fatty acids and melts at 69.2 C. Its barium salt 

 contains 19.49$ barium. 



