68 PHYSIOLOGICAL CHEMISTRY 



oxygen are taken up in an analogous manner, and this fact is 

 also used to identify fats. 



Acetyl Eqivalent. Some fats contain oxyacids, that is 

 acids containing hydroxyl groups. These groups may be re- 

 placed by acetyl groups, which in turn may be split off and 

 the resulting acetic acid titrated. The number of milligrams 

 of potassium hydrate required to neutralize the acetic acid ob- 

 tained from 1 gram of fat in which the hydroxyl groups have 

 been replaced by acetyl groups is known as the acetyl equiva- 

 lent. 



The various factors described above have been determined 

 for the important natural fats, and variations in one or more 

 of these characteristic constants are of great service in deter- 

 mining whether a given fat or oil is pure, or has been adul- 

 terated. 



Important Fats. Tristearin, tripalmitin and triolein are the 

 three fats occurring most frequently in natural fats. 



Tristearin, or stearin, as it is often called, melts first at 

 about 55, resolidifies and melts again at about 71. It is a 

 hard, flaky material, and the least soluble of the three. It is 

 obtained from tallow. Mixed with a little paraffin to make it 

 less brittle it is moulded into candles. Free stearic acid is found 

 in old pus, in gangrenous or tuberculous masses, etc., where 

 'decomposition of fat has taken place. It is found as its alkali 

 soap in blood, bile, etc., and as its calcium soap in the feces. 



Tripalmitin, or palmitin, is found in all animal and most 

 vegetable fats, notably in palm oil, whence it derives its name. 

 It predominates in human fat. It melts first at about 50, re- 

 solidifies, and melts again at about 66. 



Triolein, or olein, is found in animal, and to a greater extent 

 in vegetable fats and oils. It melts at 6 C, and is thus a 

 liquid (an oil) at room temperature. The presence of olein 

 lowers the melting point of natural fats. If exposed to the air 

 olein quickly becomes rancid. Oleic acid, by reason of its 

 double bond will take up iodine as described above. 



Butter. Butter, the fat obtained from cream, contains about 



