86 PHYSIOLOGICAL CHEMISTRY 



ular weight are not volatile and remain behind. The number 

 of cubic centimeters of N/10 alkali required to neutralize the 

 volatile fatty acids from 5 grams of fat is called the Reichert- 

 Meissl number. For butter fat the number is 25-30. 



Iodine Number. The acids which contain double bonds, such 

 as oleic acid will take up iodine or bromine, adding on two 

 atoms of the halogen for each double bond. By this process it 

 is possible to determine how much unsaturated acid is present 

 in a fat. The weight of iodine in centigrams taken up by a 

 gram of oil is known as the iodine number. Hydrogen and 

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 been replaced by acetyl groups is known as the acetyl equiva- 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



