CONSTITUTION 



cules of the latter. The composition of the mannitol olein, 

 or mannitol stearin, corresponds with that of a mixture of 

 dioleates or distearates of mannitan and isomannide. It has 

 been shown by feeding experiments that mannitol olein is 

 utilized by animals to the same extent as olive oil, but there 

 is no evidence that mannitol fats occur in nature.* 



The classification and identification of fats is based upon 

 the acids which they contain. Thus it is found that whereas 

 beef suet and mutton fat consist chiefly of esters of the higher 

 fatty acids, such as palmitic and stearic acids, butter contains 

 a considerable quantity of the lower members of this same 

 fatty series such, for example, as butyric, caproic, caprylic, and 

 capric acids ; these acids, which are low boiling liquids readily 

 volatile with steam, are known as volatile fatty acids, and their 

 presence or absence in a given sample of fat may be used for 

 characterizing the fat. Thus, for example, the estimation of 

 the amount of volatile fatty acid serves to distinguish genuine 

 butter from its substitute margarine, which is relatively poor 

 in volatile acids and contains chiefly higher fatty acids. 



The more important members of the fatty acid series are 

 given in the following list : — 



It should be noted that these acids all conform to the 

 general formula for the fatty acids, CnH2n02, in which " n " 

 may have any value, odd or even, but only those in which 

 " n " is an even number are found to occur naturally in fats ; 



♦Halliburton, Drummond, and Cannan : " Biochem. Journ.," 1919. 



13, 301. 



t These acids do not occur in fats. 



