Chemical Constituents of the Animal Body. 181 



whereas the latter name is generally applied to sub- 

 stances with relatively high melting points (that is, are 

 solid at room temperature). These terms have, however, 

 no very definite significance ; all substances to which they 

 are applied can be included under the designation of 

 " fats," which may be defined chemically as the glyceryl 

 esters of fatty acids. They yield, therefore, on hydro- 

 lysis a mixture of glycerol and a fatty acid. This hydro- 

 lysis may be accomplished either by water under high 

 pressure and at temperatures above the boiling point (at 

 200), in which case a mixture of glycerol and free fatty 

 acid is obtained, or by means of alkalis, which produce 

 a mixture of glycerol and the sodium salt of the fatty acid 

 or " soap." As an example of hydrolysis, the scission of 

 the tristearic ester of glycerol or " tristearin " into stearic 

 acid and glycerol may be quoted 



C 17 H 36 CO-0 CH 2 CH 3 .OH 



CH-i 



C 17 H 35 CO-0-CH + 3H 2 = 3C 17 H 36 -CO.OH + CH-OH 



V/iyJtl^OvJ (J VAIJ.JJ C/JUg'OJjL 



The various fats differ from one another in that they 

 yield different acids on hydrolysis. All yield glycerol, but 

 the fatty acids of fats obtained from different sources are 

 very diverse. A natural fat may be either a mixture of 

 glycerol esters (glycerides) or it may consist of mixed 

 glycerides that is, the hydrogens of the hydroxyl group 

 in glycerol may be replaced by different acid radicles. 

 Fats from different sources can be characterized by the 

 determination of certain properties of the acids yielded by 

 hydrolysis. The methods adopted for the characterization 

 of fats are of considerable technological importance in that 

 they serve for detection of adulteration (for example, 

 detection of margarine in a butter sample), and they have 



