THE PATS. 25 



The acids which enter into the fats resemble the latter 

 in many of their properties. They differ from them in 

 having a slight acid reaction, the fats being neutral. They 

 can also be distinguished by their not giving the irritating 

 odor of acrolein as the fats do when they are heated. This 

 is produced by the decomposition of glycerin, either in a 

 fat or when heated alone. It is most easily obtained by 

 adding before heating some substance which will assist in 

 removing the water. The chemical change is 



C 3 H 5 (OH) 3 2H 2 = C 3 H 4 0. 



The free acids can be neutralized by even weak alkalies, 

 forming the soaps. 



The animal and many of the vegetable oils are true fats, 

 differing only in that they are liquids at ordinary tempera- 

 tures instead of solids. We must, however, distinguish be- 

 tween these and the essential or volatile oils which are 

 found in plants, but which are not fats. The fats produce 

 spots on paper which are not volatile and do not disappear 

 on standing. The essential oils will disappear when left 

 exposed to the air. The mineral oils belong to an entirely- 

 different class of compounds, and do not contain oxygen. 



The fats are named from the acid which they contain. 

 Thus the compound of stearic acid is called stearin; of 

 palmitic acid, palmitin; and of oleic acid, olein. Some- 

 times the prefix tri is used with these names, as tristearin, 

 etc. They differ principally in their melting-points, olein 

 being a liquid at ordinary temperatures; palmitin and 

 stearin, solids, the former melting more easily than the 

 latter. In the animal body these fats are usually mixed, 

 the consistence of the fat varying with the composition. 

 Thus, the fat of the ox, or tallow, is a firmer solid than 

 the fat of the hog, or lard, because it contains less of the 



