CHAPTER IV 



FATS AND FIXED OILS 



General Properties. Fats and oils are two names for one 

 and the same class of compounds. In popular language, 

 those members of the group which are solid at ordinary tem- 

 peratures are called fats, and those which are liquids under 

 like conditions are called oils. By comparatively slight 

 changes of temperature fats are changed into oils and vice versct. 

 Those of which the melting point is about i5C. (the ordinary 

 temperature) naturally assume the condition of fats in winter 

 and of oils in summer. 



Waxes and volatile oils are not included in this group. 

 They resemble the fats and fixed oils more or less closely as 

 regards solubility and other external properties, but are clearly 

 distinguished from them by difference in chemical composition. 



Fats and oils are obtained both from plants and animals. 

 In the latter they form a not inconsiderable part of the tissue 

 substance. They take rank as one of the most important 

 classes of compounds produced exclusively by agricultural 

 methods. Fats have been prepared synthetically, but the pro- 

 cesses are of scientific interest only. They are easily assimi- 

 lated by animals and have a higher fuel value (p. 120) than 

 any other constituent of the food. They are used also for 

 many other purposes, and, in accordance with the laws of supply 

 and demand, they command a relatively high price in the 

 market. 



Fats and oils are all unctuous or greasy substances of low 

 specific gravity they are all lighter than water and of 

 variable but relatively low melting point. When pure, they 

 are colourless or pale yellow in the liquid state, and white in 



39 



