DERIVATIVES OF ALCOHOLS AND ACIDS 6l 



chemical constants depending upon the amounts of the different 

 acids present. 



Physical Constants. — The esters of the different acids have 

 distinctive properties such as melting point, specific gravity, re- 

 fractive index, etc. The relative amounts of the different esters 

 present give to the fats these definite characters which are 

 termed the physical constants. For example, fats containing 

 large amounts of palmitic or stearic acid esters are solid at ordi- 

 nary temperatures, as in heef fat, mutton tallow, palm oil, etc. 

 Those containing large amounts of the esters of oleic acid, or 

 linolic acid, eighteen carbon acids of a new series, are liquids as 

 in olive oil, cottonseed oil and linseed oil. Such fats as human 

 fat and butter fat lie between these two extremes. 



Chemical Constants. — Oleic acid, linolic acid and linolenic 

 acid not only form esters that are liquid but the acids them- 

 selves possess distinct chemical properties which they impart 

 to the esters. They all belong to series of acids differing from 

 the acetic acid series in that while the latter are saturated the 

 former are unsaturated. They are related to unsaturated hydro- 

 carbons like ethylene, which has been referred to in connection 

 with ethylene bromide or symmetrical dibrom ethane. These 

 hydrocarbons, acids and fats containing esters of the latter, all 

 show their property of unsaturation by readily taking up 

 bromine or iodine by addition. The amount of iodine, there- 

 fore, which a fat absorbs by addition rests upon the amount of 

 the esters of these acids present. This gives us a chemical con- 

 stant termed the iodine value by which fats are identified. 



Other chemical constants known as saponification value, 

 amount of insoluble acids and amount of volatile acids all depend 

 upon the amount of particular groups of acids combined in the 

 fat, and are used, together with the iodine value and the physical 

 constants already mentioned, for purposes of identification and 

 analysis. This brief mention of the basis of analytical methods 

 applying to fats and oils is all that is is desirable to make. 



In connection with plant constituents (p. 278) we shall con- 

 sider the important vegetable oils as to their occurrence and use. 



