44 THE CHEMISTRY OF CATTLE FEEDING 



(C) Series C n H2, l _ 6 O 2 . The most important member of 

 this series is linolenic acid, C 18 H 30 O 2 . It occurs along 

 with linoleic acid in linseed, cotton seed, and other vegetable 

 oils. 



Drying Oils. All these unsaturated compounds are 

 capable of forming addition products with halogens, oxygen, 

 hydrogen, hydroxyl, etc., and this property is retained also 

 by the salts and glycerides. The amount of iodine which 

 a fat can absorb is therefore a measure of the amount of 

 unsaturated acid in it. This value the " iodine number " 

 is approximately constant for any given natural fat, " and 

 forms one of the most important means of identifying them. 

 According to the formulae, two parts of linolenic acid t can 

 absorb, i.e. combine with, as much iodine as three parts of 

 linoleic acid, or six parts of oleic acid. The compounds 

 formed are of course halogen substitution products of stearic 

 acid. 



Oleic acid, linoleic acid, and linolenic acid, when saturated 

 with hydrogen, all form stearic acid. By addition of oxygen 

 or hydroxyl, oxy or hydroxy stearic acids are produced. Such 

 compounds are formed on exposure to air more rapidly at 

 higher temperatures and by the action of mild oxidising 

 agents. The products are solid substances of a resinous 

 character. Oils which contain much unsaturated acids have 

 the property of drying. Linseed oil exhibits this character in 

 very marked degree, and is therefore largely used as a medium 

 in the preparation of paints. Those which contain smaller 

 quantities of unsaturated acids are described as semi-drying 

 oils. The drying property is not noticeable in oils which con- 

 tain even considerable quantities of oleic acid, but no linoleic 

 or linolenic acids. 



Hempseed, poppy seed, and sunflower seed oils are classed 

 as drying oils ; those of cotton seed, rape seed (colza), sesame, 

 soja bean, maize, and beechnuts are classed as semi-drying 

 oils ; while earthnut oil (arachis), almond oil, and olive oil are 

 typical non-drying oils. 



Compound Types. Lecithin is a wax-like substance which 

 occurs normally in nerve and brain tissues, in the yolk of egg, 



