20 FATS, OILS, AND WAXES 



ing : linseed oil, cedar-nut oil, hempseed, walnut, poppy-seed, 

 and sunflower oil. These oils exhibit to a greater or less 

 degree the tendency to absorb oxygen from the air, thereby 

 drying up and leaving an elastic skin, a property which is 

 made use of industrially in the manufacture of oil paints. 

 These drying oils are composed chiefly of the glycerides of 

 the unsaturated acids of linolic and linolenic series and contain 

 only relatively small quantities of oleic acid. Owing to the 

 large amount of unsaturated acids which they contain their 

 iodine value (p. 29) is very high (120-200). 



In addition to the above there is also a third group of 

 vegetable oils known as the semi-drying oils whose iodine value 

 and drying properties lie midway between those of the drying 

 and non-drying oils. They differ from the true drying oils in 

 containing no acids of the linolenic series and from the non- 

 drying oils in containing linolic acid. The oils belonging to 

 this category fall naturally into two sub-groups : — 



(i) The cotton-seed oil group, to which belong Soja-bean 

 oil, maize oil, pumpkin, water-melon and melon-seed oils, 

 beech-nut oil, cotton-seed, sesame and croton oils, and the 

 lesser known oils of the apple, pear, orange, barley and rye seeds. 



(2) The rape oil group comprising garden cress, hedge 

 mustard, wild radish, black mustard seed, white mustard 

 seed, radish seed and rape or colza oil. 



The oils of the latter sub-group have a lower saponification 

 value (p. 29) than any other vegetable oils, and arachidic acid 

 seems to be a normal constituent of them all. 



To determine whether an oil is a drying one or not, a drop 

 is spread on a glass plate, such as a microscope slip, and left 

 for several days at atmospheric temperature. Non-drying oils 

 such as olive and castor oils are unaltered after about eighteen 

 days : semi-drying oils such as cotton-seed, sesame and rape 

 oil are more or less dry, but still sticky in from seven to 

 eight days, whereas real drying oils like poppy and especially 

 linseed are quite dry in from three to six days. 



GENERAL PROPERTIES AND REACTIONS OF FATS. 



(i) All fats, both solid and liquid, are soluble in ether, light 

 petroleum, carbon tetrachloride, chloroform and carbon di- 



