30 FATS, OILS, AND WAXES 



the air tend to thicken, owing partly to polymerization and 

 partly to oxidation ; in some cases the oil actually dries up, 

 leaving a more or less hard mass or a thin elastic film. 



Those oils which only thicken, without actually becoming 

 hard or dry, are called non-drying oils. They are composed 

 for the most part of triolein (cf. p. lo), and contain only 

 small quantities of solid fatty acids ; to this class of oils belong 

 the following : olive oil, almond oil, arachis or pea-nut oil, 

 quince oil, cherry-, plum-, peach-, and apricot-kernel oil, 

 wheatmeal oil, rice, tea-seed oil, and hazel-nut oil. 



Two further oils, namely, castor oil and grape-seed oil, are 

 also included in this group of non-drying oils, but they have 

 a slightly different composition from the other members of 

 this group. They are characterized by possessing a consider- 

 able percentage of glycerides of hydroxylated fatty acids, such 

 as dihydroxystearic acid, a fact which is brought out clearly 

 by their high acetyl values (p. 27). 



In contrast with these non-drying oils are the so-called 

 drying oils, among the more important of which are the follow- 

 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. 23) 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, 



