SAPONIFIABLE OILS, FATS AND WAXES	177
action does not take place readily and most of the oils of pro-
nounced drying properties are found to contain considerable
quantities of simple or mixed glycerides of linolic or linolenic
acids, more highly unsaturated compounds than oleic acid.
Linolic Acid, Ci8H3202, contains two pairs of doubly linked
carbon atoms:
This acid will absorb four atoms of halogen or two atoms of
oxygen. It gives marked drying properties to oils, linolin being
abundant in linseed, soybean and poppy seed oils.
Linolenic Acid} dsHsoOs, probably to be represented as
CH8-CH2-CH = CH-CH2-CH = CH-CH2-CH = CH- (CH2) 7COOH.
This acid possesses three sets of double bonds and will absorb
six halogen atoms or three oxygen atoms. It occurs as simple
or mixed glycerides in linseed oil and, together with linolic acid,
plays the most important part in the hardening or "drying"
of this oil when it is exposed to the air. An isomer, isolinolenic
acid, also occurs as a constituent of the glycerides of drying oils.
Ricinoleic Acid, Ci8H3403, is hydroxyoleic acid and, like oleic
acid itself, contains only one pair of doubly linked carbon atoms.
It will not readily absorb oxygen from the air and it does not
impart drying properties to an oil? It is, however, an important
constituent of castor oil and will be mentioned later, in the
discussion of acetyl value.
The five acids named above serve to illustrate the principle that
only those unsaturated acids which contain more than one pair
of doubly bonded carbon atoms are important from the stand-
point of drying. Also an interesting, although perhaps unex-
pected fact is that trebly linked carbon atoms do not, under
ordinary conditions, absorb halogens or oxygen to the point of
complete saturation, only two atoms of halogen or one of oxygen
adding to each such pair.
Solvent. — Absorption of halogen by oil cannot readily take
place unless there is present some solvent which can dissolve
both oil and halogen. The halogen solution earliest used for
this purpose was of iodine and mercuric chloride in alcohol.
This has been almost entirely replaced by a solution of either
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