MEDIUMS AND ADHESIVES 37
of iodine chloride, calculated as iodine, which is capable of being absorbed by the
substance.
The principal acids occurring in fats and oils are:
NAME FORMULA DOUBLE BONDS OCCURRENCE
Palmitic Ci6H3202 o Widely distributed; probably the most
abundant of the natural fatty acids.
Stearic CisHseC^ o Widely distributed but not present in
large amounts in most fats and oils.
Oleic CisHs^ i Present in most fats and oils.
Linoleic CigffeC^ 2 Present in most fats and oils.
Eleostearic CisKW^ 2 Present mainly in tung oil.
Linolenic QsHsoOa 3 Present mainly in linseed and perilla oil.
The saturated and unsaturated fatty acids consist of long chains of carbon atoms
strung together with an acidic or carboxyl (-COOH) group at one end of the
molecule. In nearly all cases each carbon atom is attached only to two other
carbon atoms, the remaining valences being satisfied by hydrogen atoms or left
in some cases unsatisfied (ethylenic linkage or double bond). The majority of
the fatty acids are thus * straight-chain,' aliphatic compounds; either branched-
chain or closed-chain fatty acids are exceptional. Another curious and striking
feature of the natural fatty acids is that, almost, and perhaps entirely, without
exception, they contain an even number of carbon atoms. Since one molecule of
glycerine is combined with three molecules of fatty acid, either simple triglycerides,
where all the fatty acid radicals are alike, or mixed triglycerides, consisting of
two or three different fatty acid constituents, are possible. It is unusual to find
any simple triglyceride in a vegetable oil unless a considerable excess of one fatty
acid is present. Various mixed triglycerides, dilinoleo-linolenin, oleo-linoleo-
linolenin, etc., have been obtained from linseed oil but there is no evidence of the
presence of simple glycerides such as triolein. In the case of tung oil, which con-
tains 75 to 85 per cent of eleostearic acid, the triglyceride of this acid can be
separated, the remaining acids presumably being present in the form of mixed
glycerides.
The accompanying table gives the source and some of the more important
analytical characteristics of the principal drying oils. The specific'gravity is the
ratio of the mass (weight) of the oil to the mass of an equal volume of water at
4° C. The oils are all lighter than water; their specific gravities do not vary widely.
The refractive power of oils varies more and is chiefly governed by the proportion
and degree of unsaturated matter present. Both the saponification number and
the acid number depend on the amount of acid present; the saponification number
is a measure of the amount of combined acid, Le., glyceride, and the acid number
is a measure of the amount of free acid. Numerically, the saponification number
is the number of milligrams of caustic alkali (KOH) required to saponify (see