, C_{13}H_{27}.COOH, in cocoanut oil and
spermaceti.

(6) Palmitic acid, C_{15}H_{31}.COOH, in palm oil and many
fats.

(7) Stearic acid, C_{17}H_{35}.COOH, in most fats and oils.

Intervening members of this series, such as caprylic acid,
C_{7}H_{15}.COOH, and lauric acid, C_{11}H_{23}.COOH, are also found in
smaller quantities in cocoanut and palm nut oils, in butter fat, and in
spermaceti; while higher members of the series, as arachidic acid,
C_{19}H_{39}.COOH, and lignoceric acid, C_{23}H_{47}.COOH, are found in
peanut oil; and cerotic acid, C_{25}H_{51}.COOH, and melissic acid,
C_{29}H_{59}.COOH, in beeswax and carnauba wax.

Unsaturated Acids:

(_b_) Oleic acid series--general formula, C_{_n_}H_{2_n_-1}.COOH.

(1) Crotonic acid, C_{3}H_{5}.COOH, occurs in croton oil.

(2) Oleic acid, C_{17}H_{33}.COOH, occurs in many fats and
oils.

(3) Brassic acid, C_{21}H_{41}.COOH, occurs in rape-seed oil.

(4) Ricinoleic acid, C_{17}H_{32}OH.COOH, occurs in castor oil.

(_c_) Linoleic acid series--general formula, C_{_n_}H_{2_n_-3}.COOH.

(1) Linoleic acid, C_{17}H_{31}.COOH, occurs in linseed and
other drying oils.

(_d_) Linolenic acid series--general formula, C_{_n_}H_{2_n_-5}.COOH.

(1) Linolenic acid, C_{17}H_{29}.COOH, occurs in many drying
oils.

It will be observed that all of these acids contain a multiple of two total
carbon atoms. No acid containing an uneven number of carbon atoms has been
found in a natural fat. Furthermore, the acids which occur most commonly in
natural fats are those which contain eighteen carbon atoms; in fact, more
than 80 per cent of the glycerides which compose all animal and vegetable
fats are those of the C_{18} acids. This fact, in addition to the one that
the sugars and starches all contain multiples of six carbon atoms in their
molecules, indicates a very great biological significance of the chain of
six carbon atoms. This has been alluded to in connection with the
discussion of the biological significance of molecular configuration (see
page 57) and will be mentioned again in other connections.

THE ALCOHOLS WHICH OCCUR IN NATURAL FATS

=Glycerol=, as has been pointed out, is by far the most common alcoholic
constituent of natural fats and oils. This substance, wh