170 QUANTITATIVE AGRICULTURAL ANALYSIS

Remove the tube from the bath and place in the latter a second tube of
alcohol and water. The latter, having been cooled in ice water, is sufficiently
low m temperature to cool the bath to the desired point. Add another disc"
of fat and regulate the temperature so as to reach a maximum of 1.5° above
the melting point us already determined. Bun a third determination, which
should agree closely with the second.

The disc of fat should not be allowed to touch the side of the tube, in any
determination.

Iodine Absorption Number. — The iodine absorption number
is the per cent of halogen, expressed as iodine, absorbed by
the fat or oil when subjected to the action of a halogen solution
under specified conditions. The absorption takes place because
of the presence of glycerides of unsaturated acids, which contain
double or triple bonded carbon atoms.

This action is analogous to the absorption of oxygen. In the
latter case saturated oxygen compounds are formed, often hard
and resinous in nature. Absorption of oxygen from the air
in this way is known as "drying," although the term is mis-
applied, since no real drying occurs. The determination of halo-
gen absorption number is, in a general way, a measure of drying
proportion and it serves as a distinction between the somewhat
arbitrary classes of drying, serai-drying and non-drying oils.

Of the unsaturated acids whose glycerides commonly occur in
fats or oils the following important members may be mentioned:

Olcic Acid, OiKHruOjj.— The unsaturated character of this
acid is indicated by the formula

CH,(CH2)7CH « CH(CH2)7COOH.

Olein, the triglyceridc of this acid, occurs to some extent in all
oils and fats, but especially in the former. The empirical formula
of the triglyceride is

Olein is liquid at ordinary temperatures and its presence in oils
in responsible, in a large number of cases, for their liquid character.
Oleio. acid will absorb two atoms of bromine, iodine or chlorine,
or one molecule of iodines monochloride ?'or monobromide, the
double bonded carbon atoms thus becoming saturated. Simi-
larly, cither oleic acid or olcin might be expected to absorb
oxygen and to give drying properties to a fat or oil but this