Page 4 - ALLEN'S COMMERCIAL ORGANIC ANALYSIS A TREATISE ON THE PROPERTIES, MODES OF ASSAYING... VOL II

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FIXED OILS, FATS, AND WAXES .
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BY C. AINSWORTH MITCHELL, B. A. (Ox0N.), F. I. C.

GENERAL PROPERTIES AND ANALYTICAL METHODS.
o Under the names of fixed oils, fatty oils, fats, and waxes are classed
many substances occurring in animal and vegetable structures.
The term fixed or fatty oil is generally used for such members of
the group as remain liquid at ordinary temperatures. Those having
this character contain a relatively large proportion of olein or other
compounds of low m. p., but beyond this there is no absolute dis-
tinction between fixed oils and fats.
The waxes possess well-defined physical characters, and differ in
chemical composition from the true fats. They are, however, in
many respects closely related to them, and are conveniently described
in the same division.
The following are the general properties characterising the true fats
and fixed oils:
r. When pure, most of them are colourless or pale yellow. Impure
and commercial oils vary in colour from light yellow to red, and even
to brown and black. Many vegetable oils have a distinct shade of
green from the presence of chlorophyll, and show absorption spectra,
which is never the case with oils of animal origin.
2. Their smell and taste are often peculiar, and are characteristic of
their origin. As these characters become less perceptible the more
completely the oil is purified, they may be due to the presence of as-
sociated foreign matters not readily removed, rather than to the con-
stituents of the oil.
3. If dropped in a liquid condition on paper they leave a perma-
nent grease-spot, unless they are crystalline and hard enough to be
rubbed off.
4. They are not fluorescent and, as a rule, have but little rotatory
action on a ray of polarised light. Castor and croton oils, however, are
dextrorotatory.