336 THE CHEMISTEY OF ESSENTIAL OILS
OILS OF EUCALYPTUS.The oils distilled from the leaves of the various species of this*
enormous genus of the Myrtacea are in many cases so dissimilar that
the generic term, eucalyptus oil, is meaningless unless the species is
quoted. Several species, however, yield oils which are very similar to
each other, and on this basis the British Pharmacopoeia describes as
eucalyptus oil the product obtained from Eucalyptus globulus and other
species, giving definite characters of the oil, the two chief of which are
the presence of a due, proportion of cineol (eucalyptol) and the absence
of appreciable quantities of the terpene phellandrene. The great varia-
bility in the oil from different trees will make it convenient to deal with
this oil, firstly with reference to the cineol value, and afterwards with
especial reference to the characters of the oils from those trees which
in many cases are in no way similar to "Eucalyptus oil, British
Pharmacopoeia". It is commonly believed that whatever medicinal
value this oil has—and it has been, and still is, used to a very large
extent as a prophylactic against influenza—is due to the presence of
cineol, or, as it was previously called, eucalyptol, so that whether this
be, in fact, the valuable constituent or not, in commerce the value of
the oil is made to depend on its cineol-content.
The quantitative determination of the cineol present is a matter of
importance. The earlier attempts in this direction gave distinctly too
low results. The chief method adopted was a series of fractionations,
retaining all those fractions distilling between 173° and 190°, cooling
them to - 15°, filtering off the still liquid portion, retaining the
crystals of cineol, refractionating the liquid portions and again freez-
ing, subsequently weighing the crystallised cineol. An average
loss of about 10 per cent., in the author's opinion, takes place in
this process, assuming the oil to contain 50 to 60 per cent, of
cineol. Scammeirs process above-mentioned has been more success-
fully applied to the quantitative determination of cineol, and yields
the most satisfactory results so far, although an error- of several per
cent, is scarcely avoidable.
There is some diversity of opinion on this matter, some chemists
maintaining that a very close result is obtained, others that very large
errors occur. In the author's opinion, in the hands of a careful worker,
results accurate to within 3 per cent, can be obtained, so long as the
cineol-content is not below 40 per cent. If the amount of cineol is
lower than this, the method is inapplicable, and the oil must previously
be fractionated.
To a known weight of oil from 1 to 1-J- times its weight of phosphoric
acid of specific gravity 1'75 should be added, drop by drop, the oil
being kept cold and continually stirred. The crystalline magma
formed is pressed between filter paper, after as much as possible has
drained off; and when the adherent terpenes and phosphoric acid
have been removed as far as possible, the crystals are decomposed
by hot water in a graduated tube. On cooling, the cineol is measured,
and from its specific gravity (
- the weight is easiiy calculated.
The separated cineol should readily crystallise on cooling to - 3°,
otherwise it must be regarded as impure and the process repeated.
Oils rich in cineol yield a correspondingly high fraction distilling
between 170° and 190°.
- the weight is easiiy calculated.