\ //
\ //
C
|
CH_{3}

A discussion of the evidence which supports these formulas as properly
represented the molecular arrangements of the various isomeric forms would
be out of place here, as its only particular interest is in connection with
the medicinal effects of the different compounds. It is clear, however,
that they are six-membered hydrocarbon rings, with additional hydrocarbon
groups attached to one or more of the carbon atoms in the ring.

Different modifications, or varieties, of the terpenes constitute the main
proportions of the oils of turpentine, bergamot, lemon, fir needles,
eucalyptus, fennel, pennyroyal, etc.

The =oxygenated essential oils= may be either alcohols, aldehydes, ketones,
acids, esters, or phenols, derived from either five-membered or
six-membered closed-ring hydrocarbons. They are usually present in the
plant oil in mixtures with each other or with a terpene. Since most of them
have pronounced physiological or medicinal properties, their structure has
been well worked out, in most cases; but it seems to be hardly worth while
to present these matters in detail here, as they are of interest chiefly on
account of their medicinal properties rather than their botanical
functions.

_Borneol_, C_{10}H_{17}OH, and _menthol_, C_{10}H_{19}OH, are typical
_alcohols_. The latter is a crystalline substance, which melts at 42 deg.,
which is present in peppermint oil, both as the free alcohol and as an
ester of acetic acid.

Amyl acetate, CH_{3}.COOC_{5}H_{11}, and linalyl acetate,
CH_{3}.COOC_{10}H_{17}, the latter occurring in the oils of lavender and
bergamot, are typical esters classed as essential oils.

As examples of the _aldehyde_ oils, benzoic aldehyde, C_{6}H_{5}CHO, "oil
of bitter almonds," and cinnamic aldehyde, C_{6}H_{5}CH=CHCHO, found in the
oils of cinnamon and cassia, may be cited.

Camphor, C_{10}H_{16}O, is a _ketone_, having the following structural
formula:

H
|
C
/ | \
/ | \
/ | \