," 206 pages, _Monographs_ on
Biochemistry, London, 1913.

SOUTHCOMBE, J. E.--"Chemistry of the Oil Industries," 204 pages, 13 figs.,
London, 1918.

CHAPTER XI

ESSENTIAL OILS AND RESINS

Included in this group are all those substances to which the characteristic
odors of plants are due, along with others similar in structure and
possessing characteristic resinous properties. They have no such uniformity
in composition as is exhibited by the oils which are included among the
fats and waxes; but belong to several widely different chemical groups.
Furthermore, there is no sharp dividing line between the essential oils and
certain esters of organic acids on the one hand and the fats on the other.
For example, if an aromatic fluid essence is a light fluid, non-viscid, and
easily volatile, it is usually classed with the organic esters; denser
liquid substances, of oily or waxy consistency, and with comparatively
slight odor and taste are usually fats, while oils of similar physical
properties but possessing strong characteristic odors are classed as
essential oils, regardless of their chemical composition.

Included in this general class are compounds having a great variety of
chemical structures; e.g., hydrocarbons, alcohols, phenols, organic
sulfides and sulfocyanides, etc. Many of these compounds are crystalline
solids at ordinary temperatures, but melt to oily fluids at higher
temperatures. The characteristic property which assigns any given plant
extract to this group is that it has a strikingly characteristic odor or
taste, often accompanied by some definite physiological effect, or
medicinal property.

These compounds may be either secretions or excretions of plants, sometimes
normally present in the healthy tissue, and sometimes produced as the
result of injury or disease.

The essential oils and the resins often occur associated together in the
plant; or, the resins may develop from the oily juice of the plant after
exposure to the air.

THE ESSENTIAL OILS

These may be divided, according to their chemical composition, into two
major groups; (1) the hydrocarbon oils, or terpenes, and (2) the oxygenated
and sulfuretted oils.

The =terpenes= are of three different types, namely: (_a_) the
hemiterpenes, C_{5}H_{8}, unsaturated compounds of the valerylene series,
of which _isoprene_ (found in crude rubber) is the b