Maclean, H.—"Lecithin and Allied Substances," 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.