CLASSIFICATION OF VOLATILE OILS BASED ON THEIR ODORS AND CONSTITUENTS.

Volatile oils obtained from plants possess a great variety of odors, with no two exactly alike, although many are very closely related. A classification of these oils based on their odors is not satisfactory, since many which would not be considered as related if judged only by the sense of smell have chemical relationships, containing substances belonging to the same general class of chemical compounds. For our purpose volatile oils are divided into the following classes, basing the divisions upon odors and constituents. These groups comprise the majority of oils, but they are not arranged in the order of their importance:

(1) Camphoraceous oils, possessing a characteristic camphorlike odor, with camphor or camphor-related compounds predominating, as in the oils obtained from the camphor tree and from many of the sages. The products obtained from camphoraceous oils are extensively employed in the arts and in medicine.

(2) Terebinthinate oils, having a characteristic turpentinelike odor. These oils are obtained largely from the pine family, the turpentines of commerce being examples. They are composed largely of terpene hydrocarbons and find extensive application in the paint and varnish industries.

(3) Sulphur-containing oils, a small group characterized by extremely disagreeable and offensive odors, such as those of mustard, asafetida, garlic, and onion. These oils contain as their chief constituents sulphids, sulphocyanates, or nitriles, and are used principally for medicinal purposes.

(4) Phenol and phenol-related oils, containing phenols or phenol derivatives and characterized by strong, persistent odors, some very pungent and others pleasant. Owing to their phenolic constituents the density of these oils is usually very high. Common examples of this class are the oils of thyme, cloves, cinnamon, sassafras, anise, fennel, and the monardas. The usefulness of phenol and phenol-related oils depends largely upon their antiseptic properties, the principal constituents being thymol, carvacrol, eugenol, anethol, chavicol, safrol, and their derivatives.

(5) Oils containing esters or alcohols, by far the largest group, consisting of the fragrant oils which are used principally for perfumery purposes, although some find a use in medicine. The chief constituents of these oils are usually alcohols and esters, some few containing aldehydes, ketones, oxids, and lactones. Prominent here are the alcohols menthol, linalool, geraniol, citronellol, sabinol and their esters, benzyl alcohol and its esters, and anthranilic acid and its esters, forming the chief constituents of the oils of peppermint, lavender, geranium and rose, citronella, savine, ylang-ylang, and orange flowers, respectively. Other constituents are the aldehydes citral and citronellal from lemon and lemon-grass oils, and the ketones thujone, menthone, pulegone, carvone, and methyl heptenone from the oils of wormwood, peppermint, pennyroyal, caraway, and rue. The oxid cineol from eucalyptus and many other oils, and the lactone sedanolid from celery oil are further examples.

All volatile oils capable of being isolated from wild aromatic plants will fall into one or more of the foregoing divisions although, it must be understood, the classification is far from being satisfactory. It will, however, serve to elucidate the fact that although plant odors are of a very variable character they still possess some relationship.