The revolution in the science of chemistry, which began at the end of the eighteenth century with the work of A. Lavoisier (1743-1794), resulted in a new and illuminating approach to the investigation of the nature of essential oils. It is of interest that the first really important modern investigation in the field was devoted to the oldest essential oil known, oil of turpentine. Submitting the oil to elementary analysis, J. J. Houton de la Billardiere found the ratio of carbon to hydrogen to be five to eight the same ratio that was later established for all hemiterpenes, terpenes, sesquiterpenes and polyterpenes. The investigator published his results in a pharmaceutical periodical, the Journal dc Pharmacic ([1818]).
The systematic study of essential oils may be said to have begun with the analysis of a number of stearoptenes by the great French chemist, J. B. Dumas (1800-1884), who had started his career as a pharmacist. He published his first treatise devoted to essential oils in Licbig's Annalen der Pharmacie ([1833], 245). Of considerable importance in the further development of the chemistry of volatile oils were the investigations of the French chemist, M. Berthelot (1827-1907), devoted primarily to the hydrocarbons contained in these oils. About 1866 the name Terpene was mentioned in a textbook written by Fr. A. Kekul6 (1829-1896) who apparently coined this term. In 1875 one of the greatest English chemists emerging from pharmacy, W. Tilden (1842-1926), introduced nitrosyl chloride as a reagent for terpenes, a reaction perfected and used to such an extent and with such excellent results by the German chemist, O. Wallach (1847-1931), that the renowned Swis pharmacognosist, Fr. A. Fliickiger (1828-1894), called Wallach the Messiah of the terpenes.

This very active and far-reaching research was the result, as well as the cause, of the wide expansion in the use of essential oils during the latter half of the nineteenth century; and it is difficult to decide which ranks first, the result or the cause. Gradually the use of essential oils in medicinal drugs became quite subordinate to their employment in the production of perfumes, beverages, foodstuffs, etc. The work of O. Wallach and his pupils, and of F. W. Semmler (1860-1931) and collaborators, on terpenes and terpene derivatives introduced what might properly be called the "Elizal>ethan Age" of the essential oil industry. Discovery followed discovery ; one essential oil after the other was thoroughly investigated and its composition elucidated. Newly identified constituents were synthesized, and many of them manufactured commercially. Our industry of synthetic and isolate aromatics had its origin mainly in the work of these great explorers. Illustrious names, such as 0. Aschan, E. Gildemeistcr, H. Walbaum, S. Bertram, A. Hesse, C. Kleber, E. Kremers, H. Barbier, L. Bouveault and E. Charabot, etc., are connected with these classical investigations, which are still being carried on with ever greater results by some of our greatest contemporary scientists, including L. Ruzicka in Zurich arid J. L. Simonsen in London, not to mention other diligent workers in the United States, in the British Commonwealth, the U.S.S.R., Switzerland, France, Germany and in the Far East. It should be emphasized in this connection that Wallach's work also laid the foundation for another important chapter in the chemistry of essentia oils, viz., the analysis or assay of products which, because of their high price, are prone to fraudulent manipulation and adulteration by unscrupuous producers or dealers.

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