Introduction. The majority of essential oils have always been obtained by steam distillation or, in the more general sense, by hydrodistillation.1 The practical problems connected with distillation of aromatic plants are, therefore, of utmost importance to the actual producer of essential oils. Yet our present-day technical literature, especially English literature, is surprisingly meager in regard to data and information which might serve as a really practical and reliable guide. This shortcoming has been felt severely, especially during the years of World War II, when prospective producers in North, Central and South America sought advice concerning the distillation of oils which, due to war conditions, could no longer be imported from Europe and Asia. Encouraged by countless inquiries from almost every part of the Western Hemisphere, the author finally decided to compile a comprehensive paper on this topic which would incorporate not only his own experience of many years in the field, but also the most important phases gathered from the literature published so far. There exist on this subject two really outstanding books, viz., the classical work of Dr. von Rechenberg, who spent a lifetime in the actual distillation of essential oils and on systematic research pertaining to the physical phenomena and laws underlying distillation. These works have never been translated from their German text, are now out of print and, due to the ravages of World War II, not readily available. This author would consider it an irreplaceable loss to our industries if the most important parts of these books, at least those dealing with the practical aspects of essential oil distillation, were not preserved for posterity. Unfortunately, the lucid writings of Professor von Rechenberg have not attained sufficient attention outside of Germany. In more than one way they are so fundamental and exact that they require no modification. This author has, therefore, translated parts of von Rechenberg's treatises, with a view to incorporating some of the most essential features into his own text. These books are recommended: C. von Rechenberg, "Theorie der Gewinnung und Trennung der atherischen Ole," Schimmei & Co., Miltitz bei Leipzig, 1910.
This chapter by Ernest Guenther.
1 The term "hydrodistillation" is used by von Rechenberg as referring to distillation with water vapors (steam).

C. von Rechenberg, "Einfache und Fraktionierte Destination in Theorie und Praxis,” Schimmel & Co., Miltitz bei Leipzig, 1923.
A much smaller book, "Die Fabrikation und Verarbeitung von atherischen Olen," by Max Folsch, Hartleben's Verlag, Wien und Leipzig, 1930, leans on von Rechenberg's text but adds much practical advice.
Those interested particularly in the distillation of colonial oils and in field distillation requiring simple apparatus are referred to Gattefosse's "Distillation des Plantes Aromatiques," Librarie Centrale des Sciences, Paris, 1926.
"Aspects of the Theory of Distillation as Applied to Essential Oils," have been described by Leslie Bloomfield in a series of comprehensive papers which appeared in the Perfumery and Essential Oil Record, Vol. 27 (1936), 131, 177, 294, 334, 368, 404, 443, 483; Vol. 28 (1937), 24, 59.
"A Treatise on Distillation," by Thos. H. Durrans, was published also in the Perfumery and Essential Oil Record, June, 1920, 154 to 198.
The mathematical and physical principles connected writh steam distillation in general are discussed in "Wasserdampf-destillation," by N. Schoorl, which appeared in Rec. Trav. Chim. 62 (1943), 341-379.
This chapter will be divided into two parts, the first dealing with the fundamental or theoretical principles underlying all distillation processes, and the second treating more specifically the practical aspects of distillation as applied directly in the essential oil industry.

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