The developments in the chemistry of essential oils have been so rapid and so extensive during the past few years, that I need make no apology for the appearance of the present work, in which I have attempted to group together the more important of the published facts connected with the subject. No attempt has been made, in the chapters dealing with the preparation and analysis of essential oils, to do more than give an outline of the principles involved, as, in the first place, each subject could well occupy a volume by itself, and in the second place, no man with mere text-book knowledge would undertake either branch of the subject in a practical manner. The treatment of the essential oils systematically has necessitated the suppression of much doubtful and disputed work, and whilst great care has been taken to keep the facts up to date, it has been necessary to only touch slightly on a good deal of quite recent disputed work, which must be regarded as still in an unsettled condition.