“essential (or volatile) oils” is due to the fact that the volatile vegetable aromatic substances cause a stain on paper similar to that produced by oils and fats; but the stain made by the former disappears spontaneously after some time, while that due to true oils and fats persists. The disappearance of the stain depends on the evaporation of the vegetable aromatic substances—a quality not possessed by fats. Hence the volatile vegetable aromatic substances, in contradistinction from non-volatile fixed or fatty oils, have been designated as essential or volatile or ethereal oils. Inasmuch as the latter terms are the ordinary trade names for these substances, we are compelled to retain them despite their incorrectness. The French name for essential oils is _essences_; “essence de lavande,” for instance, is the French name for essential oil of lavender, and not for an alcoholic solution of the oil, as might be inferred from the usually accepted meaning of the English terms “essence of lavender,” “essence of peppermint,” etc., which mean solutions of these essential oils in alcohol. As the localities where the raw materials—that is, the aromatic plants—are cultivated on a large scale naturally constitute the places of manufacture of essential oils, we find in southern France and in England the most extensive factories devoted exclusively to the preparation of perfumes. In the countries named, a favorable influence is exerted, too, by their situation near the sea, as well as by their trade with tropical lands from which additional aromatic plants are imported. We have stated above that the manufacture of essential oils forms almost a monopoly in France and England; but there is no doubt that this country (the United States) likewise possesses many localities favorable to the cultivation of certain aromatic plants and the preparation of essential oils from them, so that this branch of industry could be carried on at a profit. For this reason we have in our descriptions devoted some attention to the conditions of growth required by such plants as might be raised here. We even find that some advantages are derived from the hot-house cultivation of some tropical plants. An exact knowledge of the chemical properties of a substance is in all cases the first and fundamental condition for its preparation; it would appear necessary, therefore, that we should endeavor to gain complete information about the nature of vegetable aromatic substances before we enter upon the description of the various methods of their preparation. THE CHEMICAL CONSTITUTION OF VEGETABLE AROMATIC SUBSTANCES. The sources of the odors derived from the vegetable kingdom can be divided, as stated above, into so-called essential oils, balsams, gum-resins or soft resins, and hard resins. Since the latter bear a certain relation to the essential oils from which they are formed through chemical combinations, we must consider them first.