According to their elementary composition the volatile oils may be divided into three principal divisions:-- 1. Volatile oils free from oxygen, terpene (camphene), or hydrocarbons. 2. Oxygenated volatile oils. 3. Volatile oils containing sulphur. On account of the facility with which most of the volatile oils absorb oxygen, oils originally free from oxygen are frequently a mixture of hydrocarbons and combinations containing oxygen. The volatile oils varying so much in their physical as well as their chemical properties, a suitable classification of them has thus far been unsuccessful. Most of the volatile oils contain a liquid hydrocarbon, _terpene_, which is characterized neither by special taste nor odor, nor is the peculiarity of a volatile oil dependent on it. In the direct distillation of a volatile oil, for instance, lemon oil, this hydrocarbon (citrene), passes first over and can, therefore, be readily separated from the constituents on which depend the peculiarity of lemon oil, and which distil over at a higher temperature. The specific character of an oil is generally due to the portion of the oil containing oxygen. Hence, manufacturers have endeavored to free several of the volatile oils, used for perfumery and the preparation of food, from the worthless terpene and at the same time to obtain them in a concentrated form. Carvol is, for instance, caraway oil freed from carvene (terpene). These concentrated oils are not only purer and more agreeable in odor and taste and more readily soluble in dilute alcohol, but, being more concentrated, an equal volume of them goes much further than ordinary volatile oil. In the price lists these oils are designated as _extra strong_, _patented_, _concentrated_, _highly concentrated oils_ or _essences_. All the terpenes occurring in the various oils are combinations having the formula C_{10}H_{16}, or polymeric with it, C_{15}H_{24}, C_{20}H_{32}, etc. These terpenes exhibiting certain deviations in regard to their properties, odor, specific gravity, and boiling points, nearly as many terpenes as there are volatile oils have been distinguished. It is, however, very likely that these deviations may be traced back to fortuitous circumstances, for example, to the admixture of foreign substances occurring together with the terpenes, and that, by a more accurate examination, the number of terpenes entitled to be considered pure chemical combinations will be considerably reduced. By Wallach's labors, the identity of several terpenes formerly considered distinct, has already been established, whilst many others have been found to possess properties in common. According to the nature and quantity of the odoriferous substances contained in the plants, various methods, namely, _expression_, _distillation_, _extraction_, _maceration_, and _absorption_, are employed for the purpose of obtaining them.