This is the term used to describe the ease and speed with which any substance disperses on contact with the air. It refers particularly to the time taken for a liquid to evaporate, and can be scientifically measured. Essential oils, and in fact all aromatic substances, are highly volatile, i.e. they evaporate very quickly, and this is an intrinsic part of their aromatic nature, for our noses can only detect odours in the form of a vapour or gas.
Essential oils, though all highly volatile, do not all evaporate at the same speed, and this difference in the time taken to evaporate directly affects the length of time the smell of the oil will linger, and the time it will take to be absorbed into the body when applied to the skin. Those oils which take longest to evaporate (i.e., the least volatile) will continue to perfume the skin, or any substance to which they are applied, for many hours and in a few cases even days, while those which are the most volatile will disappear relatively quickly.
In any blend of oils, the oil which is most volatile is the one which will be most easily detected when first smelling the blend, while the slowest, or least volatile, is the smell which will last after the others have faded. In perfumery, these differences are classified on a scale analogous to a musical scale, and the most volatile substances are described as Top Notes, the least volatile and most long-lasting as Bass Notes, with a graduated scale of Middle Notes between. Some aromatherapists refer to such scales when making blends, and may even see different therapeutic properties attaching to the Top, Middle and Bass notes.
However, a major disadvantage of this approach is that it is somewhat subjective. Perfumiers are not all agreed as to where to place various oils on such a scale, and this is hardly surprising when we consider how much an oil can vary from season to season, according to the weather; or from place to place due to variations in soil and climate.