490 Preparations and Compositions. Part III.

Chap. XX.—VOLATILE OILS.

Substances which differ in volatility, may be separatedfrom each other by applying a degree of heat capable of con-verting the most volatile into vapour, and by again conden-sing this vapour in a proper apparatus. Water is convertedinto vapour at 212°, and may be separated by distillationfrom the earthy and saline matters which it always containsin a natural state. But it is evident, that if any substanceswhich are as volatile as water, be exposed to the same degreeof heat, either by immersing them in boiling water, or ex-posing them to the action of its steam, they will rise with it indistillation. In this way the camphor and volatile oils ofvegetable substances are separated from the more fixed prin-ciples.

Volatile oils are obtained only from odoriferous substances ;but not equally from all of this class, nor in quantity propor-tional to their degree of odour. Some, which, if we were toreason from analogy, should seem very well fitted for this pro-cess, yield extremely little oil, and others none at all. Rosesand chamomile flowers, whose strong and lasting smell pro-mises abundance, are found to contain but a small quantity ofoil; the violet and jessamine flower, which perfume the airwith their odour, lose their smell upon the gentlest coction,and do not afford any oil on being distilled, unless immensequantities are submitted to the operation at once : while savin,whose disagreeable scent extends to no great distance, givesout the largest proportion of volatile oil of almost any vege-table known.

Nor is the same plant equally fit for this operation, whenproduced in different soils or seasons, or at different times oftheir growth. Some yield more oil if gathered when theflowers begin to fall off than at any other time. Of this wehave examples in lavender and rue; others, as sage, afford thelargest quantity when young, before they have sent forth anyflowers ; and others, as thyme, when the flowers have just ap-peared. All fragrant herbs yield a larger proportion of oil,when produced in dry soils, and in warm summers, than inopposite circumstances. On the other hand, some of the dis-agreeable strong-scented plants, as wormwood, are said tocontain most oil in rainy seasons, and when growing in moistrich grounds.