The essential oils of lemons and oranges of commerce, and of some other fruits, are chiefly obtained by submitting the yellow rind to powerful pressure; but in this way they are not so white, nor do they keep so well, as when distilled, although in the case of the fruits referred to the oils are more fragrant than when prepared by any other method.
The London College excluded the usual directions for the preparation of the essential oils from their Pharmacopœia of 1851, on the ground that these substances are seldom prepared by the druggist or apothecary, or at all on the small scale.
“The fruit of anise, caraway, and juniper, the flowers of chamomile, lavender, and elder, the berries of allspice, the tops of rosemary, and the entire recent plants of the other herbs, are to be employed.” “Put any one of these into an alembic, then pour in as much water as will cover it, and distil the oil into a large vessel, kept cool.” (Ph. L. 1836.)
The Edinburgh College directs—“As much water only is to be employed as will prevent empyreuma during the distillation. The distillation may be immediately commenced after a proper maceration, and the oil afterwards separated from the water,” in the manner already noticed.
“It is also necessary to observe, in preparing these oils, as well as the distilled waters, that the quality of the substances, their texture, the season of the year, and similar circumstances, must occasion so many differences, that it is scarcely possible to give any certain and general rules which shall strictly apply to every example. Many things, therefore, must be regulated by the judgment of the operator.”
The Dublin College directs the vegetable substances to be macerated in the still with about 5 times their weight of water, for 24 hours, when one half of the water is to be drawn over. The oil having been separated from this in the usual manner, it is to be returned to the still, and the same quantity drawn over, as before, from which the oil must again be separated.
Chevallier gives the following rules for the distillation of essential oils:—
1. Operate upon as large quantities as possible, in order to obtain a greater product, and one of finer quality.
2. Conduct the distillation rapidly.
3. Divide the substances minutely, in order to facilitate the extrication of the oil.