Distillation is called the process of evaporation, and subsequent condensation of the vapor of fluids, by means of a still and condenser or other similar apparatus. Fractional distillation is the separation of substances having different boiling points, by distilling the mixture with a gradually increasing heat, and collecting the products which come over at different temperatures in separate receivers.

In the foregoing article we have already mentioned a process of distillation in recovering alcohol from exhausted drugs

This Chapter on distillation refers to the general principles of the art of distilling, as applied in the laboratory of a factory for carbonated saccharine beverages. Distillation may there be resorted to in two respects:

1, for the preparation of essential oils and extracts.

2, for the rectification of extracts, essences and tinctures, obtained by percolation, for the purpose of rendering them purer.

The volatile oils are generally procured by distilling the odoriferous substances along with water; but in a few instances they are obtained by expression, and some by a new process, viz., by extraction with petroleum-ether or benzin, as particularly directed later on.

From the National Dispensatory we extract the following directions for their preparation:

"In a few instances, where the volatile oils are separated in cells of the epidermal tissue, and not associated with resinous or fatty matters, as in the fruits of many Aurantiacem, they may be prepared by expressing that tissue, after removing it by grating. By far the largest number, however, require to be distilled with water. Herbaceous plants, flowers, and leaves which are not of a leathery nature, usually need no previous comminution, but firmer parts of plants, and those containing the volatile oil in the inner tissues, must be more or less ground or broken. Though fresh plants are usually readily softened and permeated by water, a short maceration before the distillation begins is rather advantageous, and may be regarded as very desirable if dried plants are employed; it is unnecessary in case the volatile oils are to be distilled off from resinous compounds; but whenever the volatile oils do not pre-exist, but are produced by the reaction of two principles in the presence of water, a prolonged maceration in cool or luke-warm water is required.

"The distillation is accomplished in stills made of copper or iron, a sufficient quantity of water being introduced to cover the material and prevent empyreuma; the latter object is more completely attained if a perforated false bottom is placed a few inches above the bottom of the still, and the material packed upon it. Direct heat is then applied until the water boils briskly, and is maintained at this temperature until the distillate is no longer charged with volatile oil. Some volatile oils are obtained of better quality and greater fragrance, if the direct application of fire to the still is avoided, and the volatilization is accomplished by the introduction, near the bottom, of steam under pressure; a wooden tank may then be converted into a still by cutting a suitable aperture in the top, and surmounting it with a still-head. Steam distillation is particularly applicable for flowers, like those of the orange, lavender, chamomile, etc., and for the labiate and other herbs".

Although volatile oils have a higher boiling point than water, the majority boiling above 140° C. (284° F.), their vapors diffuse readily in the vapors of water at the boiling temperature of the latter, and are thus easily carried over. Greater difficulty, however, is experienced with those volatile oils which have either an exceptionally high boiling point or a specific gravity near or exceeding that of water; the addition to the water of about 3 per cent, of table salt is then advisable, whereby the boiling point is somewhat raised, and, as the water distills over, it may be gradually increased to about 109.5° C (229° F.).

The volatile oils of cloves, cinnamon, santal wood, etc., are thus more readily obtained than with water alone.

"Cohobation is the returning of the aqueous distillate from which the volatile oil has been separated, either upon the same or upon fresh portions of material, and distilling again. The process is rendered necessary with cloves, santal wood, etc., the firm tissue of which, containing the volatile oils, is not easily ruptured or thoroughly permeated by water, and with the petals of rose and similar articles which contain only a minute proportion of volatile oil.