In pharmacy it is the separation of the more volatile from the more solid parts of any substance by means of heat. The operation itself consists of the condensing and collecting the lighter parts of bodies, previously rarefied by heat, and thus separated from the less volatile. Re-distilling a fluid several times from fresh parcels of the same kind, is called cohobatio; but little advantage is derived from this practice. When distillation is repeated, in order to purify or separate the matter distilled from some parts of less value, it is called rectification. Distillation with an alembic or a common still is called per ascensum, because the vapours rise and are condensed in the upper part of the vessel; and in this way all distillations may be performed that require no greater heat than boiling water. When a greater heat is required, retorts may be used; and as from their shape the volatile parts can only escape through the side, it is called per latus. When the heat is applied above the bodies to be distilled, and the lighter parts forced downwards, it is called per descensum: this method is now never used in pharmacy, though occasionally in the arts. When the volatile parts, rarefied by distillation, are dry, the operation is called sublimation. When no more heat is applied than is necessary just to raise a vapour, which when condensed only falls in drops, it is called a cold distillation: roses and other substances valued only for their flavour, and which do not admit of drying, are advantageously distilled in this way; and the dry cake, left after distilling roses, is well adapted for making a.decoction or syrup: in this kind of distillation, the subject should neither be bruised nor have any water added to it: they should be gathered with the morning dew- upon them.: and a retort placed in a sand bath, with a receiver, is the best apparatus. The worm still is more frequently used, and called the still, because the materials boil; it communicates with a leaden spiral tube (the worm), placed in a tub filled with cold water (the refrigeratory); in this worm the vapours are condensed, and run out in a small stream into whatever vessel is placed to receive it.
The end of distilling is the separation of volatile substances from those with which they were mixed; as in obtaining vinous spirits, essential oils, volatile spirits. etc or for the more speedy or effectual combination of such bodies as require a boiling heat for their union.
As a great object in distillation is to apply no more heat than is necessary to accomplish our intention, retorts are sometimes used, and are placed on an open fire. They are placed also in sand or in water, that the 4 D 2 heat "may be more certainly adjusted to the degree of volatility which the subject to be distilled possesses. In distilling water, the menstruum should be attended to, as well as the heat to be applied; for, as some essential oils require the full heat of boiling water, they cannot be raised by the use of spirits of wine: this happens in distilling oil of cinnamon, and some other ponderous oils.
Retorts are proper when the subject to be distilled would corrode the metal of a still, as in the preparing a mineral acid, or other corrosive matters. Earthen vessels are sometimes used, and, on some few occasions, iron ones. But these, and many other observations on this subject, are fully noticed under the articles where an attention to them may be required.
Distillatio per descensum. See Descensio.