DISTAFF, in the early forms of spinning, the “rock” or short stick round one end of which the flax, cotton or wool is loosely wound, and from which it is spun off by the spindle. The word is derived from the Old English distaef, the first part of which is connected with dizen, in modern English seen in “bedizen,” to deck out or embellish, originally “to equip the distaff with flax, &c.,” cf. the German dialectal word Diesse, flax. The last part of the word is “staff.” “Distaff” from early times has been used to symbolize woman’s work (cf. the use of “spinster” for an unmarried woman); thus the “distaff” or “spindle” side of a family refers to the female branch, as opposed to the “spear” or male branch. The 7th of January, the day after Epiphany, was formerly known as St Distaff’s day, as women then began work again after the Christmas holiday.
DISTILLATION (from the Lat. distillare, more correctly destillare, to drop or trickle down), an operation consisting in the conversion of a substance or mixture of substances into vapours which are afterwards condensed to the liquid form; it has for its object the separation or purification of substances by taking advantage of differences in volatility. The apparatus consists of three parts:—the “retort” or “still,” in which the substance is heated; the “condenser,” in which the vapours are condensed; and the “receiver,” in which the condensed vapours are collected. Generally the components of a mixture will be vaporized in the order of their boiling-points; consequently if the condensates or “fractions” corresponding to definite ranges of temperature be separately collected, it is obvious that a more or less partial separation of the components will be effected. If the substance operated upon be practically pure to start with, or the product of distillation be nearly of constant composition, the operation is termed “purification by distillation” or “rectification”; the latter term is particularly used in the spirit industry. If a complex mixture be operated upon, and a separation effected by collecting the distillates in several portions, the operation is termed “fractional distillation.” Since many substances decompose either at, or below, their boiling-points under ordinary atmospheric pressure, it is necessary to lower the boiling-point by reducing the pressure if it be desired to distil them. This variation is termed “distillation under reduced pressure or in a vacuum.” The vaporization of a substance below its normal boiling-point can also be effected by blowing in steam or some other vapour; this operation is termed “distillation with steam.” “Dry distillation” is the term used when solid substances which do not liquefy on heating are operated upon; “sublimation” is the term used when a solid distils without the intervention of a liquid phase.
Distillation appears to have been practised at very remote times. The Alexandrians prepared oil of turpentine by distilling pine-resin; Zosimus of Panopolis, a voluminous writer of the 5th century a.d., speaks of the distillation of a “divine water” or “panacea” (probably from the complex mixture of calcium polysulphides, thiosulphate, &c., and free sulphur, which is obtained by boiling sulphur with lime and water) and advises “the efficient luting of the apparatus, for otherwise the valuable properties would be lost.” The Arabians greatly improved the earlier apparatus, naming one form the alembic (q.v.); they discovered many ethereal oils by distilling plants and plant juices, alcohol by the distillation of wine, and also distilled water. The alchemists gave great attention to the method, as is shown by the many discoveries made. Nitric, hydrochloric and sulphuric acids, all more or less impure, were better studied; and many ethereal oils were discovered. Prior to about the 18th century three forms of distillation were practised: (1) destillatio per ascensum, in which the retort was heated from the bottom, and the vapours escaped from the top; (2) destillatio per latus, in which the vapours escaped from the side; (3) destillatio per descensum, in which the retort was heated at the top, and the vapours led off by a pipe passing through the bottom. According to K. B. Hoffmann the earliest mention of destillatio per descensum occurs in the writings of Aetius, a Greek physician who flourished at about the end of the 5th century.
In modern times the laboratory practice of distillation was greatly facilitated by the introduction of the condenser named after Justus von Liebig; A. Kolbe and E. Frankland introduced the “reflux condenser,” i.e. a condenser so placed that the condensed vapours return to the distilling flask, a device permitting the continued boiling of a substance with little loss; W. Dittmar and R. Anschütz, independently of one another, introduced “distillation under reduced pressure”; and “fractional distillation” was greatly aided by the columns of Wurtz (1855), E. Linnemann (1871), and of J. A. Le Bel and A. Henninger (1874). In chemical technology enormous strides have been made, as is apparent from the coal-gas, coal-tar, mineral oil, spirits and mineral acids industries.
The subject is here treated under the following subdivisions: (1) ordinary distillation, (2) distillation under reduced pressure, (3) fractional distillation, (4) distillation with steam, (5) theory of distillation, (6) dry distillation, (7) distillation in chemical technology and (8) commercial distillation of water.
| Fig. 1. |
1. Ordinary Distillation.—The apparatus generally used is shown in fig. 1. The substance is heated in a retort a, which consists of a large bulb drawn out at the top to form a long neck; it may also be provided with a tubulure, or opening, which permits the charging of the retort, and also the insertion of a thermometer b. The retort may be replaced by a distilling flask, which is a round-bottomed flask (generally with a lengthened neck) provided with an inclined side tube. The neck of the retort, or side tube of the flask, is connected to the condenser c by an ordinary or rubber cork, according to the nature of the substance distilled; ordinary corks soaked in paraffin wax are very effective when ordinary or rubber corks cannot be used. Sometimes an “adapter” is used; this is simply a tapering tube, the side tube being corked into the wider end, and the condenser on to the narrower end. The thermometer is placed so that the bulb is near the neck of the retort or the side tube of the distilling flask. It generally happens that much of the mercury column is outside the flask and consequently at a lower temperature than the bulb, hence a correction of the observed temperature is necessary. If N be the length of the unheated mercury column in degrees, t the temperature of this column (generally determined by a small thermometer placed with its bulb at the middle of the column), and T the temperature recorded by the thermometer, then the corrected temperature of the vapour is T + 0.000143 (T − t) N (T. E. Thorpe, Journ. Chem. Soc., 1880, p. 159).
| Fig. 2. |