DISTILLATION (from the

See also:

• LAT

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: (I) destillatio per ascensum, in which the retort was heated from the bottom, and the vapours escaped from the top; (2) destillatio per lotus, 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 . first type; N2O4-22NO2, of the second (see CHEMICAL ACTION) . Electrolytic or ionic dissociation is the separation of a substance in solution into ions (see ELECTROLYSIS; SOLUTION) .