Distillation is the name given to the process of applying heat to a liquid, or, it may be, a solid, in order that certain constituents may pass away in vapour, and, by suitable arrangements, be obtained in the form of a liquid. When the vapour does not condense as a liquid, but only as a fine dust or flour, the process is called Sublimation (q.v.). The natural evaporation of water, by the heat of the sun, or warm air currents, the ascent of the vapour into the colder regions of the atmosphere, and the condensation there into clouds and mists, with the subsequent rain-shower, form together the grandest example of distillation. The apparatus for artificial distillation essentially consists of three parts, the Still (or RETORT, q.v.), Condenser, and Receiver. The still is made of glass, copper, iron, or earthenware, according to the nature of the substances to be placed in it. In experimental chemical work, glass is almost the only material admissible, while in the preparation of alcohol from grain, copper-stills are commonly employed. The condenser is made in an infinity of forms, the object being to condense the vapours disengaged from the still as rapidly and effectually as possible. For this purpose it is important that the condenser shall expose a large surface to the cooling medium, water or air. Owing to its rapidity in conducting heat, and the thinness of pipes made of copper, this metal is generally employed in the construction of condensers. Whatever the form, the principle is the same—viz. that the hot vapours pass through a tube or vessel surrounded with cold water, which running in a constant stream, passes away more or less warm after it has done its work. The receiver merely consists of a suitable vessel in which to receive the distillate. As the forms of apparatus are capable of infinite variety, so the conditions of distillation are innumerable; the best defined types going under the names of Fractional Distillation, Destructive Distillation, and Distillation in Vacuo.
In Fractional Distillation a mixture of liquids having different boiling-points is put in the still and heat is applied. If the vapours were allowed to pass over together and the distillate to be received in a single receiver, it is evident that nothing would be gained; for the different liquids would re-mingle in the receiver. If, however, the temperature be raised very slowly, and especially if the vapour requires to rise through a high head, or tube, before passing into the condenser, it will be found that the more volatile liquids pass over first in a state of comparative purity, and while the others follow as the heat is increased. If the receiver be frequently changed, a series of fractions, or portions of liquid, are obtained, corresponding to the different substances present in the still. For example, if a mixture of glycerine, water, alcohol, chloroform, and ether were distilled, the ether would distil first, then the chloroform, next the alcohol, and lastly, the water, while nearly all the glycerine would be left behind. This then is fractional distillation.
Destructive Distillation, or Dry Distillation as it is sometimes called, is best exemplified when coal is heated in an iron still or retort, as in the manufacture of gas. Now, in coal there are only traces of moisture capable of distillation, but when strongly heated, the coal is destroyed, or decomposed, and a large number of substances distil over, some of which, like lighting gas, are permanently in the gaseous state, others like Creasote (q.v.) are liquid, while others, such as naphthaline, are solid bodies at the ordinary temperature. Here no distillation takes place until the substance heated is destroyed, hence the term destructive distillation. Further examples are the distillation of wood in close vessels, at a red heat, when charcoal is left in the vessel, and wood-vinegar, wood-spirit, tar, &c. pass over in vapour, and are condensed; and the heating of bones in similar retorts, when animal charcoal is left in the retort, and Dippel's animal oil distils over. See CHARCOAL, COAL-TAR, and DIPPEL'S ANIMAL OIL.
Distillation in Vacuo.—When water is heated in a kettle it eventually boils, and if a thermometer be plunged into it, the mercury is seen to remain stationary at a temperature of about ( C.). It will be noticed, however, that when the barometer is low, the temperature of the boiling water is somewhat below ( C.), and that when the barometer is very high, the temperature is also high. On the summit of Mont Blanc, it is found that water boils at about ( C.); while in a vessel from which the air has been removed by an air-pump, it continues to boil even when the temperature falls down to the freezing-point (see BOILING.) There are many substances which are injured by heat, such as extract of malt and sugar, and when solutions of these bodies require to be evaporated on a large scale, they are always distilled in vacuo, so that the water may be removed without unduly heating the vessel.
There is another very interesting method of distillation whereby liquids which, alone, cannot be distilled without decomposition, are made to distil in presence of the vapour of some other liquid. Thus, glycerine is not, practically speaking, capable of being distilled, but when heated with high-pressure steam, it readily distils over, and can thus be purified. The great object of distillation is purification, and in the manufacture of the fragrant volatile oils of lemon, lavender, &c., it is necessary to distil them along with water so as to avoid overheating of the oils, and consequent injury. Not only is the flavour improved by this treatment, but they distil at a lower temperature along with water than when alone. The term distillation in excise language refers to the distillation of alcohol alone. For information on this subject, see FERMENTATION, ALCOHOL, BRANDY, GIN, SPIRITS, and especially WHISKY; under which latter head the subject of the duties payable and of illicit distillation are also treated.