The art of obtaining in a separate state, by the application of heat, the more volatile parts of bodies; but the term is generally limited to signify the separation of volatile liquids, for when the volatile product is obtained from a solid, and assumes a solid form, the operation is termed sublimation. In the distillation of liquids the most volatile parts rising in vapour first are conducted to variously disposed refrigerators, usually composed of metal, and surrounded with cold water, which, abstracting a portion of heat from the vapour, it becomes condensed, and assumes a liquid form. One of the principal applications of the art of distillation is the preparation of spirituous liquors, which is usually divided into two branches. The first, termed distillation, consists in separating the spirituous parts of fermented liquors, mixed with a large portion of water, from the fixed or nonvolatile parts; and in the latter branch, termed rectification, the spirit is concentrated and purified principally by means of redistillation.
Having already treated largely upon the various methods of effecting this, under the head Alcohol, we shall in this place limit ourselves principally to the description of the preparatory process of distillation, together with some of the most approved apparatus employed therein.
In London and its neighbourhood the process of forming the wash for distillation is the same as in brewing for beer, except that no hops are used, and that instead of boiling the wort they pump it into coolers, and afterwards draw it into backs, to be then fermented with yeast. During the fermentation, considerable attention should be paid to the temperature of the liquid, which should be steadily maintained at about 70° Fahr., and the fermentation is continued until the liquor grows fine and pungent to the taste, but not so long as to allow acetous fermentation to commence. In this state the wash is put into the still (of which it should occupy about three-fourths), and distilled with a gentle fire as long as any spirit comes over, which is generally until about half the wash is consumed. The form of the common still is too well known to need any particular description. It generally consists of a large boiler made of copper, and fixed in masonry over a fire-place. The boiler has a head, or capital, as it is called, which is of a globular form, to which is soldered a neck, forming an arch curved downwards, and fits into what is called the worm: this is a long tube, made generally of pewter, of a gradually increasing diameter; it is curled round in a spiral form, and enclosed in a tub, which is kept filled with cold water during distillation.
That celebrated philosopher and mechanic, the late Mr. Watt, having ascertained that liquids boil in vacuo at a much lower temperature than when under the pressure of the atmosphere, endeavoured to turn this circumstance to advantage in distillation, under the idea that less fuel and also less water for condensation would be required; but found, by experiment, little or no advantage in this respect, the latent heat of the vapour being nearly the same, whether formed in vacuo or under the pressure of the atmosphere. The idea of distillation in vacuo was subsequently taken up by Mr. Tritton, as affording a means of preventing any empyreumatic flavour being imparted to the spirit by the burning of any matter contained in the still, as a heat considerably less than 212o Fahr, is sufficient to cause the wash to boil rapidly in vacuo. The annexed diagram exhibits a section of Mr. Tritton's apparatus for distilling in vacuo. A is the body of the still; B is a water bath, into which the body of the still is immersed; C is the head or capital, D the neck of the same, which, curving downwards, is connected with a pipe that enters the condensing vessel E; F is a refrigeratory or close vessel, containing cold water, for converting into liquid the spirituous vapours, which, having been raised in the still, are contained in the vessel E.
From the bottom of the vessel E a pipe issues, for conveying the liquid and the vapour not yet condensed into vessel G, which being surrounded with cold water contained in the vessel H, acts also as a refrigeratory, and reduces the whole of the remaining vapour into a liquid state. l is an air pump for effecting a vacuum in the vessels A E G; K is a stop-cock for cutting off the communication between the vessels E and G, when the contents of G are drawn off by the cock M, by which means a vacuum is preserved during that operation in the vessel E and the still A. L is an air cock, to admit air into the vessel G, to allow the contents to run out at M; N is the discharge cock to the still A. It will be seen that the greatest heat to which the matter in the still can be subjected can never exceed 212° Fahr.; but upon the pressure of the atmosphere being removed by means of the air-pump, the distillation is effected at the low temperature of 132° Fahr., by which means all injury to the flavour of the spirit, by carbonization of the matters contained in the still, is entirely avoided.
Dr. Arnott, in his work on the Elements of Physics, proposes a mode of distilling or evaporating in vacuo, without the aid of an air pump, by simply establishing a communication between a close distilling or evaporating vessel, and the top of a water barometer. The principle of this method will be readily comprehended by referring to the annexed diagram and its accompanying explanation, a is the evaporating vessel or still, the neck of which communicates with a strong vessel b, forming the top of the barometer; from the under side of b proceeds a tube, plunging in a small vessel d, situated 36 feet below the bottom of b. The cocks at d and e being shut, the vessel b and the descending pipe are to be filled with water through a cock c at the top; then this cock being shut, and the cock at d opened, the water will sink down out of the vessel b until the column in the tube be only 34 feet high, as at f, that being the height which the atmosphere will support. On opening a communication between the vessel a and the vacuum in b, the operation goes on as desired, and the steam arising from a may be constantly condensed by allowing a small stream of water to run through b from above, in cases where it is sought to concentrate any liquid in a in vacuo; but for distillation, where the condensed vapour is the product which is sought, the water must be applied externally to b, by placing that vessel in another vessel g, kept constantly full of water.