1902 Encyclopedia > Distillation


DISTILLATION, a generic term for a class of chemical operations which all agree in this, that the substance operated upon is heated in a close vessel (" retort," " still") and thereby wholly or partially converted into vapour, which vapour is then condensed, by the application of cold, in another apparatus (the " condenser ") connected with the vessel, and allowed to collect in a third portion of the apparatus, called a " receiver." In most cases the substance is a liquid, or assumes the liquid form previous to emitting vapours, and the product obtained (the "distillate ")is also in greater proportion liquid. The comparatively few and special cases of distillation, wherein solids are converted into vapours which condense directly from the gaseous into the solid form, are designated " sublimations." Thus we speak of the " distillation " of water or of spirits, while we speak of the " sublimation " of sal-ammoniac. Distillations may be divided into two classes—viz., 1st, those which are not, and 2d, those which are, accompanied by chemical changes. The word "distillation," in a narrower sense, is generally understood to apply to the first class only. The second might be called " destructive distillations," if it were not customary to reserve this term for the particular case in which the substance operated on consists of vegetable or animal matter which is being decomposed by the application of heat alone, i.e., without the aid of re-agents.

The general object of simple distillation is the separation of substances of different degrees of volatility. The apparatus used varies very much according to the nature of the substance operated on and of the product extracted, and according to the scale on which the operation is carried out. Of the various contrivances used in chemical laboratories, the simplest is a glass retort, the descending neck of which is inserted into, and goes to near the bottom of, a slanting globular flask. The retort serves for the reception of the substance to be distilled, and is heated by means of char-coal or gas fire ; the vapours pass into the flask, which is kept cool by a continuous current of cold water running over it, or, in the case of more volatile substances, by being immersed in ice or some freezing mixture. This somewhat primitive arrangement works satisfactorily only when the vapours are easily condensible, and when the product is meant to be collected as a whole. In the majority of cases, however, the distillate has to be " fractionated," i.e., collected in a number of separate, consecutive portions; and it is then desirable that the apparatus should be so constructed as to enable one at any moment to examine the distillate as it is coming over. For this purpose it is necessary to condense the vapours on their way to, and not within, the receiver, so that the latter can, at any time, be removed and replaced by another. The condenser most generally used in chemical laboratories is that known as Liebig's condenser. It consists of a straight glass or metal tube, 1 to 3 feet long and J to 1 inch wide, fitted co-axially, by means of corks or india-rubber tubes, into a wider tube (made of glass or iron) which communicates at the lower end with a water tap, and at the upper with a sink, so that a stream of cold water can be made to run against the current of the vapour. The condenser tube is fixed in a slanting position, and the vapours made to enter at the upper end. The dimensions of the condenser and rate of water-flow depend on the speed at which the vapour is driven over, and on the temperature of that vapour, and, last not least, on the latent heat of the vapour and specific heat of the distillate. To show the importance of the last-named point, let us compare the quantities of heat to be withdrawn from 1 lb of steam and 1 lb of bromine vapour respectively, to reduce them to liquids at 0° C. We have in the case of water and bromine—
Water. Bromine.
For the temperature of the vapour 100° 63°
For the latent heat 536° 45°'6
For the specific heat of the liquids 1° 0°'106
For the total heats of the vapours 636" 52°'3
The withdrawal of 52'3 units of heat from lib of bromine vapour reduces it to liquid bromine at 0° C. By the with-drawal of (-j^- x 52-3 =) 83 units from the steam, as an easy calculation shows, only 0T6S) of liquid water, of even 100°, could be produced—hence more than 0-84 lb of steam remains uncondensed (at a temperature of about 96° G, assuming the steam to remain saturated, and to have the temperature of the condensed water). But obviously a condenser under all circumstances is the more efficacious the greater its surface and the thinner its body. It is also obvious, cceteris paribus, that the most suitable material for a condenser tube is that which conducts heat best. Hence a metal tube will generally condense more rapidly than one of glass, and for metal tubes copper is better than tin, and silver better than either. In chemical laboratories glass is the only material which is quite generally applicable. In chemical works, on the other hand, glass, on account of its fragility, is rarely used; condensers there, wherever possible, are made of metal, usually fashioned into spirals (" worms ") and set in tub-shaped refrigerators Where acids have to be condensed, stoneware worms are generally employed. In the distillation of acetic acid pla-tinum worms, notwithstanding their high price, have been found to work best, and in the long run to be cheapest.

The theory and successful execution of the process assume their greatest simplicity when the substances to be separated differ so greatly in their volatility that, without appreciable error, one can be assumed to be non-volatile at the boiling point of the other. A good illustration of this special case is afforded by the customary process used for the purification of water. A natural sweet water may in general be assumed to consist of three parts—1st, water proper, which always forms something like 98 per cent, or more of the whole; 2d, non-volatile salts ; 3d, gases. To obtain pure water from such material, we need only boil it in a distillation apparatus, so as to raise from it dry steam, which steam when condensed yields water con-taminated only with the gases. To expel these all that is necessary is to again boil it for a short time; the gases go off with the first portions of steam, so that the residue, when allowed to cool in absence of air, constitutes pure water. To pass to a less simple case, let us assume that the substance to be distilled is a solution of ether in water, and the object is the separation of these two bodies. Ether boils at 35° O, water at 100° C. The elastic force of saturated steam at 35° is 42 mm., = -/^ = ygth of an atmosphere. Assuming now the mixture to be distilled from a flask, what will go on ? Neglecting for the sake of simplicity the small tension of the steam at 35°, we should expect that at first the ether would simply boil away, so to speak, from a bath of warm water at 35° C. ; that the vapour would be pure ether, and maintain that composition until all the ether had boiled off ; then there would be a break—the tempera-ture of the liquid would gradually rise to 100°, and the water then distil over in its turn. And so it is approxi-mately, but not exactly. Our theory obviously neglects some important points. Water at 35° has a tension of -j^th atmosphere, ether of one atmosphere; hence the two saturated vapours together should press with a force of l-j5gth atmosphere—in other words, the mixture should com-mence to boil at less than 35°. This, however (as in the majority of analogous cases), is not confirmed by experi-ment. The mixture commences to boil at a little above 35°, and the boiling point rises steadily as the proportion of ether in the liquid decreases. Now, a priori, we should presume that at every given moment the volumes of ether and water in the vapour should be, approximately at least, proportional to the respective vapour tensions at the temperature at which the mixture happens to boil. Thus, for instance, assuming at the first that the liquid boils at 40° O., when the two tensions are equal to 910 and 55 mm. respectively, the vapour will contain ^0^rgg = 0 94 of its volume of ether vapour, and 0'06 of its volume of steam, supposing both substances to have the same chances of forming saturated vapour, which, of course, holds only so long as they both are present in appreciable quantities. We easily see that, as the distillation progresses, the ether vapour must get more and more largely charged with vapour of water, until at last what goes off is steam, con-taminated with less and less of ether vapour. A thermometer placed near the entrance end of the condenser will, of course, record lower than one plunged into the boiling liquid, because the vapour in rising undergoes partial condensation, and the thermometer being bedewed with the condensed vapour will approximately indicate tho boiling point of that dew, i.e., of that which is just going over. The composition of the vapour as above given must not be confounded with the composition by weight of the distillate. To obtain the latter we must multiply each of the two volumes by the density of the respective vapour, or, what comes to the same thing, by its molecular weight as expressed by the chemical formula. In our case the vapour volume ratio

water 55 ether 91~0
corresponds to the weight ratio
55 x H2Q 55 x 18 Jl_ 901 xC4H1()0~ 910 x 74 - 68 nearly"

This consideration strips of its apparently anomalous character what we observe when vegetable substances con-taining essential oils are distilled with water, when we find that these oils, although boiling far above 100° C, go over with the first fractions of the water. Take the case of lemon oil, which boils at about 174° C. The molecular weight of the oil is 136 = C10H16; its vapour tension at 100° is 70 mm. Hence what goes over at first when lemon, peel is distilled with water should contain oil and water in the proportion—

Oil of Lemons. Water.
Mol. Vap. Mol. Vap. wt tension. \vt. tension.
136 x 70 : 18 x 760 = 12 : 17 (nearly). The oil, although the less volatile substance of the two, being present in small quantity, but finely diffused, is soon completely driven over. No doubt the latent heats of vaporization of the two constituents have some-thing to do with the composition of the vapour formed, as the chance of every particle of the mixture to be" vaporized is obviously the greater the less its latent heat of vaporization.

After what has been said it will be clear that in the dis-tillation of a mixture of two substances of approximately equal molecular weight and latent heats of vaporization, supposing neither to predominate overwhelmingly over the other, the one with the lower boiling point will predominate in the early, and the other will gradually accumulate in the later, fractions of the distillate. And similarly with mixtures of three or more bodies. The further the respec-tive boiling points are removed from one another the more complete a separation can be effected ; but in no case is the separation perfect. It is, however, easily seen that the analytic effect of a distillation can be increased by causing the vapour, before it reaches the condenser, to undergo partial condensation, when naturally the less volatile parts chiefly will run back. This artifice is largely employed by chemists, technical as well as scientific. The simplest mode is to let the vapour ascend through a long, vertical tube before it reaches the condenser, and to distil so slowly that a sufficiently large fraction of the vapour originally formed fails to survive the ascent through the cooling influence of the atmosphere. A more effective method is to let the condensed vapour accumulate in a series of small receptacles inserted between flask and condenser, constructed so that the vapour cannot pass through the receptacles without bubbling through their liquid contents, and so that the liquid in the receptacles cannot rise above a certain level, the excess flowing back into the next lower receptacle or into the still. But the most effective method is to let the vapour ascend through a slanting condenser kept by means of a bath at a certain temperature, which is controlled so that while the liquid in the flask boils rapidly, the dis-tillation only just progresses and no more.

The general principles thus stated regarding fractional distillation are liable to not a few exceptions, of which the following may be cited as examples. A solution of one part of hydrochloric acid gas in four parts of water boils (constant) at 110° C.—i.e., 10° above the boiling point of water, although the acid constituent is an almost permanent gas. This, however, is easily explained; there can be no doubt that such an acid is a mixture of real hydrates, i.e., does not contain either free water or free hydrochloric acid. A similar explanation applies to the case of aqueous oil of vitriol, which boils the further above 100° the stronger it is, although the vapour may be, and in the case of acids contain-ing less than 84 per cent, of real acid really is, pure steam. The following cases, however, can scarcely be disposed of by the assumption of the interference of chemical action. Propyl alcohol boils at 97° C, water at 100°; and yet a mixture of the two, as Pierre and Puchot found, when distilled always commences to boil at 88°'5 with formation of a distillate of the approximate composition C3H80 + 2-78H20 ; and this particular aqueous alcohol boils without apparent decomposition at 88°-3. Some time later Dittmar and Steuart made a precisely analogous observation with regard to aqueous allyl alcohol. A strong temptation exists to explain these anomalies by the assumption of definite hydrates in the aqueous alcohols, and this hypothesis would serve in the meantime were it not for the curious fact, discovered by the two French chemists named, that amyl alcohol and water (two liquids which do not mix), when distilled simultaneously out of the same retort, go over at a constant temperature less than 100°, and with formation of a distillate which, although it is not even a mixture, has a constant composition. The most natural explanation of these phenomena is to assume them to be owing, not to chemical action, but rather to an exceptional absence of chemical affinity between the two components of the mixture, which for once gives the physical forces fair play.

DRY (DESTRUCTIVE) DISTILLATION.—Of the great number of chemical operations falling under this head, we can notice only those which are carried out industrially for the manufacture of useful products. Of such the most important are those in which wood, coal, shale, and bones form the materials operated upon. But as these processes form so many important industries, which have all special articles devoted to them, we must confine ourselves here to summing up shortly the features common to all.

In all cases the " retorts " consist of iron or fire-clay semi-cylinders placed horizontally in a furnace and con-nected by iron pipes with refrigerators, and through these with gas-holders. Within these retorts the materials are brought up, more or less gradually, to a red heat, which is maintained until the formation of vapours practically ceases. Each of the materials named is a complex mixture of different chemical species. Wood consists mainly of cellulose and other carbo-hydrates, i.e., bodies composed of carbon and the elements of water; in coal and shale the combustible part consists of compounds of carbon and hydrogen, or carbon, hydrogen, and oxygen, richer in carbon than the components of wood; bones consist of about half of incombustible and infusible phosphate of lime (bone earth) and half of organic matter, of which the greater part is gelatine (compounds of carbon, nitrogen, hydrogen, and oxygen), and the lesser is fat (compounds of carbon, hydrogen, and oxygen). The chemical decomposi-tion in each case is highly complex. An infinite variety of products is invariably formed, which, however, always readily divide into three :—1st, a non-volatile residue, con-sisting of mineral matter and elementary carbon (" wood charcoal," " coke," &c.) which, in the case of animal matter, contains chemically combined nitrogen; 2d, a part condensible at ordinary temperatures which always readily separates into two distinct layers, viz. :—(a) an aqueous portion (" tar-water "), and (b) a semifluid, viscid, oily, or resinous portion (" tar"); and 3d, a gaseous por-tion.

The "tar-water" is the one, of all the four products, of which the qualitative composition most directly depends on the nature of the material distilled. In the case of wood it has an acid reaction, from the presence in it of acetic acid, which is associated (amongst many other things) with acetone and methyl alcohol. In the case of coal it is alkaline, from ammonia, present as carbonate, sulphide, sulphocyanide, and in other forms. Alcohols and oxygenated acids are absent.

The " tar" is a complex mixture of carbon com-pounds, all combustible, but, although all directly derived from a vapour, not by any means all of them volatile. (Regarding the components, see TAR.) The quantity and quality of the tar naturally depend on the kind of material used, but perhaps yet more on the mode in which the dis-tillation is conducted. Thus, for instance, a coal tar pro-duced at low temperature contains a considerable per-centage of paraffins. If, on the other hand, the dis-tillation is conducted at a high temperature, the paraffins are almost absent, while the proportion of benzols con-siderably increases. A similar remark applies to the gaseous portion, as will readily be understood when we say that all volatile tar constituents, when passed through red hot tubes, are decomposed with formation of hydrogen and gaseous hydrocarbons, which latter again, when submitted to the same operation, are all liable to undergo dissociation into simpler compounds and associa-tion into more complex.

DISTILLATION OF WATER.—The continual interchange and circulation of water, between oceans and other great reservoirs of water on the one hand and dry land on the other, may be regarded as a process of distillation. Rain is thus a form of distilled water; and when it falls through a pure atmosphere it is found to possess the softness and freedom from dissolved salts characteristic of water artificially distilled. Rain water, however, absorbs a considerable proportion of air and some carbonic acid from the air, and also frequently contains ammonia, salts, and free acids.

Water of that purity which can be secured only by dis-tillation is of indispensable value in many operations both of scientific and industrial chemistry. The apparatus and process for distilling ordinary water are very simple. The body of the still is made of copper, with a head and worm, or condensing apparatus, either of copper or tin. The first portion of the distillate brings over the gases dissolved in the water, ammonia, and other volatile impurities, and is consequently rejected, and scarcely two-fifths of the entire quantity of water can be with safety used as pure distilled water.

Among the innumerable schemes which have been pro-posed for the production of a potable fresh water from the salt water of the ocean, two or three dependent on simul-taneous distillation and aeration have been found, in practice, to produce most satisfactory results. Of course the simple distillation of sea water, and the production thereby of a certain proportion of chemically fresh water, is a very simple problem; but it is found that water which is merely evaporated and recondensed has a very disagree-able empyreumatic odour, and a most repulsive flat taste, and it is only after long exposure to pure atmospheric air, with continued agitation, or repeated pouring from one vessel to another, that it becomes sufficiently aerated to lose its unpleasant taste and smell and become drinkable. The water, moreover, till it is saturated with gases, readily absorbs noxious vapours to which it may be exposed. For the successful preparation of potable water from sea water, therefore, the following conditions are essential:—1st, aeration of the distilled product so that it may be immedi-ately available for drinking purposes; 2d, economy of coal to obtain the maximum of water with the minimum expenditure of fuel; and 3d, simplicity of working parts, to secure the apparatus from breaking down, and enable unskilled attendants to work it with safety. Among the forms of apparatus which have most fully satisfied these conditions are the inventions of Dr Normandy and of Chaplin of Glasgow. While these have met with most acceptance in the United Kingdom, the apparatus of Rocher of Nantes, and that patented by Gall! and Mazeline of Havre, have been highly appreciated by French maritime authorities.

Normandy's apparatus, while leaving nothing to be desired in point of economy of fuel and quality of water produced, is very complex in its structure, consisting of very numer-ous working parts, with elaborate arrangements of pipes, cocks, and other fittings. It is consequently expensive, and requires for its working the careful attention of an ex-perienced workman. It consists of three essential parts, in addition to any convenient form of boiler from which steam under a certain amount of pressure may be obtained. These parts are called respectively the evaporator, the con-denser, and the refrigerator. These are all closed cylindrical vessels, permeated internally with sheaves of pipes, through which pipes the steam generated percolates, condenses, and is aerated as explained below. The refrigerator is a horizontal vessel above which the condenser and the evaporator are placed in a vertical position. When the apparatus is in operation the refrigerator and condenser are filled with sea water, and a constant current is main-tained which enters by the refrigerator, passes upwards through the condenser, and is discharged by an overflow pipe at a level a little above the top of the condenser. The evaporator is filled only to about two-thirds of its height with water from the condenser, and the admission and regulation of its contents are governed by a stop-cock on the pipe communicating between the two vessels. The vessels being so prepared, superheated steam is admitted by a pipe leading from the boiler into the top of the evaporator, and, passing through the sheaf of pipes immersed in water, is there condensed. The condensed water passes direct from the evaporator into the pipes of the refrigerator, in which it is cooled to the temperature of the surrounding sea water. Here then is produced pure distilled but non-aerated water; and the means by which it is aerated and rendered fit for immediate use may be now traced. The superheated steam in permeating the pipes in the evaporator heats and vaporizes a portion of the water around them. The steam so generated passes into the sheaf of pipes in the condenser, in which, as already explained, a current of water is constantly rising and pass-ing away by the overflow pipe. The condensation of the steam within the pipes, again, communicates a high temperature to the upper stratum of water in the condenser. As water at a temperature of 54°-5 C. parts with its dis-solved air and carbonic acid gas, a stream of water is con-tinually rising to the upper part of the condenser at a temperature more than sufficient to liberate these gases, and by means of a pipe these pass over into the upper part of the evaporator, and there mingle with and supersaturate the steam generated in that vessel. Instead, therefore, of it being simply steam which passes from the evaporator to the tubes of the condenser, it is a mixture of steam and gases, the latter being in sufficient quantity not only to supersaturate the steam with which they are mixed, but also fully to aerate the condensed steam which passed direct from the evaporator into the refrigerator. The super-aerated condensed steam passes from the pipes in the condenser into those in the refrigerator, where it meets the non-aerated water from the evaporator pipes, the course of which has already been traced. Here the two products mingle, cool down to the temperature of the sea, and passing outwards through a filter, may be drawn off as pure aerated water of excellent quality. In Dr Normandy's apparatus the combustion of 1 lb of coal yields from 14 to 20 lb of potable water. The apparatus is extensively adopted in the British navy, the Ounard line, and many other important emigraut and mercantile lines.

Chaplin's apparatus, which was invented and patented later, has also, since 1865, been sanctioned for use on emi-grant, troop, and passenger vessels. The apparatus possesses the great merit of simplicity and compactness, in con-sequence of which it is comparatively cheap and not liable to derangement. In addition to a boiler for generating steam from sea water the apparatus consists of an aerator, a con-denser, and a filter. The condenser is a cylinder, usually of cast iron with an internal worm pipe of copper, which is found to be the only really suitable metal for this use, The steam to be condensed is admitted to this worm or coil through the aerator. This part of the apparatus—the aerator—is really the essential feature in the invention, and consists simply of a series of holes perforated around the steam inlet pipe at the point where it enters the con-denser. The steam passing down in a powerful jet draws with it through these holes a proportion of atmospheric air sufficient to properly aerate the water for drinking purposes. The steam and air thoroughly commingled are together condensed as they pass through the coils of the worm,—cold sea water passing in to the condenser at its lowest end, and rising upwards and flowing away at the top. After passing through the filter placed directly under the condenser, the aerated water is delivered or stored ready for use, clear, bright, colourless, palatable, and devoid of odour, at a temperature of about 15° C. The cold sea water for condensing may be forced into the condenser by a special steam pump attached to the apparatus—a plan usually followed on sailing vessels—or any other convenient pumping arrangement may be resorted to. The steam for condensation is, in steamers, frequently supplied from the engine boilers; but generally it is preferable to employ a special small upright boiler, or to use the boilers attached to steam winches. Chaplin's apparatus has been adopted by many important British and Continental shipping com-panies, among others by the Peninsular and Oriental, the Inman, the North German Lloyd, and the Hamburg American Companies.

DISTILLATION OF SPIRITS.—Notwithstanding the enormous scale on which this industry is now prosecuted, it is only in modern and comparatively recent times that it has attained to the important position which it now occupies. The art of separating alcoholic spirit from fermented liquors appears, however, to have been known in the far East from the most remote antiquity. It is supposed to have been first known to, and practised by, the Chinese, whence a knowledge of the art gradually travelled westward. A rude kind of still, which is yet employed, has been used for obtaining ardent spirits in Ceylon from time immemorial. The name alcohol indicates that a knowledge of the method of preparing that substance probably came to Western Europe, like much more chemical knowledge, through the Arabs. Albucasis, who lived in the 12th century, is spoken of as the first Western philo-sopher who taught the art of distillation as applied to the preparation of spirits ; and in the 13th century Baymond Lully was not only well acquainted with the process, but also knew the method of concentrating it into what he denominated aqua ardens by means of potassic carbonate. At the time when Henry II.—in the 12th century— invaded and conquered Ireland, the inhabitants were in the habit of making and using an alcoholic liquor—usquebagh (uisge-bedtha, water of life), a term since abbreviated into whisky, which consequently is synonymous with the classical aqua vitce. It is further a noticeable fact that Captain Cook found, among the inhabitants of the Pacific Islands discovered by him, a knowledge of the art of distilling spirit from alcoholic infusions.

The preparation of ardent spirit involves two separate series of operations:—1st, the making of an alcoholic solution by means of vinous fermentation ; and 2d, the concentration of the alcoholic solution so obtained by the process of dis-tillation and rectification.
All substances in nature which contain sugar in any of its forms are susceptible of undergoing vinous fermentation, and may therefore be used as sources of alcohol. Further, all starchy substances and ligneous tissue, seeing that by various chemical processes starch and cellulose may be converted into grape sugar, may also be used for the pre-paration of alcohol. It is thus obvious that the variety of organic substances, especially of the vegetable kingdom, from which alcohol may be elaborated is almost endless ; and in practice it is found that the sources employed are very numerous. Commercially, distilled alcoholic liquors are manufactured of varying strength, or proportion of alcohol to water, according as the spirit is intended to be used for drinking purposes or for employment in the arts. The standard by which excise duty on alcoholic liquor is charged in Great Britain is proof spirit, in which the alcohol and water are in almost equal proportions by weight, there being in 100 parts 49-24 of absolute alcohol, and 50'76 of water. Distilled spirits are said to be " over proof" when the proportion of alcohol is greater, and " under proof " when there is more water present than is indicated by " proof." Thus a spirit 11 over proof (o.p.) is a compound which requires the addition of 11 volumes of water to every hundred to reduce it to proof strength; and similarly 10 under proof (u.p.) indicates a liquor from every 100 gallons of which 10 gallons of water must be withdrawn to bring it to proof strength. Spirit for drink-ing is seldom sold at more than 11 over proof, from which it varies downward to 25 and more under proof. Bum, however, is manufactured and imported as highly concen-trated as from 10 to 43 over proof. Spirit of wine as used in the arts must be at least 43 over proof, and generally it is sold at from 54 to 64 over proof.

The alcoholic liquors enumerated below are those most commonly distilled for drinking or medicinal purposes. Brandy, when genuine, is a spirit chiefly distilled in France from wine. Bum is made from molasses or treacle, and is. distilled in the West Indies, and generally in all countries where the sugar cane is cultivated. From fermented infusions of grain, malted and unmalted, and chiefly from barley, whisky is distilled, and that spirit when " silent " or flavourless is the basis of flavoured spirits, such as gin and factitious or British brandy. Arrack is an Oriental spirit distilled from " toddy," or the fermented juice of certain palm trees, and also from rice, which grain is the source of sak6, the national spirit of the Japanese. Potato brandy is very extensively prepared from the fecula of potatoes in Germany and Russia, and is a spirit much used for fortifying wines, and for making factitious wine, as well as in the arts. Beet root, carrots, Jerusalem artichokes, and several other saccharine roots are also used for the distilla-tion of spirit on a commercial scale. The only example of a spirit drawn from animal sources is the koumiss of the Tartars, which is distilled from the fermented milk of mares.

The modifications of stills or of distilling apparatus used in the preparation of alcoholic liquor are exceedingly i numerous, and many of the later inventions are of most complicated structure. The simple and primitive varieties of apparatus yield only a comparatively weak spirit on the first distillation, while the effect of the complex appliances now generally used is to produce, in one operation, a highly concentrated spirit, and that with a great saving of fuel, time, and labour. All varieties of distillatory apparatus resolve themsel ves under these heads:—1 st, stills heated and worked by the direct application of the heat of a fire ; 2d, stills worked by the action of steam blown direct into the alcoholic solution from a steam boiler; and 3d, stills heated by steam passing in coiled pipes through the alcoholic solutions to be acted upon.

To the first of these classes—stills heated by direct fire— belong the earliest and simplest forms of distillatory apparatus; and for producing particular classes of alcoholic liquor, stills very simple in their construction are yet employed. The common still is a flat-bottomed, close vessel of copper, with a high head to prevent the fluid within boiling over. To the top of this head a tube is connected, which is carried in a spiral form round the inside of a tub or barrel (the condenser or refrigerator), filled with cold water, and from its twisted form this tube receives the name of the " worm." The tube terminates at the bottom of the barrel, passing through it to the outside, and is con-ducted into the vessel termed the receiver, a stopcock, or more commonly a vessel termed a "safe," being usuallyplaced on the tube where it leaves the refrigerator. In distilling with an apparatus of this simple construction, it is obvious that at the beginning of the operation, when the wash or liquid to be distilled is rich in alcohol, and its boiling point consequently low, the distillate will pass over at a low temperature and contain a high percentage of alcohol. But as the operation progresses, the boiling point of the mixture in the still rises, the heat has therefore to be forced, and the quantity of watery vapour which passes over with the alcohol is proportionately increased. As the wash or liquid in the still continually weakens, a point is arrived at when the value of the weak distillate produced will not balance the expenditure on fuel for maintaining the heat of distillation.

One of the earliest devices for economizing the heat of distillation consisted in interposing between the still and the refrigerator a wash warmer, or vessel charged with liquid ready for distillation. Through this vessel the pipe conveying the hot vapours to the refrigerator coil passed, and the vapours, partly condensing there, heated up the wash, which was thus prepared to pass into the still at an elevated temperature. The " pot" stills, in which the markedly flavoured Irish whisky is made, are of this con-struction. In the great establishment of the Banagher Distillery Company, King's co., Ireland, simple stills of a capacity of 20,000 gallons are erected having a rousing apparatus within them to keep the wash in agitation so as to prevent solid particles from settling on the bottom and burning. Beyond a wash warmer, or intermediate charger interposed between the still and the condenser, there is no other appliance attached to the apparatus. The first dis-tillate from the still is termed " low wines," and passes into the "low wines receiver," whence it passes into No. 1 " low wine still " to undergo a second distillation. The product of the second distillation, under the name of " faints or feints," is caught in the " faints receiver," from which it passes to No. 2 low wines still, and from this it is discharged as Irish whisky.

The introduction of another principle into distillatory apparatus is illustrated by Dorn's still, which was intro-duced into Germany in the early part of the century, and is yet much used in smaller establishments in that country. In that apparatus the vessel, of copper, interposed between the still and the condenser is divided horizontally into two unequal compartments by a diaphragm of copper. The upper and larger portion acts as a wash warmer (German, Vorwdrmer), and through it the pipe from the still body coils, opening into the lower division. For a time the whole distillate condenses in this division, but as the temperature of the wash in the upper division rises, and the heat of the more watery distillate from the still also increases, the condensed liquor in the lower division in its turn begins to boil, and undergoes a second distillation or rectification, the vapours from it passing onwards to be con-densed in the ordinary refrigerator. In many forms of distillatory apparatus two or more such rectifiers are placed between the primary still and the final condenser. The principle of the rectifier is easily understood. Supposing the operation of distilling to commence, the vapours which condense in rectifier No. 1 are much richer in alcohol than the liquid remaining in the still. The boiling point of the condensed liquid is consequently proportionately lower, and the vapour from the still passing into it gradually raises it to the boiling point, so that in its turn rectifier No. 1 distills into rectifier No 2 a liquid of still higher alcoholic richness. The relation of No. 2 to No. 1 is the same as that of No. 1 to the still body, and thus the concentration and redis-tillation might be carried on to any practicable or desired extent.

Another principle brought into play in complex stills for the separation of stronger from weaker alcoholic solutions consists of dephlegmation, or the submitting of the vapour to a temperature so regulated that a portion of it, and that of course the most watery, is condensed and separated, running back into the still or into a special vessel, whilst the richly alcoholic vapour passes on to the rectifier or con-denser. In Dorn's still the wide and lofty head attached acts as a dephlegmator, watery vapours condensing on it, and thence falling back into the body; but in the more recent forms of apparatus—such as those of Pistorius and Siemens—special dephlegmators of an elaborate nature are introduced.

Of the second class of stills—those in which the opera-tion is conducted by the heat of steam generated in a boiler, and forced into the apparatus—the Coffey still may be taken as an example. It is the form most frequently adopted in Great Britain for the manufacture of " silent" spirit, and it is generally recognized as the best and most economical device for preparing a highly concentrated spirit in a single operation. The Coffey still may further be regarded as a type of continuous distilling apparatus, as in it the necessity for withdrawing exhausted solutions and recharging the still with fresh wash is avoided. Beginning, as the Coffey still does, with the steam of pure water, the principle of rectification formerly alluded to is here carried out from the first step. The watery vapour becomes more and more highly charged with alcoholic fumes, till in the end the strongest spirit falls, condensed, into the receiver. In Coffey's apparatus the wash is exposed in a series of shallow chambers, placed one over the other, to the vapour of steam, which rises through the perforated bottoms of each chamber, and carries off the alcoholic vapours into the condenser. This condenser also consists of a series of chambers separated from each other by per-forated plates, and is so contrived that the cold wash passing in pipes through these chambers, in its way to feed the other series of chambers, acts as the condenser to the vapour of the alcohol, the wash being gradually heated thereby, as it passes through the successive chambers. The still, therefore, consists essentially of three separate but connected parts. The first is a large square receiver at the base, which receives the spent wash after it has been deprived of its alcohol by passing through the series of evaporating chambers : the second, a large, square, upright box, termed an " analyzer," contains the series of evaporat-ing chambers, each communicating with the one below by means of a valved tube, which allows fluid to escape from the upper to the lower chamber only, and having the dividing partition of each chamber perforated with fine apertures, to allow the steam which is admitted from below to pass from chamber to chamber through the shallow layer of wash of each. A safety or escape valve is also fitted to each chamber. The already heated wash enters the upper-most of these chambers in a continuous regulated stream, is gradually deprived of its alcohol by the steam as it passes from chamber to chamber, and at last escapes into the lower large receiver, from which it flows off after attaining a certain depth. The third part of the apparatus also consists of a square upright box, termed a " condenser," divided into compartments by means of finely perforated plates, and in each chamber is a link of the tube which carries the cold wash onwards to supply the evaporating chambers just described. The alcoholic vapours escaping from the upper-most of the evaporating chambers are carried by pipes to the lowermost of these chambers, and are partly condensed by each successive chamber being colder than the one below it, in consequence of the wash entering the pipes from above, and only getting gradually heated by contact with the alcoholic vapour as it advances from chamber to chamber. As in the lowest of these chambers the heat is greatest, the alcoholic vapour or the condensed spirit con-tains a large amount of water ; but as the chambers are successively cooler, the alcoholic vapour and condensed spirit at last arrive at a temperature only sufficient to con-vert spirit of the strength wished into vapour, and by an adaptation of valves, the substitution of an impervious parti-tion for the perforated plate, and the admission of the alcoholic vapour into the chambers cooled by the passage of the cold wash in its contained pipes, that spirituous vapour is condensed, and the spirit is drawn off at one operation, of the very strength which it ought to have, and of the utmost purity.

Flat-bottomed and fire-heated stills are considered the best for the distillation of malt spirit, as by them the flavour is preserved. Coffey's still, on the other hand, is the best for the distillation of grain spirit, as by it a spirit is obtained almost entirely destitute of flavour, and of a strength varying from 55 to 70 over proof. Spirit produced of this high strength evaporates at such a low temperature that scarcely any of the volatile oils on which the peculiar flavour of spirits depends are evaporated with it, hence the reason why it is not adapted for the dis-tillation of malt whisky, which requires a certain amount of these oils to give, it its requisite flavour. The spirit produced by Coffey's still is, therefore, chiefly used for making gin and factitious brandy by the rectifiers, or for being mixed with malt whiskies by the wholesale dealers.

As the preparation of alcoholic spirit is the most important industry in which the operation of distillation occupies a prominent place, the establishments in which the manufacture is conducted are known as distilleries. But there are many other important industries in which distillation is an essential feature, being in them employed either for the separation, purification, or concentration of various products. A large proportion of the essential oils are, for example, obtained by the distillation of the substances containing them from water or a mixture of salt and water. The treatment of other bodies in which distillation plays a part will be found under their respective headings. (w. D.—j. PA.)

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