392

DISTILLATION AND CONDENSATION.

[188.

shown in fig. 142, surrounded by a considerable volume of cold water the vapour passes from the boiler into the worm, is condensed, and runs off at the lower aperture into suitable receivers.

Fresh additions of cold water are continually required in the refrigeratory, as the worm and tub are called. The heat is greatest in the upper coils, where the hot vapour enters; and as the heated water, from its diminished density, remains at the top, it is necessary, in supplying the fresh water for cooling, to allow it to enter at the bottom of the vessel, while the heated portions flow off at the upper part. The object of giving to the steam-pipe an ascending direction, as it passes to the condenser, is to insure the return to the boiler of any particles of liquid which may have been mechanically carried up by the breaking of the bubbles in the act of ebullition.

Various modifications of condenser are employed in the laboratory. A convenient form of the apparatus is that known as Liebig's. It consists of an outer metallic tube, through the axis of which a glass tube is passed, and is supported by perforated corks: the space between the two tubes is filled with water, which is continually renewed by cold water supplied by a funnel near the lower extremity, while the hot water escapes at the other

188.]

CONDENSATION.

393

end. The method of using it is sufficiently indicated by the annexed figure.

When the products of distillation are not very volatile, it is often found convenient to make use of the evaporation of water from the neck of the retort as a means of condensation. Fig. 144

shows a method by which this can be effected, the neck of the retort being prolonged by the addition of the conical tube or adapter. Pieces of blotting-paper are used to distribute the water

394

COFFEY'S STILL.

[188. as it trickles slowly from the funnel, the throat of which is obstructed by a plug of tow: the superfluous water is carried off into a jug or other vessel placed to receive it, by means of a fillet of tow twisted round the neck of the retort. The progress of the distillation is hastened by covering the arch of the retort with a cap of brown paper or of tin-plate; a chamber of hot air is thus maintained in contact with the upper part of the retort, and the vapour is prevented from condensing where it would necessarily return again into the mass of liquid undergoing distillation.

The complete separation of two liquids which differ in volatility cannot, however, be effected by mere distillation, because a certain proportion of the less volatile one always passes over with that which is the more volatile. The separation of alcohol and water, for example, is never completely effected by distillation; because at 173°1 (78°4 C.) (the boiling-point of alcohol) the pressure of aqueous vapour is still considerable; indeed it is sufficient to balance a column of mercury nearly 13 inches (330mm) in height. In the first distillation of the fermented liquor, a considerable portion of water, therefore, comes over with the spirit. The less the amount of spirit originally contained in the liquid, the larger is the portion of water in the distilled liquor. By a second distillation the portion of water in the distillate is reduced; and the process may be repeated with like effect until the reduction of the portion of water in each successive product of distillation no longer compensates for the waste and expense of the operation. An ingenious method of dispensing with the necessity for these frequent and costly rectifications was devised by a Frenchman of the name of Adam. By its means he succeeded in carrying the concentration at a single operation to the highest point attainable by mere distillation. The principle of this invention consists in connecting together a number of rectifying chambers, in such a manner that the vapour driven off from the chamber nearest the fire shall be condensed in the second, and by the heat given out in its condensation shall cause the more volatile portions of the liquid of the second to distil into the third chamber, and those of the third into a fourth, and so on till a sufficient degree of concentration is effected.

The most effective method of attaining this object is exhibited in the form of apparatus called from its inventor, Coffey's still. Fig. 145 represents a section of one of these stills. B B' is the body of the still, which is made of copper, and enclosed in a case of wood, to prevent loss of heat: upon the body two columns, D F, H K, are supported; o is the vessel from which the liquor for distillation is raised by the pump Q; the liquor enters the column H K, by the long spiral pipe L L, by which it is ultimately conveyed, through the pipe m to

188.]

the top of the column D F. direct heat of a fire, but is not shown in the figure. The steam enters the body of the still through the pipe A; the amount of steam admitted being regulated by a valve, the handle of which is shown at F. B B' is divided into two chambers by means of a copper shelf, pierced with numerous small holes, which allow the passage of steam upwards, though they are sufficiently small to prevent the descent of any considerable quantity of liquid which may be resting upon the shelf. The steam is at first condensed in the cold liquid of the lower chamber, but it quickly raises this liquid to the boiling-point, driving off the alcoholic portions first, as they are the most volatile.

This vapour traverses the liquid which rests in B', on the perforated shelf, and gradually raises it to the boiling-point, driving off from it the alcohol in vapour; this vapour passes off by a pipe z, to the bottom of the column D F. This column is divided into a series of compartments, by perforated shelves of copper; each of these shelves is provided with a pipe for carrying off the liquid to the shelf below. This pipe is long enough to dip below the surface of the liquid on the shelf beneath it, and projects about an inch above the upper surface of the shelf to which it is attached; a stratum of liquid about an inch in depth is thus retained upon each shelf, and is traversed by the vapours which ascend from the shelf next below it. The wash or liquid for distillation, having become heated during its passage through the spiral pipe in the column H K, falls upon the uppermost perforated shelf in D F, flows off at the farthest end of that shelf, and then falls upon the next shelf; thence it passes to the third, and so on in succession to each shelf: as it descends, it encounters the ascending vapours, which at each successive step of the ascent become more and more alcoholic-the wash, as it descends becoming weaker and weaker, until when it reaches the vessel B B', it is wholly deprived of spirit. If the quantity of the ascending vapour should become at any time too great to pass through the perforations in the shelves, the pressure opens the valves T T, which are provided for security in each shelf. The vapour having reached the top of the column D F, is conveyed by the steam-pipe II N, to the bottom of the finishing column or rectifier H K. The lower part of this column, as high as the pipe Y, is constructed exactly upon the same plan as the column D F, but in each compartment between the shelves the spiral pipe L L makes three or four convolutions, and thus becomes warmed by the ascending heated vapours. In this second column the spirituous liquid distilled over from the first column undergoes a successive rectification upon each of the lower shelves, and becomes more and more concentrated by the ascent of the alcoholic vapours, which, by

396

EVAPORATION.

[188. their condensation at each successive stage, emit sufficient of the heat previously held latent to effect the distillation of the more volatile portions of the liquid by which they are condensed. The five upper shelves of this column merely act as a condenser for the alcoholic vapours; these shelves are not perforated, and are attached to the alternate sides of the column, leaving a narrow passage at one end of each shelf, so as to oblige the vapours to describe a zigzag direction: the pipe y carries off the finished spirit into proper receivers ; the pipe R carries off any uncondensed spirituous vapour to a refrigeratory, whilst the weak spirit which reaches the lower part of the column is returned by the pipes to the vessel o. The spent wash, as it accumulates in B B', is drawn off at intervals, and the still can thus continue its operations without intermission.

In chemical investigation recourse is frequently had to fractional distillation for the partial separation of liquids of different volatilities. When a retort is employed a thermometer is adapted by a cork to the tubulure, the bulb being placed above the liquid, but below the lower edge of the neck of the retort. Unless there is a considerable difference between the volatilities of the bodies an incomplete separation is effected, much of the less volatile bodies passing over with those of lower boiling-points. The separation is made much more complete by changing the form of apparatus, imitating, in fact, on a small scale, the action of Coffey's still. For this purpose a flask may be employed, to the neck of which is adapted a glass tube of about a half or three quarters of an inch in diameter, on which five or six bulbs have been blown, and which is surmounted by a small glass alembic head. A thermometer is placed in the tubulure of the alembic head, and another thermometer may be inserted through a side tube into the lowest bulb. When the distillation is carried on slowly the vapour of the most volatile body passes over first, those of lower boiling-point condensing on the bulbs, and flowing back into the flask. The contractions between the bulbs check the diffusion of the different vapours, and the more volatile liquid is being continually carried upwards from the condensed liquid in the bulbs by the rising vapours. When the two thermometers indicate the same temperature a homogeneous liquid is passing over; a rise of the lower one indicates that a second vapour is commencing its passage. Several more complex forms of this appa

ratus have been devised.

(189) Evaporation.-All liquids, at temperatures far below their points of ebullition, emit vapour by the tranquil process of evaporation. The amount of vapour given off at a constant temperature differs greatly in different liquids, and is dependent upon the temperature at which each liquid boils.

The great difference in the volatility of liquids at the same temperature is strikingly shown by filling a number of barometer-tubes (fig. 146) with mercury,