passing gradually and insensibly from deep red to an attenuated violet, in the following order, as described by Newton, and since very generally concurred in, red, orange, yellow, green, blue, indigo, violet. This experiment, which first opens the analysis of light, is easily made by letting a beam of sunlight pass through a small circular hole in a shutter, in a darkened room, on a glass prism such as above described, the refracted and dispersed beam being received on the opposite wall, ceiling, or floor, according to the position of the prism.

When the image of the sun, or a star, candle, &c., is thus formed by admission through a small hole, and the refraction of the prism, the coloured space Gg, which has the same angular breadth in a direction parallel to the axis of the prism as it would have had if the prism were removed and the light received directly on the screen at the same distance from the place of the prism (the screen being in both cases supposed to be held perpendicularly to the incident light), but which is considerably elongated in the perpendicular direction, is called the spectrum; and that angle of the prism B A C the sides containing which BA, A C, have been traversed by the ray D E F G, is called the refracting angle of the prism. Suppose, now, that a small orifice, o, is made in the screen at some point of the spectrum, so that rays of any particular colour, green for example, may be transmitted through it; and let the transmitted portion be again submitted to refraction through another prism, this beam being supposed very small, to ensure its purity or near uniformity of colour. It will not, after refraction, be again decomposed, or undergo any alteration of colour; but if the first prism be turned round its axis, which will cause different colours in succession to fall on o, while the direction of the ray incident on the second prism remains unchanged, being that of a line joining o with the first prism, it will be found that as we pass from the red to the violet the ray will be more and more bent round by the refraction of the second prism. This shows that light incident on the first prism, when once decomposed into homogeneous elements by refraction, is then, at least by refraction, not further decomposable, but each element retains its own colour and its own refrangibility, or disposition to be bent by refraction.

This experiment will not perfectly succeed with sun-light as above described, because however small be the hole in the window the incident beam will not be a mere ray, on account of the finite angular diameter of the sun, but a cone of very sensible angle, so that the first spectrum will be impure, from the finite size and consequent overlapping of the coloured circles corresponding to the elementary kinds of light. It might be rendered pure, but at great expense of light, by limiting the beam by a screen with a small hole placed at some distance from the first hole. But it is far better, according to the method first described by Newton, to combine the prism (which in this case must be placed at a considerable distance from the hole in the window) with a convex lens, and receive the spectrum on a screen placed at the focus of the lens conjugate to the hole in the window. The lens alone collects into a point the divergent pencil of rays of any one kind; the prism alone bends it round as a whole, but differently for the different kinds, without (at least in the position of minimum deviation) affecting the divergency; and the two combined will give a pure and brilliant spectrum, but very narrow. To obtain an equally pure and brilliant spectrum, but of considerable breadth, we have only to replace the hole by a narrow aperture parallel to the axis of the prism. If our object be merely to see a pure spectrum, without placing objects in it, we may replace the lens and the screen by the eye and the retina, that is, merely view, through a prism applied to the eye, a slit transmitting light. In this way very pleasing and instructive experiments may easily be made on the action of absorbing media (such as coloured glasses, &c.) on light. Instead of the naked eye, a telescope may be employed; and in this way accurate measures may be taken, supposing the telescope to be properly mounted, and furnished with cross wires.

We have seen that compound light, the sun's for example, may be decomposed into its homogeneous constituent rays by refraction through a transparent prism. Conversely it may be recompounded into light similar to the original, merely by making the rays. thus separated, by another refraction to occupy the same place. This may be effected by placing a prism of exactly similar material and form to that already used, with its refracting angle turned in a direction opposite to that of the former, so that the near faces of both prisms may be parallel; for the rays entering the second prism are in the same condition as if we supposed their direction inverted, that they may repass through the first; and therefore they emerge in a similar compound ray with the original, which may also be easily confirmed by experiment.

The rays issuing from the second face of the refracting prism, may also be collected by means of a large convex lens, placed at a considerable distance from the prism. If the rays after passing through the lens be received on a screen of white paper, which is first held close to the lens and then moved away, the spectrum will contract in width, until at the focus conjugate to the prism the colours disappear, the original compound white light being reproduced, after which the colours reappear in the reverse order.

The prismatic analysis of light, together with the phenomena relative to the transmission and absorption of light, enabled Newton to conclude that the colours of natural bodies are not inherent qualities of those bodies, but depend on their powers of reflecting, transmitting, or absorbing the rays of some colours more than others from the compound

light incident on them; for all bodies placed in homogeneous light of any colour appear themselves to be of that colour, though they are most luminous (making allowance for the different intensity of different parts of the spectrum) when placed in that coloured light which they reflect most copiously. Hence also arise the different colours of coloured liquids or glasses. [ABSORPTION.]

Many of the prismatic colours may be imitated by mixing colours taken, as they lie in the spectrum, of greater and less refrangibility, as orange from red and yellow, &c., but such compound colours are not identical with the homogeneous light of the same colour, being immediately decomposed when viewed through a prism.

If the original prism B A C be turned gradually round its axis, presenting always to the incident light the same refracting angle ▲, the spectrum Gg may be made to descend towards K, but after arriving at a certain point where the deviation, that is the inclination of DE produced to F G, is a minimum, it then re-ascends, and it is usual to make the chromatic experiments in this definite position of minimum deviation. This occurs when the position of the prism is such that the angles of incidence and emergence, or their complements DE B, G F C, are equal; for when the moving point & has reached its lowest place, it is for a moment in the condition of a fixed point like the point D, through which we may suppose the incident beam admitted; hence rays proceeding from D, notwithstanding a small variation of incidence arising from the rotation of the prism, reach G, as if it were a fixed point; and since in dioptrics it is of no consequence to the path in what direction we suppose the rays to move, it follows that rays proceeding from G, notwithstanding a small alteration of the angle CFG, would arrive at the fixed point or orifice D; and consequently the data for the determination of the angles D E B, G F C, in the position of minimum deviation, are precisely the same, and therefore these angles must then be equal.

This being premised, the following easy calculation will give the necessary angle of incidence to produce a minimum deviation.

H

M

E

F

B

formed by the interior ray E F with both sides of the prism are equal, Since the angles of incidence and emergence are equal, the angles or the triangle A E F is an isosceles; let 2 a be the refracting angle of which being the complement of AEM, is necessarily the angle of the prism, then drawing A м perpendicular to E F, we have EAM = α, refraction; if, therefore, u be the index of refraction for rays of any given colour, the angle of incidence P, corresponding to a minimum deviation, is given by the equation,

The angles i and D, and consequently the value of μ, admit of very exact determination by using a telescope properly mounted, provided we can find in the spectrum an object sufficiently precise to fix on. The transition from one colour into another is far too gradual to allow us to fix on the limit which separates them; but fortunately we have been put in possession of perfectly definite standard objects by the beautiful discovery made by Wollaston and Fraunhofer of the existence of dark spaces, narrow bands transverse to the length of the spectrum, and now generally designated Fraunhofer's lines.

These bands are best observable by forming the spectrum of a luminous line instead of a point, by means of a prism of great purity, and viewing it through a telescope of good magnifying power, though some of them may, when carefully pointed out, be recognised by the unassisted eye, and after one recognition are in future easily found; and for naked eye observations a prism of moderate purity will suffice. They are spaces totally deficient of light, of very unequal width, and exceedingly numerous; it is also to be remarked that these bands, always the same in number and relative position for the same light, are different, or altogether wanting, when the source of light is varied. Thus sunlight, moon-light, planet-light, sky-light, derived from a common source, have the same lines, but several of the fixed stars have dark lines of their own, while artificial lights rarely if ever exhibit dark

lines, but frequently show bright lines, of which the light of the electric spark is almost wholly made up.

In the formula (2) suppose the angle of the prism i, and consequently the deviation D, to be very small, then

D= (μ—1) i, . ... (3)

a formula which may be readily shown to remain true so long as the angle of incidence is small, even though the angles of incidence and emergence should no longer be equal.

Suppose now that while D, relate to rays of mean refrangibility they become D,,, and D.,, for two definite kinds of rays chosen as nearly as may be at the red and violet extremities of the spectrum. The formula (3) shows that the difference of deviation D-D1, or 8 D, that is the length of the spectrum, is expressed by (μ) i or Sμ.i. The ratio of the difference of deviation du.i to the mean deviation (μ-1) i, or 1, depends (for given selected rays) only on the nature of the substance of which the prism is made, and is called the dispersive power of the substance.

δμ

L

Newton supposed that all substances disperse light in the same proportion as they refract it, and concluded that in refracting telescopes it was impossible to get rid of the defects arising from the chromatic dispersion of the object glass. Mr. Hall was the first to point out Newton's mistake, and to apply the fact of the difference of dispersive power of substances to the construction of an achromatic telescope (Herschel's Light,' art. 425); but the discovery fell into oblivion, and it was not until after the fact had been rediscovered by Dollond, and reapplied to the same object, that the achromatic tele scope came into general use. The mode in which the compensation is effected by the use of two lenses may be readily understood from the following considerations.

Imagine a single ray to be transmitted through a convex lens in a direction nearly parallel to its axis, but at a good distance from its centre. If tangent planes be drawn at the two points where the ray cuts the surfaces, the refraction of this single ray will be the same as if the lens were replaced by a slender wedge or prism of the same material bounded by those tangent planes. Consequently the ray will be not only deflected as a whole, but " dispersed." If now we consider all the rays emanating from a distant point in the axis of the lens we readily see that the violet rays will be brought to a point or focus in the axis of the lens sooner than the green, the green than the red, &c. At no one distance will all the rays be brought to a focus together, and consequently the image will be confused.

Suppose now we have two slender prisms, composed of different materials, in contact with one another or nearly so, with their angles i, i turned in contrary directions; and let a ray of white light be incident nearly perpendicularly upon the system. The deviations produced by the two prisms being (-1) i and (-1) i', the whole deviation will be

• ...

(4)

while the difference of deviation of the red and violet rays will be δμ . ἐπδμ' . ' Unless therefore du: μ-1 :: du': u'-1, that is unless the dispersive powers of the two substances are the same, the difference of deviation may be destroyed without at the same time destroying the common deviation of the two kinds of rays. The outstanding deviation will be in the direction of that produced by the prism of smaller dispersive power.

Suppose now that a compound lens is formed consisting of a convex and concave of substances differing in dispersive power; and imagine the course of a ray incident towards the edge in a direction nearly parallel to the axis. By drawing tangent planes as before, the lenses may, as regards the course of this single ray, be replaced by a pair of prisms turned in contrary directions. The small chromatic variations of the points of incidence, and consequently of the angles of the prisms, arising from the dispersion of the ray during its passage through the lenses, may be altogether neglected. Now the deviations being on the one hand as (u-1)i to (u'-1), and on the other inversely as the focal lengths F, F' for parallel rays, we have (μ—1) F.i=(-1) F'.i, and substituting in (4) equated to zero we find

or in order that the dispersion may be corrected, the focal lengths must be as the dispersive powers. The dispersive power of substance is most accurately determined by forming the substance into a prism of considerable angle (supposing it sufficiently homogeneous), and determining, as before explained, the refractive indices for two properly selected and perfectly definite points of the spectrum, such as two fixed lines. The ratio of the dispersive powers of two substances, which is all that is required for the construction of an achromatic object-glass, may, however, be determined by different methods of compensation. One of the simplest, at least in theory, which has been much employed by Dollond and practical opticians up to the present day, consists in forming two prisms of the substances with small angles, and altering by trial the angle of one of the prisms until an object seen through both appears

a free as possible from fringes of colour, when the dispersive powers are inversely as the deviations produced by the two prisms respectively.

When different substances are formed into slender prisms through which light passes nearly perpendicularly, not only does the separation the different substances, but the ratio of the angular extent of one of the extreme rays bear to the mean deviation a different ratio in portion of the spectrum to that of another portion changes from substance to substance. Thus if three definite points, (such as three fixed lines,) be taken in the red, the green, and the violet, the ratio of the separation of the violet from the green to that of the green from the red will be greater in flint glass than in crown. This want of proportionality is termed the irrationality of dispersion, and the outstanding spectrum formed when two prisms as nearly as possible compensate each other, which is coloured green on one side and purple (from a mixture of red and blue) on the other, is called a secondary spectrum. This irrationality prevents the compensation in the case of a double object glass from being perfect, and constitutes one of the chief obstacles to the perfection of large refracting telescopes. The rainbow is a beautiful natural exhibition of the dispersion of light into spectral colours. [RAINBOW.]

Two simple propositions relative to the effect of chromatic dispersion in a single lens are here subjoined.

To find the longitudinal chromatic aberration of a lens, or the interval on the axis between the foci of extreme red and violet rays. Let the red rays converge to the point R, and the violet to the point v in the axis.

dispersion. To find, in the same case, the radius of the circle of least chromatic

By referring to the same figure, we may observe that the foci B, V are respectively the vertices of red and violet conical surfaces, having the lens as a common base. Let these surfaces intersect in a circle, of which the radius is DE; then it is plain that all the intermediate coloured rays pass through this circle. It is therefore that of least dispersion :

=3

CR

2

F

CA, and for

The preceding figure representing a plane section of the whole system taken through the axis, it is obvious that, from the smallness of R V relative to CR, the angles C V B, CRA are sensibly equal, or the triangle V RD is exceedingly nearly isosceles, and therefore DE CA h f bisects VR, or ER = and DE ER, 2 h parallel incident rays DE = CA. · 2 DISPOSITION, in the law of Scotland, is the name given to an instrument, or as it would be termed in England, deed poll, by which a party solemnly makes over to another real property. It may be used as a title to moveables alone, but it is in the law of real property that it is of most frequent use and of highest importance, the conveyance of moveables being usually by assignation or assignment. When a new feu or fief is created, it is by charter or contract of feu, containing a disposition in itself, or disposition in feu; but when a feu, fief, or estate is transferred from one holder to another it is by Disposition, in which all the conditions on which the property is to change hands are set forth. Being given to the disponee, it is in his hands a personal obligation by the disponer to give him a full title, and contains the warrants for getting the title made real by registration, and by obtaining the superior's sanction to the new investiture. As heritable property cannot be bequeathed by testament in Scotland

[WILL], the usual form of family settlements in which such pro perty is disposed of is the disposition.

DISSECTION. The art of separating the parts of organised bodies in such a manner as to display their structure. It is an art equally applicable to both divisions of the organic kingdom, and indispensable alike to the discovery of the structure of plants and animals. The grounds on which, for the well-being of the community, every facility should be afforded to the cultivation of this art, as far as regards human dissection, have been already stated. [ANATOMY ACT.] It is satisfactory to observe that the prejudices which formerly obstructed this practice are rapidly disappearing, and that even the most uneducated are beginning to appreciate its great importance and its signal utility. DISSEISIN. [SEISIN.]

DISSENTERS, the general name for the various Protestant religious sects in this country that disagree in doctrine, discipline, or mode of worship with the Established Church. The Jews and Roman Catholics are not commonly called dissenters. The origin of Protestant dissent from the Church of England is usually traced back to the year 1548, in the reign of Edward VI., when a controversy arose among the adherents of the new Reformation in consequence of the excellent Hooper (afterwards the martyr) scrupling to be consecrated as bishop of Gloucester in the customary canonical habit, which he deemed objectionable as a relic of Romanism. Hooper eventually received consecration without being attired in canonicals. At this time the two parties received the names of Conformists and Nonconformists; very soon after that of Puritans came into use as the general appellation of the dissenters, and it continued to be that by which they were commonly distinguished down to the close of the civil wars in the next century. The toleration of the dissenters, even in the most limited extent, dates only from the Revolution. During the century and a half that elapsed between the Reformation and that event, with the exception only of the short period of the Commonwealth, during which first the Presbyterians and afterwards the Independents had the ascendancy, they continued to be persecuted by a succession of restrictive and penal laws of almost constantly increasing severity. It was not till 1828 that the dissenters were raised from being a merely tolerated body to a free participation in the rights of their fellow-subjects, by the abolition of the Test and Corporation Acts. If the relaxation of the marriage law, that has since taken place, shall be followed by the abolition of Church rates, the dissenters will be placed as nearly on an equality in all respects with the adherents of the Established Church as it is possible that they should be, without the Established Church itself being abolished. In the early times of dissent the great classes of dissenters were the Presbyterians, the Independents, the Baptists, and the Quakers. The most numerous now are the Methodists, or followers of Wesley and Whitfield, some only of whom are avowedly dissenters. The Methodists are subdivided into Wesleyan Methodists, Primitive Methodists, United Free Church, &c. The minor sects of dissenters now make a long list; but many of them may be considered as only subdivisions of or included in the four leading denominations. Until the formation of the Free Church, the most numerous classes of dissenters in Scotland were those which originated in a separation from the Established Church in 1736. [ERSKINE, EBENEZER, in BIOG. DIV.] They were called generally Seceders, and were divided into Burghers, Anti-Burghers, Original Burghers, and Original Seceders. The greater number of the Burghers and Anti-Burghers united in 1820, under the designation of the United Associate Synod of the Secession Church; in 1847 this body united with the Relief Church (which originated in a separation from the Establishment in 1752), the aggregate body taking the name of the United Presbyterian Church. The Free Church of Scotland, which separated from the Establishment in 1843, forms now the most numerous body of dissenters in Scotland, although in some respects the members of the Free Church disclaim the designation of dissenters. These bodies are all Presbyterians, and differ chiefly on the theory of the relations of the Church to the State. The only considerable body of Scottish dissenters of older standing, with the exception of the Episcopalians, is that of the Cameronians, or Reformed Presbyterian Synod, who are the representatives of the Covenanters of the 17th century. The Congregationalists, or Independents, form a considersble body in Scotland; the Baptists, of whom there are several sections, are fewer in number. In Ireland, exclusive of the Roman Catholics, the principal dissenters are the Presbyterians, who are mostly confined to the province of Ulster. In the Census of 1851 an attempt was made to ascertain the number of each sect, by taking the number who attended divine service on a certain Sunday, but the result was confessedly imperfect, though it may afford a rough approximation. [CENSUS OF THE UNITED KINGDOM, vol. ii., col. 723.]

CORD.]

DISSONANCE, in music, a term synonymous with discord. [DISDISTANCE. The only remark which we need make upon this common word is that it is very frequently applied to angular distance, meaning the angle of separation which the directions of two bodies include. In the apparent sphere of the heavens, distance always means angular distance. The term apparent distance is frequently applied in

the same case.

DISTEMPER, an inferior kind of colouring, in which size is the principal vehicle employed for mixing with the colour. It is used for

both internal and external walls, but principally for the former, instead of oil colour, being a cheap substitute.

It is composed of whitening mixed with size of a coarse quality, in the proportions of twelve pounds of whitening to one of size. The size is boiled and reduced to a proper working consistency by the addition of water, after which the colour is added to form the neces sary tint. Coarser colours are used for distemper than are employed in oil-painting and colouring. Scene-painting is executed in distemper, and paper-stainers employ distemper colour in printing and staining papers for walls. The colours used in these cases are, however, of a better quality, and the size employed is made from the hide of the buffalo, or parchment cuttings. The proportions of size and whitening in paper-staining depend on the strength of the size. In five quarts of distemper, if the size be strong, one-fourth part will be sufficient; if weak, about one-half. In mixing the size and whitening much depends on the judgment of the workman. The distemper is used in a chilled state. Five quarts will stain about eighty-four yards of paper.

It is

The method of painting of the early Italian painters before the employment of oil as a vehicle, is known as tempera, from which term our word distemper as applied to painting is no doubt derived. DISTILLATION. The process of separating, by the aid of heat, a volatile from a fixed or less volatile constituent. Sometimes the volatile matter so separated condenses as a solid, and then the process is termed sublimation. [SUBLIMATION.] When the product obtained is the result of a change induced by heat upon the original substance, the operation is named destructive or dry distillation. The ordinary process of gas-manufacture, wherein certain liquid products are condensed, is a process of dry distillation. Distillation in the unqualified sense, however, signifies the volatilisation of a liquid by heat and its subsequent condensation in a separate vessel by cold. employed to separate a volatile liquid from less volatile solid or liquid matters. It is thus largely employed in the arts (see following article). In chemistry the operation is generally performed in an apparatus figured under CONDENSER. The material to be operated upon is placed in a flask or retort connected with the higher extremity of a condenser, which conducts the liquified product to a vessel called the receiver. In order to attain perfect purity from the less volatile matter, the process of distillation must in many cases be repeated, and it is then termed rectification. Some liquids, on being boiled in glass vessels, produce sudden bursts of vapour often causing the fracture of the vessel; this inconvenience may generally be remedied by placing platinum wire, or angular particles like quartz-sand, at the bottom of the vessel. Thus concentrated sulphuric acid can only be safely distilled in glass vessels, by first converting it into a magma with quartz sand. It is sometimes desirable to protect liquids from the air, or to lower their boiling points [BOILING OF LIQUIDS]; in either case distillation in vacuo is resorted to. When small quantities only of liquid are thus to be operated upon, the following is a very convenient form of apparatus :—

A B are two glass bulbs blown near the ends of a tube about one foot long, the extremities of the tube beyond the bulbs being drawn out as shown in the figure. The capillary tube attached to B is now immersed in the liquid to be distilled contained in the bottle c, and suction being applied to D, the necessary amount of fluid is raised into B, which is then so inclined as to prevent its return. The neck E of the capillary tube is now to be sealed off by the mouth-blowpipe, and the extremity D connected with an air-pump, by means of a caoutchouc joint. A vacuum being thus made in the bulbs, the tube D is fused off at F, and the distillation may now be commenced. The empty bulb A being immersed in cold water, the necessary heat is applied to B until a sufficient amount of the liquid has distilled over. The tube connecting the two bulbs must now be cut across with a triangular file, and the products removed from the bulbs.

DISTILLERY. Having, in the article DISTILLATION, explained the chemical principles on which all distilling processes necessarily rest, we shall now treat of these processes in their practical connection with manufactures, especially the distilling of ardent spirits in those great establishments known as distilleries.

The Arabians seem to have practised, in the remotest ages, the art of extracting the aromatic essences of plants and their flowers, in the form of distilled waters, to supply the luxuries of oriental baths. They are also supposed to have been the first to extract from wine a colourless intoxicating liquor by distillation. From certain passages in Pliny and Galen there can be no doubt that the Greeks and Romans were well acquainted with the distillation of aromatic waters. Indeed Nicander, a Greek poet and physician who lived 140 years before the Christian era, employs the terms außig ambix and distillation in describing the preparation of rose-water. From ambix, which signifies a pot, the Arabic name alambic or alembic is derived. The words pot and poteen are used in the same way by the modern Irish to designate a still and its spirituous product. It is obvious that distillation must have been a familiar process to the countrymen of Avicenna, since, in his treatise of catarrh, he compares the human body to an alembic; he regards the belly as the cucurbit or body, and the head as its capital, through which the humours distil, passing off by the nostrils as its beak. Arnoldus de Villa Nova, a chemical physician of the 13th century, is the first author who speaks explicitly of an intoxicating spirit obtained by the distillation of wine; and he describes it as a recent discovery. He considers it to be the universal panacea so long sought after in vain. His disciple Raymond Lully, of Majorca, declares this admirable essence of wine to be an emanation of the Divinity, an element newly revealed to man, but hidden from antiquity because the human race were then too young to need this beverage, destined to revive the energies of modern decrepitude. He further imagined that the discovery of this aqua vitæ, as it was called, indicated the approaching consummation of all things-the end of the world. In his Chemical Theatre,' written towards the close of the 13th century, Lully describes the distillation of ardent spirits thus: "Limpid and well-flavoured red or white wine is to be digested during twenty days in a close vessel by the heat of fermenting horse-dung, and to be then distilled in a sand-bath with a very gentle fire. The true water of life will come over in precious drops, which being rectified by three or four successive distillations, will afford the wonderful quintessence of wine.... . To prove its purity," adds he, "if a rag be dipped in it, and kindled, it will not become moist, but consume away." The only substances employed in this country in the manufacture of ardent spirits upon the great scale are different kinds of corn, such as barley, rye, wheat, oats, buckwheat, and maize. Peas and beans have been occasionally used in small quantity. The principles in these grains from which the spirits are indirectly produced are starch and a little sweet mucilage, which, by a peculiar process called mashing, are converted into a species of sugar. It is the sugar so formed which is the immediate generator of alcohol, by the process of fermentation. In mashing one or more kinds of corn, a greater or smaller proportion of malt is always mixed with the raw grain; and sometimes malt alone is used, as in the production of malt whiskey. The process of malting is that incipient growth called germination, in which, by the disengagement of a portion of the carbon of the starch, in the form of carbonic acid, the ultimate vegetable elements become combined in such a proportion as to constitute a species of sugar. Malting is the most effectual method of converting starch into sugar; although chemists are acquainted with other and very singular modes of effecting this transformation. By mashing, a larger or smaller proportion of the fecula of the corn is thereby converted into sugar, and thus brought into a state fit for producing alcohol by fermentation.

.

The manufacture of ardent spirit, whether known as whiskey or by any other name, consists in three distinct operations: first, mashing; second, fermentation; third, distillation.

1. Mashing.-Either malt alone, or malt mixed with other grain, and coarsely ground, is put into the mash-tun, along with a proper proportion of hot water; and the mixture is subjected to agitation by a mechanical revolving apparatus, similar to that employed in the breweries for the manufacture of beer. When malt alone is used, the water first run into the mash-tun among the meal has usually a temperature of 160° or 165° Fahrenheit; but when a considerable proportion of raw grain is mixed with the malt, the water is let on at a lower temperature, as from 145° to 155. The following quantities have been found to afford a good product of whiskey in a wellconducted Scotch distillery ::

252 bushels of malt, at 40 pounds per bushel.

948

barley, 533 oats, 474 rye,

53

1500

From each bushel of the above mixed meal 23 gallons of proof whiskey (specific gravity 0-921) may be obtained, or 183 gallons per quarter. A few distillers are skilful enough to extract 20 gallons per quarter from such a mixture. Ten imperial gallons may be con

sidered a fair proportion of water to be introduced into the mash-tun for every bushel of meal at the first infusion. After two or three hours' agitation, the whole is left to repose for an hour and a half, and then the worts are drawn off to about one-third the volume of water employed, the rest being entangled in a pasty state among the farina. About two-thirds of the first quantity of water is now let into the tun, but at a temperature somewhat higher; and the agitation is renewed for nearly half an hour. A second period of infusion or repose ensues, after which these second worts are drawn off. Both infusions must be cooled as quickly as possible down to the temperature of 80° or 70° Fahr., otherwise they are apt to run into the acetous fermentation by the rapid absorption of oxygen. This refrigeration is usually effected by exposing the wort for some time in large shallow coolers, placed near the top of the building, where it may be freely exposed to the aërial currents. But it is sometimes cooled by being passed through serpentine tubes surrounded with cold water, or by the agency of ventilators blowing over its surface in extensive cisterns only three or four inches deep.

After the second wort is drawn off, a third quantity of water, fully as great as the first, but nearly boiling hot, is run into the mash-tun, and well incorporated with the magma by agitation; after repose, this third wort is also drawn off, cooled, and either directly mixed with the preceding worts, or after it has been concentrated by boiling down; in most cases, however, it is reserved, and used instead of water for the first infusion of a fresh quantity of meal. The mashing and fermentation are jointly called brewing, and the period in which they are carried on is by law kept quite distinct from the distilling period, the one occupying usually one week, and the other the next in rotation. About 150 gallons of wort or wash are obtained from each quarter of corn employed.

The first of the above worts will have generally the density of 1.078 when the grain is good and the mashing is well managed, and the second a density of 1·054, so that the mixture will have a specific gravity somewhat above 1060, and will contain about 60 pounds of extract per barrel. Now, by the excise rules, 100 gallons of such wort ought to yield one gallon of proof spirit for every five degrees of attenuation which its specific gravity undergoes in the fermenting tun, so that if it falls from 1060 to 1000, 12 gallons of proof spirit are supposed to be generated, and must be accounted for by the distiller. After an alteration in the excise laws some years ago, the distillers were allowed to ferment worts of less density than they previously could, and have been able to effect a more productive fermentation. They have been also enabled thereby to reduce the proportion of malt in the mixed meal. Formerly they were accustomed to use three parts of malt to four parts of barley, or two to three, but they soon diminished the malt to one-fifth, and latterly to one-eighth, or one-tenth, of the whole grain. One principal use of malt, besides its furnishing the saccharine ferment called diastase, is to keep the mash magma porous, and facilitate the drainage of the worts.

2. Fermentation.-This is undoubtedly the most intricate, as it is the most important process in distillation. Experiments have proved that the quantity of saccharine matter converted into alcohol is dependent upon the proportion of ferment or yeast introduced into the worts; if too little be used a portion of the sugar will remain undecomposed; and if too much, the spirits will contract a disagreeable taste. In general, the worts are let down at the specific gravity of 1050 or 1060, and at a temperature varying from 60° to 70° Fahr. For every 100 gallons one gallon of good porter yeast is immediately poured in and thoroughly incorporated by agitation with a stirrer. When by attenuation the density is diminished to 1035, one half gallon more is added, and another half gallon at the density of 1·025, after which the worts usually receive no further addition of yeast. The temperature of the fermenting mass rises soon after the introduction of the yeast 8 or 10 degrees, and sometimes more; so that it reaches in some cases 85° or 90° Fahr. From the appearance of the froth or scum the experienced distiller can form a tolerably correct judgment as to the progress and quality of the fermentation. The greatest elevation usually takes place within thirty-six hours after the commencement of the process. The object of the manufacturer of spirits is to push the attenuation as far as possible; this so far differs from that of the beer-brewer, who wishes always to preserve a portion of the saccharine matter undecomposed to give flavour and body to his beverage. The first appearance of fermentation shows itself by a ring of froth round the edge of the vat usually within an hour after the addition of the yeast; and in the course of five hours the extrica tion of the carbonic acid from the particles throughout the whole body of the liquor causes frothy bubbles to cover its entire surface. The temperature meanwhile rises from 10 to 15 degrees, according to circumstances. The greater the mass of liquid, the lower the temperature at which it was let down into the tun, and the colder the surrounding atmosphere, the more slowly will the phenomena of fermentation be developed under a like proportion of yeast and density of the worts. In general large vats afford a better result than small ones, on account of the equality of the process. It is reckoned good work when the specific gravity comes down to 1000, or that of water; and superior work when it falls 4 or 5 below it, or to 0.995.

After thirty-six hours upon the moderate scale the yeasty froth begins to subside; and when the attenuation gets more advanced, the

greater part of it falls to the bottom on account of its density relatively to the subjacent fluid. In from forty-eight to sixty hours the liquor begins to grow clear, and becomes comparatively tranquil. It has been deemed advantageous towards the perfection of the fermentation to rouse up the wash occasionally with a proper stirrer, and in some cases to increase its temperature a few degrees by the transmission of steam through a serpentine pipe coiled round the sides of the vat. Some have imagined that a considerable portion of spirit is carried off by the great volume of carbonic acid evolved, and have proposed to save it by covering the vats air-tight, and conducting the gas through a pipe in the lids into a vessel containing water. The economy of this apparatus is not worth the expense and trouble which it occasions. The distillers content themselves with enclosing their vats after the first violence of the action under tolerably tight covers. It is found that the acetous fermentation is always proceeding simultaneously with the vinous fermentation: for judging by the usual tests there is always a slight degree of acidity in fermenting wash; that vinegar is in fact forming along with alcohol, or that while the attenuation is increasing, acetic acid is being formed. This important fact serves to show how very fallacious a test the attenuation or diminution of density is of the amount of alcohol generated and existing in a fermented wash. The acetic acid along with the undecomposed mucilaginous starch may, in fact, so far counteract the attenuating effect of the spirits as to produce a specific gravity which shall indicate 10 or 15 per cent. less spirit than is actually present in the wash.

It is computed that every 5 degrees of attenuation, as it is called, that is every diminution of the number 5 upon the specific gravity in the third place of decimals, ought to produce 1 per cent. of proof spirit, or 1 gallon out of 100; so that if the wort be set at 1.055, and come down to 1.000, 11 gallons of proof spirits are chargeable upon each 100 of such wash. In the fermentation of sugar worts, 1 gallon of proof spirits was calculated for every four degrees of attenuation; but distillation from sugar or molasses-wash is now illegal. With corn-wash there is never more than four-fifths of the saccharine matter decomposed into alcohol and carbonic acid, in the best-managed fermentation, and frequently indeed much less. In fact, each pound of real sugar may be resolved by a successful process into half a pound of alcohol, or into about one pound of proof spirit; and hence as a solution of sugar at the density of 1060 contains 15 per cent. by weight, or 16 per cent. by measure, which is nearly 17 pounds per gallon, it should yield nearly 170 pounds from 100 gallons, or 180 pounds measure equal to 18 gallons of proof spirit; whereas 100 gallons of corn-wash, fermented at the above density, are computed by the excise law to yield only 12 gallons, and seldom produce more than 13 In the huge fermenting vats used by the corn distillers of this country, the fermentation goes on far more slowly than when conducted upon the moderate scale referred to in the account of this process given above. About 1 gallon of yeast is added at first for every 100 gallons of wort, and a half gallon additional upon each of the succeeding four days, making in the whole 3 per cent,; when less can be made to suffice, the spirits will be better flavoured. The fermentation goes on during from six to twelve days, according to the modifying influence of the circumstances above enumerated. After the fifth or sixth day, the tuns are covered in, so as to obstruct, in a certain degree, the discharge of the carbonic acid: since it is supposed that this gas in excess favours fermentation. The temperature is usually greatest on the fourth or fifth day, when it sometimes rises to 85° Fahr., from the starting pitch of 60° or 56°. Whenever the attenuation has reached the lowest point by the hydrometer, the wash ought to be distilled, since immediately afterwards the alcohol begins to be converted into acetic acid. This acidification may be partially repressed by the exclusion of atmospheric oxygen.

and a small fraction.

III. Distillation.-There are few kinds of chemical apparatus which have undergone so many metamorphoses as the still and condenser. In its simplest form it has been already represented and described. [ALEMBIC.] It may be considered to have reached its highest point of perfection, as to power and rapidity of work in Scotland, at the time when the distillers paid a stipulated sum per annum to the revenue for the privilege of using a still of a certain size, and when therefore they derived a profit proportionable to the quantity of spirits they could run off in a given time. Since the year 1815, the whiskey duties have been levied on the quantity distilled, independent of the capacity of the still. This change has introduced a modification in the distilling apparatus, with the view of combining purity of product with economy of time. The body of the still is still comparatively flat, so as to expose a large surface to the fire; but the tapering upper part, corresponding to the capital of an alembic, is made very long, rising sometimes 15 or 20 feet before it terminates in the worm pipe or refrigeratory for condensation.

Great distilleries are usually mounted with two stills, a larger and a smaller. The former is the wash still, and serves to distil from the fermented worts a weak crude spirit called low wines; the latter is the low-wine still, and rectifies by a second process the product of the first distillation. The annexed cut represents a copper wash-still, having a capacity of about 20,000 gallons. In these successive distillations a quantity of fetid oil, derived from the corn, comes over along with the first and last portions received, and constitutes by its combination what

ARTS AND SCI. DIV. VOL. III.

channels, which lead to separate receivers. From these receivers the various qualities of spirit, low wines, and faints, are, for the purpose of redistillation, pumped up into charging backs, from which they are run in gauged quantities into the low-wine and spirit stills. One of the greatest improvements in modern distillation is the accomplishment of this essential analysis of the impure spirit at one operation. Chemistry had been long familiar with the pneumatic apparatus of Woulfe, without thinking of its adaptation to distillery apparatus; when Edouard Adam, an illiterate operative, after attending by accident a chemical lecture at Montpellier, where he saw that apparatus, immediately employed it for obtaining fine brandy, of any desired strength, at one and the same heat. He obtained a patent for this invention in July, 1801, and soon afterwards was enabled by his success to set up in that city a magnificent distillery, which attracted the admiration of all the practical chemists of the day. In November, 1805, he obtained a certificate of improvements whereby he could extract from wine at one process the whole of its alcohol. Adam was so overjoyed after making his first experiments, that, like another Archimedes, he ran about the streets telling everybody of the surprising results of his new invention. About the same time, Solimani, professor of chemistry at Montpellier, and Isaac Berard, distiller in the department of Gard, having contrived two distinct systems of apparatus, each most ingenious, and obtaining results little inferior to those of Adam, became in consequence formidable rivals of his fame and fortune.

The late Dr. Ure devised a form of distilling apparatus which to him appeared to combine the delicacy of the French with the solidity of the English forms. The lower the temperature of the spirituous vapour which enters into the refrigerator, the stronger and finer will the condensed spirit be; because the noxious oils are less volatile than alcohol, and come over chiefly with the aqueous vapour. A perfect still should therefore, he believed, consist of three parts: first, the cucurbit or boiler; second, the rectifier, for intercepting the greater part of the watery particles, and the whole of the corn oil; and third, the refrigerator. Three principal objects are obtained by this arrangement: first, the extraction from fermented wort or wine, at one operation, of a spirit of any desired cleanness and strength; second, a great economy of time, labour, and fuel; third, freedom from all danger of blowing up or boiling over by mismanaged firing. When a mixture of the alcohol, water, and essential oil, in the state of vapour, is passed upwards through a series of winding passages, maintained at a regulated degree of heat, from 170° to 180°, the alcohol alone, in notable proportion, retains the elastic form, and proceeds onward into the refrigeratory tube, in which these passages terminate; while the water and the oil are, in a great measure, condensed and retained in the passages, so as to drop back into the body of the still, and be discharged with the effete residuum.

The system of channels shown in fig. 2 is so contrived as to bring the compound vapours which rise from the alembic a into intimate and extensive contact with metallic surfaces, immersed in a water-bath, and maintained at any desired temperature by a self-regulating thermostat or heat-governor. The neck of the alembic tapers upwards as shown at B, fig. 1; and at c, fig. 2, it enters the bottom or ingress

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