Chapter VIII: Act 1890 (2)

The simplest way of showing dispersion is to refract a narrow beam of sunlight through a prism of glass or prismatic vessel containing water or other clear liquid. As the light is twice refracted, the dispersion is increased, and the rays, after transmission through the prism, form a divergent system, which may be allowed to fall on a sheet of white paper, forming the well-known solar spectrum. This method was employed by Sir Isaac Newton, whose experiments constitute the earliest systematic investigation of the phenomenon. Let O (fig. 2) represent a small hole in the shutter of a darkened room, and OS a narrow beam of sunlight which is allowed to fall on a white screen so as to form an image of the sun at S. If now the prism P be interposed as in the figure, the whole beam is not only refracted upward, but also spread out into the spectrum RV, the horizontal breadth of the band of colours being the same as that of the original image S. In an experiment similar to that here represented, Newton made a small hole in the screen and another small hole in a second screen placed behind the first. By slightly turning the prism P, the position of the spectrum on the first screen could be shifted sufficiently to cause light of any desired colour to pass through. Some of this light also passed through the second hole, and thus he obtained a narrow beam of practically homogeneous light in a fixed direction (the line joining the apertures in the two screens). Operating on this beam with a second prism, he found that the homogeneous light was not dispersed, and also that it was more refracted the nearer the point from which it was taken approached to the violet end of the spectrum RV. This confirmed his previous conclusion that the rays increase in refrangibility from red to violet.

Newton also made use of the method of crossed prisms, which has been found of great use in studying dispersion. The prism P (fig. 3) refracts upwards, while the prism Q, which has its refracting edge perpendicular to that of P, refracts towards the right. The combined effect of the two is to produce a spectrum sloping up from left to right. The spectrum will be straight if the two prisms are similar in dispersive property, but if one of them is constructed of a material which possesses any peculiarity in this respect it will be revealed by the curvature of the spectrum.

The coloured borders seen in the images produced by simple lenses are due to dispersion. The explanation of the colours of the rainbow, which are also due to dispersion, was given by Newton, although it was known previously to be due to refraction in the drops of rain (see RAINBOW).

According to the wave-theory of light, refraction (q.v.) is due to a change of velocity when light passes from one medium to another. The phenomenon of dispersion shows that in dispersive media the velocity is different for lights of different wave-lengths. In free space, light of all wave-lengths is propagated with the same velocity, as is shown by the fact that stars, when occulted by the moon or planets, preserve their white colour up to the last moment of disappearance, which would not be the case if one colour reached the eye later than another. The absence of colour changes in variable stars or in the appearance of new stars is further evidence of the same fact. All material media, however, are more or less dispersive. In air and other gases, at ordinary pressures, the dispersion is very small, because the refractivity is small. The dispersive powers of gases are, however, generally comparable with those of liquids and solids.

_Dispersive Power._--In order to find the amount of dispersion caused
by any given prism, the deviations produced by it on two rays of any
definite pure colours may be measured. The angle of difference between
these deviations is called the dispersion for those rays. For this
purpose the C and F lines in the spark-spectrum of hydrogen, situated
in the red and blue respectively, are usually employed. If [delta]F
and [delta]C are the angular deviations of these rays, then [delta]F -
[delta]C is called the mean dispersion of the prism. If the refracting
angle of the prism is small, then the ratio of the dispersion to the
mean deviation of the two rays is the dispersive power of the material
of the prism. Instead of the mean deviation, 1/2 ([delta]F + [delta]C),
it is more usual to take the deviation of some intermediate ray. The
exact position of the selected ray does not matter much, but the
yellow D line of sodium is the most convenient. If we denote its
deviation by [delta]D, then we may put

_Dispersive power_ = ([delta]F - [delta]C)/[delta]D (1).

This quantity may readily be expressed in terms of the refractive
indices for the three colours, for if A is the angle of the prism
(supposedly small)

[delta]C = ([mu]C - 1)A,
[delta]D = ([mu]D - 1)A,
[delta]F = ([mu]F - 1)A,

where [mu]C,[mu]D,[mu]F are the respective indices of refraction. This
gives at once

_Dispersive power_ = ([mu]F - [mu]C)/([mu]D - 1) (2).

The second of these two expressions is generally given as the
definition of dispersive power. It is more useful than (1), as the
refractive indices may be measured with a prism of any convenient
angle.

By studying the dispersion of colours in water, turpentine and crown
glass Newton was led to suppose that dispersion is proportional to
refraction. He concluded that there could be no refraction without
dispersion, and hence that achromatism was impossible of attainment
(see ABERRATION). This conclusion was proved to be erroneous when
Chester M. Hall in 1733 constructed achromatic lenses. Glasses can now
be made differing considerably both in refractivity and dispersive
power.

_Irrationality of Dispersion._--If we compare the spectrum produced by
refraction in a glass prism with that of a diffraction grating, we
find not only that the order of colours is reversed, but also that the
same colours do not occupy corresponding lengths on the two spectra,
the blue and violet being much more extended in the refraction
spectrum. The refraction spectra for different media also differ
amongst themselves. This shows that the connexion between the
refrangibility of light and its wave-length does not obey any simple
law, but depends on the nature of the refracting medium. This property
is referred to as the "irrationality of dispersion." In a diffraction
spectrum the diffraction is proportional to the wave-length, and the
spectrum is said to be "normal." If the increase of the angle of
refraction were proportional to the diminution of wave-length for a
prism of any material, the resulting spectrum would also be normal.
This, however, is not the case with ordinary refracting media, the
refrangibility generally increasing more and more rapidly as the
wave-length diminishes.

The irrationality of dispersion is well illustrated by C.
Christiansen's experiments on the dispersive properties of white
powders. If the powder of a transparent substance is immersed in a
liquid of the same refractive index, the mixture becomes transparent
and a measurement of the refractive index of the liquid gives the
refractivity of the powder. Christiansen found, in an investigation of
this kind, that the refractivity of the liquid could only be got to
match that of the powder for mono-chromatic light, and that, if white
light were used, brilliant colour effects were obtained, which varied
in a remarkable manner when small changes occurred in the refractive
index of the liquid. These effects are due to the difference in
dispersive power of the powder and the liquid. If the refractive index
is, for instance, the same for both in the case of green light, and a
source of white light is viewed through the mixture, the green
component will be completely transmitted, while the other colours are
more or less scattered by multiple reflections and refractions at the
surfaces of the powdered substance. Very striking colour changes are
observed, according to R. W. Wood, when white light is transmitted
through a paste made of powdered quartz and a mixture of carbon
bisulphide with benzol having the same refractive index as the quartz
for yellow light. In this case small temperature changes alter the
refractivity of the liquid without appreciably affecting the quartz.
R. W. Wood has studied the iridescent colours seen when a precipitate
of potassium silicofluoride is produced by adding silicofluoric acid
to a solution of potassium chloride, and found that they are due to
the same cause, the refractive index of the minute crystals
precipitated being about the same as that of the solution, which
latter can be varied by dilution.

_Anomalous Dispersion._--In some media the usual order of the colours
is changed. This curious phenomenon was noticed by W. H. Fox Talbot
about 1840, but does not seem to have become generally known. In 1860
F. P. Leroux discovered that iodine vapour refracted the red rays more
than the violet, the intermediate colours not being transmitted; and
in 1870 Christiansen found that an alcoholic solution of fuchsine
refracted the violet less than the red, the order of the successive
colours being violet, red, orange, yellow; the green being absorbed
and a dark interval occurring between the violet and red. A. Kundt
found that similar effects occur with a large number of substances, in
particular with all those which possess the property of "surface
colour," i.e., which strongly reflect light of a definite colour, as
do many of the aniline dyes. Such bodies show strong absorption bands
in those colours which they reflect, while of the transmitted light
that which is of a slightly greater wave-length than the absorbed
light has an abnormally great refrangibility, and that of a slightly
shorter wave-length an abnormally small refrangibility. The name given
to this phenomenon,--"anomalous dispersion"--is an unfortunate one, as
it has been found to obey a regular law.

In studying the dispersion of the aniline dyes, a prism with a very
small refracting angle is made of two glass plates slightly inclined
to each other and enclosing a very thin wedge of the dye, which is
either melted between the plates, or is in the form of a solution
retained in position by surface-tension. Only very thin layers are
sufficiently transparent to show the dispersion near or within an
absorption band, and a large refracting angle is not required, the
dispersion usually being very considerable. Another method, which has
been used by R. W. Wood and C. E. Magnusson, is to introduce a thin
film of the dye into one of the optical paths of a Michelson
interferometer, and to determine the consequent displacement of the
fringes. E. Mach and J. Arbes have used a method depending on total
reflection (Drude's _Theory of Optics_, p. 394).

A very remarkable example of anomalous dispersion, which was first
observed by A. Kundt, is that exhibited by the vapour of sodium. It
has not been found practicable to make a prism of this vapour in the
ordinary way by enclosing it in a glass vessel of the required shape,
as sodium vapour attacks glass, quickly rendering it opaque. A. E.
Becquerel, however, investigated the character of the dispersion by
using prism-shaped flames strongly coloured with sodium. But the best
way of exhibiting the effect is by making use of a remarkable property
of sodium vapour discovered by R. W. Wood and employed for this
purpose in a very ingenious manner. He found that when sodium is
heated in a hard glass tube, the vapour which is formed is
extraordinarily cohesive, only slowly spreading out in a cloud with
well-defined borders, which can be rendered visible by placing the
tube in front of a sodium flame, against which the cloud appears
black. If a long glass tube with plane ends, and containing some
pellets of sodium is heated in the middle by a row of burners, the
cool ends remain practically vacuous and do not become obscured. The
sodium vapour in the middle is very dense on the heated side, the
density diminishing rapidly towards the upper part of the tube, so
that, although not prismatic in form, it refracts like a prism owing
to the variation in density. Thus if a horizontal slit is illuminated
by an arc lamp, and the light-rendered parallel by a collimating
lens--is transmitted through the sodium tube and focused on the
vertical slit of a spectroscope, the effect of the sodium vapour is to
produce its refraction spectrum vertically on the slit. The image of
this seen through the glass prism of the spectroscope will appear as
in fig. 4. The whole of the light, with the exception of a small part
in the neighbourhood of the D lines, is practically undeviated, so
that it illuminates only a very short piece of the slit and is spread
out into the ordinary spectrum. But the light of slightly greater
wave-length than the D lines, being refracted strongly downward by the
sodium vapour, illuminates the bottom of the slit; while that of
slightly shorter wave-length is refracted upward and illuminates the
top of the slit. Fig. 4 represents the inverted image seen in the
telescope. The light corresponding to the D lines and the space
between them is absorbed, as evidenced by the dark interval. If the
sodium is only gently heated, so as to produce a comparatively
rarefied vapour, and a grating spectroscope employed, the spectrum
obtained is like that shown in fig. 5, which was the effect noticed by
Becquerel with the sodium flame. Here the light corresponding to the
space between the D lines is transmitted, being strongly refracted
upward near D1, and downward near D2.

The theory of anomalous dispersion has been applied in a very
interesting way by W. H. Julius to explain the "flash spectrum" seen
during a solar eclipse at the moment at which totality occurs. The
conditions of this phenomenon have been imitated in the laboratory by
Wood, and the corresponding effect obtained.

_Theories of Dispersion._--The first attempt at a mathematical theory
of dispersion was made by A. Cauchy and published in 1835. This was
based on the assumption that the medium in which the light is
propagated is discontinuous and molecular in character, the molecules
being subject to a mutual attraction. Thus, if one molecule is
disturbed from its mean position, it communicates the disturbance to
its neighbours, and so a wave is propagated. The formula arrived at by
Cauchy was

B C
n = A + --------- + --------- + ....
[lambda]2 [lambda]4

n being the refractive index, [lambda] the wave-length, and A, B, C,
&c., constants depending on the material, which diminish so rapidly
that only the first three as here written need be taken into account.
If suitable values are chosen for these constants, the formula can be
made to represent the dispersion of ordinary transparent media within
the visible spectrum very well, but when extended to the infra-red
region it often departs considerably from the truth, and it fails
altogether in cases of anomalous dispersion. There are also grave
theoretical objections to Cauchy's formula.

The modern theory of dispersion, the foundation of which was laid by
W. Sellmeier, is based upon the assumption that an interaction takes
place between ether and matter. Sellmeier adopted the elastic-solid
theory of the ether, and imagined the molecules to be attached to the
ether surrounding them, but free to vibrate about their mean positions
within a limited range. Thus the ether within the dispersive medium is
loaded with molecules which are forced to perform oscillations of the
same period as that of the transmitted wave. It can be shown
mathematically that the velocity of propagation will be greatly
increased if the frequency of the light-wave is slightly greater, and
greatly diminished if it is slightly less than the natural frequency
of the molecules; also that these effects become less and less marked
as the difference in the two frequencies increases. This is exactly in
accordance with the observed facts in the case of substances showing
anomalous dispersion. Sellmeier's theory did not take account of
absorption, and cannot be applied to calculate the dispersion within a
broad absorption band. H. von Helmholtz, working on a similar
hypothesis, but with a frictional term introduced into his equations,
obtained formulae which are applicable to cases of absorption. A
modified form of Helmholtz's equation, due to E. Ketteler and known as
the Ketteler-Helmholtz formula, has been much used in calculating
dispersion, and expresses the facts with remarkable accuracy. P. Drude
has obtained a similar formula based on the electromagnetic theory,
thus placing the theory of dispersion on a much more satisfactory
basis. The fundamental assumption is that the medium contains
positively and negatively charged ions or electrons which are acted on
by the periodic electric forces which occur in wave propagation on
Maxwell's theory. The equations finally arrived at are
____
\ D[lambda]^2([lambda]^2 - [lambda]_m^2)
n^2(1 - [kappa]^2) = 1 + > ------------------------------------------,
/___ ([lambda^2 - [lambda]_m^2) + g^2[lmabda]^2
____
\ Dg[lambda]^3
2n^2[kappa]^2 = > -------------------------------------------,
/___ ([lambda]^2 - [lambda]_m^2) + g^2[lmabda]^2

where [lambda] is the wave-length in free ether of light whose
refractive index is n, and [lambda]_m the wave-length of light of the
same period as the electron, [kappa] is a coefficient of absorption,
and D and g are constants. The sign of summation [Sigma] is used in
cases where there are several absorption bands, and consequently
several similar terms on the right-hand side, each with a different
value of [lambda]_m. This would occur if there were several kinds of
ions, each with its own natural period.

In a region where there is no absorption, we have [kappa] = 0 and
therefore g = 0, and we have only one equation, namely,

____
\ D[lambda]^2
n^2 = 1 + > --------------------------,
/___ ([lambda]^2 - [lambda]_m^2)

which is identical with Sellmeier's result. As [lambda]_m, is a
wave-length corresponding to an absorption band, this formula can be
used to find values of [lambda]_m which satisfy the observed values
of n within the region of transparency, and so to determine where the
absorption bands are situated. In this way the existence of bands in
the infrared part of the spectrum has been predicted in the case of
quartz and detected by experiments on the selective reflection of the
material.

_References._--For the theory of dispersion see P. Drude, _Theory of
Optics_ (Eng. trans.); R. W. Wood, _Physical Optics_; and A. Schuster,
_Theory of Optics_. For descriptive accounts, see Wood's _Physical
Optics_, T. Preston's _Theory of Light_, E. Edser's _Light_. The last
work contains an elementary treatment of Sellmeier's theory.
(J. R. C.)

D'ISRAELI (or DISRAELI), ISAAC (1766-1848), English man of letters, father of the earl of Beaconsfield (q.v.), was born at Enfield in May 1766. He belonged to a Jewish family which, having been driven by the Inquisition from Spain, towards the end of the 15th century, settled as merchants at Venice, and assumed the name which has become famous; it was generally spelt D'Israeli until the middle of the 19th century. In 1748 his father, Benjamin D'Israeli, then only about eighteen years of age, removed to England, where, before passing the prime of life, he amassed a competent fortune, and retired from business. He belonged to the London congregation of Spanish and Portuguese Jews, of which his son also remained a nominal member until after Benjamin D'Israeli died at the end of 1816.

The strongly marked characteristics which determined Isaac D'Israeli's career were displayed to a singular degree even in his boyhood. He spent his time over books and in long day-dreams, and evinced the strongest distaste for business and all the more bustling pursuits of life. These idiosyncrasies met with no sympathy from either of his parents, whose ambitious plans for his future career they threatened to disappoint. When he was about fourteen, in the hope of changing the bent of his mind, his father sent him to live with his agent at Amsterdam, where he worked under a tutor for four or five years. Here he studied Bayle and Voltaire, and became an ardent disciple of Rousseau. Here also he wrote a long poem against commerce, which he produced as an exposition of his opinions when, on his return to England, his father announced his intention of placing him in a commercial house at Bordeaux. Against such a destiny D'Israeli's mind strongly revolted; and he carried his poem, with a letter earnestly appealing for advice and assistance, to Samuel Johnson; but when he called again a week after to receive an answer, the packet was returned unopened--the great Doctor was on his death-bed. He also addressed a letter to Dr Vicesimus Knox, master of Tonbridge Grammar School, begging to be received into his family, that he might enjoy the benefit of his learning and experience. How this application was answered we do not know. The evident firmness of his resolve, however, was not without effect. His parents gave up their purpose for a time. He was sent to travel in France, and allowed to occupy himself as he wished; and he had the happiness of spending some months in Paris, in the society of literary men, and devoted to the literary pursuits in which he delighted.

In the beginning of 1788 he returned home, and in the next year he attacked Peter Pindar (John Wolcot) in _The Gentleman's Magazine_ in a poem in the manner of Pope, "On the Abuse of Satire." The authorship of the poem was much debated, and it was attributed by some to William Hayley, upon whom it was actually avenged, with characteristic savageness, by its victim. It is greatly to Wolcot's credit that, on learning his mistake, he sought the acquaintance of his young opponent, whose friend he remained to the end of his life. Through the success of this satire D'Israeli made the acquaintance of Henry James Pye, who helped to persuade his father that it would be a mistake to force him into a business career, and introduced him into literary circles. D'Israeli dedicated his first book, _A Defence of Poetry_, to Pye in 1790. Henceforth his life was passed in the way he best liked--in quiet and almost uninterrupted study. In 1802 he married Maria Basevi, by whom he had five children, of whom Benjamin (afterwards Lord Beaconsfield and Prime Minister of England) was the second. He was able to maintain his strenuous habits of study till he reached the advanced age of seventy-two, when he was forced, by paralysis of the optic nerve, to give up work almost entirely. He lived ten years longer, and died at his seat at Bradenham House, Buckinghamshire, on the 19th of January 1848.

Isaac D'Israeli is most celebrated as the author of the _Curiosities of Literature_ (1791, subsequent volumes in 1793, 1817, 1823 and 1834). It is a miscellany of literary and historical anecdotes, of original critical remarks, and of interesting and curious information of all kinds, animated by genuine literary feeling, taste and enthusiasm. With the _Curiosities of Literature_ may be classed D'Israeli's _Miscellanies, or Literary Recreations_ (1796), the _Calamities of Authors_ (1812-1813), and the _Quarrels of Authors_ (1814). Towards the close of his life D'Israeli projected a continuous history of English literature, three volumes of which appeared in 1841 under the title of the _Amenities of Literature_. But of all his works the most delightful is his _Essay on the Literary Character_ (1795), which, like most of his writings, abounds in illustrative anecdotes. In the famous "Pope controversy" he supported Byron and Campbell against Bowles and Hazlitt by a defence of Pope in the form of a criticism of Joseph Spence's _Anecdotes_ contributed to the _Quarterly Review_ (July 1820). In 1797 D'Israeli published three novels; one of these, _Mejnoun and Leila, the Arabian Petrarch and Laura_, was said to be the first oriental romance in English. His last novel, _Despotism, or the Fall of the Jesuits_, appeared in 1811, but none of his romances was popular. He also published a slight sketch of Jewish history, and especially of the growth of the Talmud, entitled the _Genius of Judaism_ (1833).

He was the author of two historical works--a brief defence of the literary merit and personal and political character of James I. (1816), and a learned _Commentary on the Life and Reign of King Charles I._ (1828-1831). This was recognized by the University of Oxford, which conferred upon the author the honorary degree of D.C.L. As an historian D'Israeli is distinguished by two characteristics. In the first place, he had small interest in politics, and no sympathy with the passionate fervour, or adequate appreciation of the importance, of political struggles. And, secondly, with a laborious zeal then less common than now among historians, he sought to bring to light fresh historical material by patient search for letters, diaries and other manuscripts of value which had escaped the notice of previous students. Indeed, the honour has been claimed for him of being one of the founders of the modern school of historical research.

Of the amiable personal character and the placid life of Isaac
D'Israeli a charming picture is to be found in the brief memoir
prefixed to the 1849 edition of _Curiosities of Literature_, by his
son Lord Beaconsfield.

DISS, a market town in the southern parliamentary division of Norfolk, England; near the river Waveney (the boundary with Suffolk), 95 m. N.E. by N. from London by the Great Eastern railway. Pop. of urban district (1901) 3745. The town lies pleasantly upon a hill rising above a mere, which drains to the Waveney, having its banks laid out as public gardens. The church of St Mary exhibits Decorated and Perpendicular stone and flint work. There is a corn exchange and the agricultural trade is considerable; brushes and matting are manufactured. The poet and satirist, John Skelton (d. 1529), was rector here in the later part of his life, and is doubtfully considered a native.

DISSECTION (from Lat. _dissecare_, to cut apart), the separation into parts by cutting, particularly the cutting of an animal or plant into parts for the purpose of examination or display of its structure.

DISSENTER (Lat. _dis-sentire_, to disagree), one who dissents or disagrees in matters of opinion, belief, &c. The term "dissenter" is, however, practically restricted to the special sense of a member of a religious body in England which has, for one reason or another, separated from the Established Church. Strictly, the term includes the English Roman Catholics, who in the original draft of the Relief Act of 1791 were styled "Protesting Catholic Dissenters." It is in practice, however, restricted to the "Protestant Dissenters" referred to in sec. ii. of the Toleration Act of 1688. The term is not applied to those bodies who dissent from the Established Church of Scotland; and in speaking of members of religious bodies which have seceded from established churches abroad it is usual to employ the term "dissidents" (Lat. _dissidere_, to dissent). In this connotation the terms "dissenter" and "dissenting," which had acquired a somewhat contemptuous flavour, have tended since the middle of the 19th century to be replaced by "nonconformist," a term which did not originally imply secession, but only refusal to conform in certain particulars (e.g. the wearing of the surplice) with the authorized usages of the Established Church. Still more recently the term "nonconformist" has in its turn, as the political attack on the principle of a state establishment of religion developed, tended to give place to the style of "Free Churches" and "Free Churchman." All three terms are now in use, "nonconformist" being the most usual, as it is the most colourless. (See CONGREGATIONALISM, &c.)

DISSOCIATION, a separation or dispersal, the opposite of association. In chemistry the term is given to chemical reactions in which a substance decomposes into two or more substances, and particularly to cases in which associated molecules break down into simpler molecules. Thus the reactions NH4Cl <=> NH3 + HCl, and PCl5 <=> PCl3 + Cl2 are instances of the first type; N2O4 <=> 2NO2, of the second (see CHEMICAL ACTION). Electrolytic or ionic dissociation is the separation of a substance in solution into ions (see ELECTROLYSIS; SOLUTION).

DISSOLUTION (from Lat. _dissolvere_, to break up into parts), the act of dissolving or reducing to constituent parts, especially of the bringing to an end an association such as a partnership or building society, and particularly of the termination of an assembly. A dissolution of parliament in England is thus the end of its existence, brought about by the efflux of time in accordance with the Septennial Act 1716, or by an exercise of the royal prerogative. This is done either in person, or by commission, if parliament is sitting; if prorogued, then by proclamation. The word is used as a synonym for end or death.

DISTAFF, in the early forms of spinning, the "rock" or short stick round one end of which the flax, cotton or wool is loosely wound, and from which it is spun off by the spindle. The word is derived from the Old English _distaef_, the first part of which is connected with _dizen_, in modern English seen in "bedizen," to deck out or embellish, originally "to equip the distaff with flax, &c.," cf. the German dialectal word _Diesse_, flax. The last part of the word is "staff." "Distaff" from early times has been used to symbolize woman's work (cf. the use of "spinster" for an unmarried woman); thus the "distaff" or "spindle" side of a family refers to the female branch, as opposed to the "spear" or male branch. The 7th of January, the day after Epiphany, was formerly known as St Distaff's day, as women then began work again after the Christmas holiday.

DISTILLATION (from the Lat. _distillare_, more correctly _destillare_, to drop or trickle down), an operation consisting in the conversion of a substance or mixture of substances into vapours which are afterwards condensed to the liquid form; it has for its object the separation or purification of substances by taking advantage of differences in volatility. The apparatus consists of three parts:--the "retort" or "still," in which the substance is heated; the "condenser," in which the vapours are condensed; and the "receiver," in which the condensed vapours are collected. Generally the components of a mixture will be vaporized in the order of their boiling-points; consequently if the condensates or "fractions" corresponding to definite ranges of temperature be separately collected, it is obvious that a more or less partial separation of the components will be effected. If the substance operated upon be practically pure to start with, or the product of distillation be nearly of constant composition, the operation is termed "purification by distillation" or "rectification"; the latter term is particularly used in the spirit industry. If a complex mixture be operated upon, and a separation effected by collecting the distillates in several portions, the operation is termed "fractional distillation." Since many substances decompose either at, or below, their boiling-points under ordinary atmospheric pressure, it is necessary to lower the boiling-point by reducing the pressure if it be desired to distil them. This variation is termed "distillation under reduced pressure or in a vacuum." The vaporization of a substance below its normal boiling-point can also be effected by blowing in steam or some other vapour; this operation is termed "distillation with steam." "Dry distillation" is the term used when solid substances which do not liquefy on heating are operated upon; "sublimation" is the term used when a solid distils without the intervention of a liquid phase.

Distillation appears to have been practised at very remote times. The Alexandrians prepared oil of turpentine by distilling pine-resin; Zosimus of Panopolis, a voluminous writer of the 5th century A.D., speaks of the distillation of a "divine water" or "panacea" (probably from the complex mixture of calcium polysulphides, thiosulphate, &c., and free sulphur, which is obtained by boiling sulphur with lime and water) and advises "the efficient luting of the apparatus, for otherwise the valuable properties would be lost." The Arabians greatly improved the earlier apparatus, naming one form the alembic (q.v.); they discovered many ethereal oils by distilling plants and plant juices, alcohol by the distillation of wine, and also distilled water. The alchemists gave great attention to the method, as is shown by the many discoveries made. Nitric, hydrochloric and sulphuric acids, all more or less impure, were better studied; and many ethereal oils were discovered. Prior to about the 18th century three forms of distillation were practised: (1) _destillatio per ascensum_, in which the retort was heated from the bottom, and the vapours escaped from the top; (2) _destillatio per latus_, in which the vapours escaped from the side; (3) _destillatio per descensum_, in which the retort was heated at the top, and the vapours led off by a pipe passing through the bottom. According to K. B. Hoffmann the earliest mention of destillatio per descensum occurs in the writings of Aetius, a Greek physician who flourished at about the end of the 5th century.

In modern times the laboratory practice of distillation was greatly facilitated by the introduction of the condenser named after Justus von Liebig; A. Kolbe and E. Frankland introduced the "reflux condenser," i.e. a condenser so placed that the condensed vapours return to the distilling flask, a device permitting the continued boiling of a substance with little loss; W. Dittmar and R. Anschutz, independently of one another, introduced "distillation under reduced pressure"; and "fractional distillation" was greatly aided by the columns of Wurtz (1855), E. Linnemann (1871), and of J. A. Le Bel and A. Henninger (1874). In chemical technology enormous strides have been made, as is apparent from the coal-gas, coal-tar, mineral oil, spirits and mineral acids industries.

The subject is here treated under the following subdivisions: (1) ordinary distillation, (2) distillation under reduced pressure, (3) fractional distillation, (4) distillation with steam, (5) theory of distillation, (6) dry distillation, (7) distillation in chemical technology and (8) commercial distillation of water.

1. _Ordinary Distillation._--The apparatus generally used is shown in
fig. 1. The substance is heated in a retort a, which consists of a
large bulb drawn out at the top to form a long neck; it may also be
provided with a tubulure, or opening, which permits the charging of
the retort, and also the insertion of a thermometer b. The retort may
be replaced by a distilling flask, which is a round-bottomed flask
(generally with a lengthened neck) provided with an inclined side
tube. The neck of the retort, or side tube of the flask, is connected
to the condenser c by an ordinary or rubber cork, according to the
nature of the substance distilled; ordinary corks soaked in paraffin
wax are very effective when ordinary or rubber corks cannot be used.
Sometimes an "adapter" is used; this is simply a tapering tube, the
side tube being corked into the wider end, and the condenser on to the
narrower end. The thermometer is placed so that the bulb is near the
neck of the retort or the side tube of the distilling flask. It
generally happens that much of the mercury column is outside the flask
and consequently at a lower temperature than the bulb, hence a
correction of the observed temperature is necessary. If N be the
length of the unheated mercury column in degrees, t the temperature of
this column (generally determined by a small thermometer placed with
its bulb at the middle of the column), and T the temperature recorded
by the thermometer, then the corrected temperature of the vapour is T
+ 0.000143 (T - t) N (T. E. Thorpe, _Journ. Chem. Soc._, 1880, p.
159).

The mode of heating varies with the substance to be distilled. For
highly volatile liquids, e.g. ether, ligroin, &c., immersion of the
flask in warm water suffices; for less volatile liquids a directly
heated water or sand bath is used; for other liquids the flask is
heated through wire gauze or asbestos board, or directly by a Bunsen.
The condensing apparatus must also be conditioned by the volatility.
With difficulty volatile substances, e.g. nitrobenzene, air cooling of
the retort neck or of a straight tube connected with the distilling
flask will suffice; or wet blotting-paper placed on the tube and the
receiver immersed in water may be used. For less volatile liquids the
Liebig condenser is most frequently used. In its original form, this
consists of a long tube surrounded by an outer tube so arranged that
cold water circulates in the annular space between the two. The
vapours pass through the inner tube, and the cold water enters at the
end farthest from the distilling flask. For more efficient
condensation--and also for shortening the apparatus--the central tube
may be flattened, bent into a succession of V's, or twisted into a
spiral form, the object in each case being to increase the condensing
surface. Of other common types of condenser, we may notice the
"spiral" or "worm" type, which consists of a glass, copper or tin worm
enclosed in a vessel in which water circulates; and the ball
condenser, which consists of two concentric spheres, the vapour
passing through the inner sphere and water circulating in the space
between this and the outer (in another form the vapour circulates in a
shell, on the outside and inside of which water circulates). A very
effective type is shown in fig. 2. The condensing water enters at the
top and is conducted to the bottom of the inner tube, which it fills
and then flows over the outside of the outer tube; it collects in the
bottom funnel and is then led off. The vapours pass between the inner
and outer tubes.

Practically any vessel may serve as a receiver--test tube, flask,
beaker, &c. If noxious vapours come over, it is necessary to have an
air-tight connexion between the condenser and receiver, and to provide
the latter with an outlet tube leading to an absorption column or
other contrivance in which the vapours are taken up. If the substances
operated upon decompose when heated in air, as, for example, the zinc
alkyls which inflame, the air within the apparatus is replaced by some
inert gas, e.g. nitrogen, carbon dioxide, &c., which is led in at the
distilling flask before the process is started, and a slow current
maintained during the operation.

2. _Distillation under Reduced Pressure._--This method is adopted for
substances which decompose at their boiling-points under ordinary
pressure, and, generally, when it is desirable to work at a lower
temperature. The apparatus differs very slightly from that employed in
ordinary distillation. The "receiver" must be connected on the one
side to the condenser, and on the other to the exhaust pump. A safety
vessel and a manometer are generally interposed between the pump and
receiver. For the purpose of collecting the distillates in fractions,
many forms of receivers have been devised. Bruhl's is one of the
simplest. It consists of a number of tubes mounted vertically on a
horizontal circular disk which rotates about a vertical axis in a
cylindrical vessel. This vessel has two tubulures: through one the end
of the condenser projects so as to be over one of the receiving tubes;
the other leads to the pump. By rotating the disk the tubes may be
successively brought under the end of the condenser. Boiling under
reduced pressure has one very serious drawback, viz. the liquid boils
irregularly or "bumps." W. Dittmar showed that this may be avoided by
leading a fine, steady stream of dry gas-air, carbon dioxide,
hydrogen, &c., according to the substance operated upon--through the
liquid by means of a fine capillary tube, the lower end of which
reaches to nearly the bottom of the flask. "Bumping" is common in open
boiling when the liquid is free from air bubbles and the interior of
the vessel is very smooth. It may be diminished by introducing
clippings of platinum foil, pieces of porcelain, glass beads or
garnets into the liquid. "Frothing" is another objectionable feature
with many liquids. When cold, froth can be immediately dissipated by
adding a few drops of ether. In boiling liquids its formation may be
prevented by adding paraffin wax; the wax melts and forms a ring on
the surface of the liquid, which boils tranquilly in the centre.

FIG. 3.]

3. _Fractional Distillation._--By fractional distillation is meant the
separation of a mixture having components which boil at neighbouring
temperatures. The distilling flask has an elongated neck so that the
less volatile vapours are condensed and return to the flask, while the
more volatile component passes over. The success of the operation
depends upon two factors: (1) that the heating be careful, slow and
steady, and (2) that the column attached to the flask be efficient to
sort out, as it were, the most volatile vapour. Three types of columns
are employed: (1) the elongation is simply a straight or bulb tube;
(2) the column, properly termed a "dephlegmator," is so constructed
that the vapours have to traverse a column of previously condensed
vapour; (3) the column is encircled by a jacket through which a liquid
circulates at the same temperature as the boiling-point of the most
volatile component. To the first type belongs the simple straight
tube, and the Wurtz tube (see fig. 3), which is simply a series of
bulbs blown on a tube. These forms are not of much value. Several
forms of the second type are in use. In the Linnemann column the
condensed vapours temporarily collect on platinum gauzes (a) placed at
the constrictions of a bulbed tube. In the Le Bel-Henninger form a
series of bulbs are connected consecutively by means of syphon tubes
(b) and having platinum gauzes (a) at the constrictions, so that when
a certain amount of liquid collects in any one bulb it syphons over
into the next lower bulb. The Glynsky form is simpler, having only one
syphon tube; at the constrictions it is usual to have a glass bead.
The "rod-and-disk" form of Sidney Young is a series of disks mounted
on a central spindle and surrounded by a slightly wider tube. The
"pear-shaped" form of the same author consists of a series of
pear-shaped bulbs, the narrow end of one adjoining the wider end of
the next lower one. In this class may also be placed the Hempel tube,
which is simply a straight tube filled with glass beads. Of the third
type is the Warren column consisting of a spiral kept at a constant
temperature by a liquid bath. Improved forms were devised by F. D.
Brown. Kreusler's form is easily made and manipulated. A tube closed
at the bottom is traversed by an open narrower tube, and the
arrangement is fitted in the neck of the distilling flask. Water is
led in by the inner tube, and leaves by a side tube fused on the wider
tube. Many comparisons of the effectiveness of dephlegmating columns
have been made (see Sidney Young, _Fractional Distillation_, 1903).
The pear-shaped form is the most effective, second in order is the Le
Bel-Henninger, which, in turn, is better than the Glynsky. The main
objection to the Hempel is the retention of liquid in the beads, and
the consequent inapplicability to the distillation of small
quantities.

4. _Distillation with Steam._--In this process a current of steam,
which is generated in a separate boiler and superheated, if necessary,
by circulation through a heated copper worm, is led into the
distilling vessel, and the mixed vapours condensed as in the ordinary
processes. This method is particularly successful in the case of
substances which cannot be distilled at their ordinary boiling-points
(it will be seen in the following section that distilling with steam
implies a lowering of boiling-point), and which can be readily
separated from water. Instances of its application are found in the
separation of ortho- and para-nitrophenol, the o-compound distilling
and the p- remaining behind; in the separation of aniline from the
mixture obtained by reducing nitrobenzene; of the naphthols from the
melts produced by fusing the naphthalene monosulphonic acids with
potash; and of quinoline from the reaction between aniline,
nitrobenzene, glycerin, and sulphuric acid (the product being first
steam distilled to remove any aniline, nitrobenzene, or glycerin, then
treated with alkali, and again steam distilled when quinoline comes
over). With substances prone to discolorization, as, for example,
certain amino compounds, the operation may be conducted in an
atmosphere of carbon dioxide, or the water may be saturated with
sulphuretted hydrogen. Liquids other than water may be used: thus
alcohol separates [alpha]-pipecoline and ether nitropropylene.

5. _Theory of Distillation._--The general observation that under a
constant pressure a pure substance boils at a constant temperature
leads to the conclusion that the distillate which comes over while the
thermometer records only a small variation is of practically constant
composition. On this fact depends "rectification or purification by
distillation." A liquid boils when its vapour pressure equals the
superincumbent pressure (see VAPORIZATION); consequently any process
which diminishes the external pressure must also lower the
boiling-point. In this we have the theory of "distillation under
reduced pressure." The theory of fractional distillation, or the
behaviour of liquid mixtures when heated to their boiling-points, is
more complex. For simplicity we confine ourselves to mixtures of two
components, in which experience shows that three cases are to be
recognized according as the components are (1) completely immiscible,
(2) partially miscible, (3) miscible in all proportions.

When the components are completely immiscible, the vapour pressure of
the one is not influenced by the presence of the other. The mixture
consequently distils at the temperature at which the sum of the
partial pressures equals that of the atmosphere. Both components come
over in a constant proportion until one disappears; it is then
necessary to raise the temperature in order to distil the residue. The
composition of the distillate is determinate (by Avogadro's law) if
the molecular weights and vapour pressure of the components at the
temperature of distillation be known. If M1, M2, and P1, P2 be the
molecular weights and vapour pressures of the components A and B, then
the ratio of A to B in the distillate is M1P1/M2P2. Although, as is
generally the case, one liquid (say A) is more volatile than the other
(say B), i.e. P1 greater than P2, if the molecular weight of A be much
less than that of B, then it is obvious that the ratio M1P1/M2P2 need
not be very great, and hence the less volatile liquid B would come
over in fair amount. These conditions pertain in cases where
distillation with steam is successfully practised, the relatively high
volatility of water being counterbalanced by the relatively high
molecular weight of the other component; for example, in the case of
nitrobenzene and water the ratio is 1 to 5. In general, when the
substance to be distilled has a vapour pressure of only 10 mm. at 100
deg. C., distillation with steam can be adopted, if the product can be
subsequently separated from the water.

When distilling a mixture of partially miscible components a
distillate of constant composition is obtained so long as two layers
are present, i.e. A dissolved in B and B dissolved in A, since both of
these solutions emit vapours of the same composition (this follows
since the same vapour must be in equilibrium with both solutions, for
if it were not so a cyclic system contradicting the second law of
thermodynamics would be realizable). The composition of the vapour,
however, would not be the same as that of either layer. As the
distillation proceeded one layer would diminish more rapidly than the
other until only the latter would remain; this would then distil as a
completely miscible mixture.

The distillation of completely miscible mixtures is the most common
practically and the most complex theoretically. A coordination of the
results obtained on the distillation of mixtures of this nature with
the introduction of certain theoretical considerations led to the
formation of three groups distinguished by the relative solubilities
of the vapours in the liquid components.

(i.) If the vapour of A be readily soluble in the liquid B, and the
vapour of B readily soluble in the liquid A, there will exist a
mixture of A and B which will have a lower vapour pressure than any
other mixture. The vapour pressure composition curve will be convex to
the axis of compositions, the maximum vapour pressures corresponding
to pure A and pure B, and the minimum to some mixture of A and B. On
distilling such a mixture under constant pressure, a mixture of the
two components (of variable composition) will come over until there
remains in the distilling flask the mixture of minimum vapour
pressure. This will then distil at a constant temperature. Thus nitric
acid, boiling-point 68 deg., forms a mixture with water, boiling point
100 deg., which boils at a constant temperature of 126 deg., and
contains 68% of acid. Hydrochloric acid forms a similar mixture which
boils at 110 deg. and contains 20.2% of acid. Another mixture of this
type is formic acid and water.

(ii.) If the vapours be sparingly soluble in the liquids there will
exist a mixture having a greater vapour pressure than that of any
other mixture. The vapour pressure-composition curve will now be
concave to the axis of composition, the minima corresponding to the
pure components. On distilling such a mixture, a mixture of constant
composition will distil first, leaving in the distilling flask one or
other of the components according to the composition of the mixture.
An example is propyl alcohol and water. At one time it was thought
that these mixtures of constant boiling-point (an extended list is
given in Young's _Fractional Distillation_) were definite compounds.
The above theory, coupled with such facts as the variation of the
composition of the constant boiling-point fraction with the pressure
under which the mixture is distilled, the proportionality of the
density of all mixtures to their composition, &c., shows this to be
erroneous.

(iii.) If the vapour of A be readily soluble in liquid B, and the
vapour of B sparingly soluble in liquid A, and if the vapour pressure
of A be greater than that of B, then the vapour pressures of mixtures
of A and B will continually diminish as one passes from 100% A to 100%
B. The vapour tension may approximate to a linear function of the
composition, and the curve will then be practically a straight line.
On distilling such a mixture pure A will come over first, followed by
mixtures in which the quantity of B continually increases;
consequently by a sufficient number of distillations A and B can be
completely separated. Examples are water and methyl or ethyl alcohol.

Van't Hoff (_Theoretical and Physical Chemistry_, vol. i. p. 51)
illustrates the five cases on one diagram. In fig. 4 let AB be the
axis of composition, AP be the vapour pressure of pure A, BQ the
vapour pressure of pure B. For immiscible liquids the vapour pressure
curve is the horizontal line ab, described so that aP = QB and bQ =
AP. For partially miscible liquids the curve is Pa1b1Q. The horizontal
line a1b1 corresponds to the two layers of liquid, and the inclined
lines Pa1Qb1 to solutions of B in A and of A in B. The curves Pa4Q,
having a minimum at a4, Pa3Q, having a maximum at a3, and Pa5Q, with
neither a maximum nor minimum, correspond to the types i., ii., iii.
of completely miscible mixtures.

6. _Dry Distillation._--In this process the substance operated upon is
invariably a solid, the vapours being condensed and collected as in
the other methods. When the substance operated upon is of uncertain
composition, as, for example, coal, wood, coal-tar, &c., the term
destructive distillation is employed. A more general designation is
"pyrogenic processes," which also includes such operations as leading
vapours through red-hot tubes and condensing the products. We may also
consider here cases of sublimation wherein a solid vaporizes and the
vapour condenses without the occurrence of the liquid phase.

Dry distillation is extremely wasteful even when definite substances
or mixtures, such as calcium acetate which yields acetone, are dealt
with, valueless by-products being obtained and the condensate usually
requiring much purification. Prior to 1830, little was known of the
process other than that organic compounds generally yielded tarry and
solid matters, but the discoveries of Liebig and Dumas (of acetone
from acetates), of Mitscherlich (of benzene from benzoates) and of
Persoz (of methane from acetates and lime) brought the operation into
common laboratory practice. For efficiency the operation must be
conducted with small quantities; caking may be prevented by mixing the
substance with sand or powdered pumice, or, better, with iron filings,
which also renders the decomposition more regular by increasing the
conductivity of the mass. The most favourable retort is a shallow iron
pan heated in a sand bath, and provided with a screwed-down lid
bearing the delivery tube. Sidney Young has suggested conducting the
operation in a current of carbon dioxide which sweeps out the vapours
as they are evolved, and also heating in a vapour bath, e.g. of
sulphur.