toluidine; and it is essential for the production of magenta that this
substance should be operated on along with the aniline. Whether the
presence of some toluidine is also necessary for the production of mauve
and other colours is not yet known, but they are always prepared from
commercial benzol. It is certain that pure aniline yields no magenta,
neither does pure toluidine; but a mixture supplies it in abundance. For
the preparation of magenta the best proportions for this mixture would
be about three parts of aniline to one of toluidine; but, in practice,
it is not necessary to obtain the two substances separately, as benzol,
mixed with a sufficient quantity of toluol, may be obtained by
regulating the distillation. The apparatus used in the production of
magenta is shown in Fig. 352. It consists of a large iron pot set over a
furnace in brickwork, and having a lid with a stuffing-box, through
which passes a spindle carrying a stirrer. A bent tube rises from the
lid, and is connected with a worm surrounded by cold water, for the
purpose of condensing the aniline which is vapourized in the process.
The aniline, containing a due amount of toluidine, is mixed in this
apparatus with about one and a half times its weight of a saturated
solution of arsenic acid (H_{3}AsO_{4}). The fire is lighted and kept up
for several hours: water first, and lastly aniline, distil over. When
the operation is ended, steam is blown through the apparatus, thus
carrying off an additional portion of aniline. The crude product is then
boiled with water, the solution filtered, and common salt added, which
precipitates an impure magenta. This is afterwards dissolved and
recrystallized several times. The crystals of this magenta—like those of
many of the coal-colour products—have a peculiar greenish metallic
lustre; they dissolve in warm water, forming a deep purplish-red
solution. The chemical composition of magenta has been investigated by
Dr. Hofman, who found it to be a salt of an organic base, to which he
gave the name of “rosaniline.” This rosaniline is easily obtained from
magenta by addition to its solution of an alkali. While all its salts
are intensely coloured, rosaniline itself is a perfectly colourless
substance, becoming reddened by exposure to the air, as it absorbs
carbonic acid, thus passing to the condition of a salt. Rosaniline,
then, displays its chromatic powers only when it is combined with an
acid. This property is sometimes shown at lectures in a striking manner
by dipping a piece of paper into a colourless solution of rosaniline,
and exposing it to the air, when, as the rosaniline absorbs carbonic
acid, the paper changes from white to red. A more elegant form of the
same experiment is to dip a white rose into a solution of rosaniline
containing a little ammonia. As the ammonia escapes, or is expelled by a
current of warm air, the same kind of action occurs, and the white rose
changes to red—as if by magic, the emblem of the House of York is
transformed into the badge of Lancaster! The chemical nature of
rosaniline is regarded as analogous to that of ammonia—it is, in fact,
looked upon by chemists as a sort of ammonia, in each particle of which
some atoms of hydrogen have been replaced by certain _groups_ of carbon
and hydrogen atoms—some of these groups being derived from the aniline
and others from the toluidine. The particular salt of rosaniline which
constitutes the crude product of the action on the aniline and
toluidine, depends on the substance employed to effect the oxidation. If
a chloride, the resulting product is chloride of rosaniline; if a
nitrate, it is the nitrate; and so on. The magenta which is formed in
the first instance by the process we have described is an arseniate of
rosaniline; but in the subsequent processes, it is converted into the
chloride—the salt usually sold as magenta. Other salts of rosaniline are
made on the large scale—especially the acetate, the beautiful crystals
of which have the advantage of being very soluble.
Magenta attaches itself strongly to animal fibres, but the colour is
somewhat fugacious under the action of sunlight. It is used not only as
a dye, but more largely as the raw material from which a number of other
beautiful colours are obtained. For this reason it is manufactured on an
enormous scale, thousands of tons being produced annually, and the money
value of the colour produced from it must be reckoned by thousands of
pounds. Yet aniline was a few years ago merely a curiosity never met
with out of the laboratory of the scientific chemist. It is stated that
a single firm now makes more than twelve tons of aniline weekly, and on
its premises may be seen tanks, in each of which 30,000 gallons of
magenta solution is depositing its crystals. If a salt of rosaniline be
heated with aniline, the colour changes gradually through purple to
blue, while ammonia is at the same time given off. This is the colour
known as aniline blue, “bleu du Lyons,” &c. In its preparation it has
been found that the best results are obtained by employing the salt of
some weak acid—acetate of rosaniline, for example—and pure aniline, that
is, aniline free from toluidine. The operation is conducted in iron pots
very similar to those used in making magenta, but smaller. These pots
are not set over a fire, but a number of them are placed in a large
vessel containing oil, by which they can be maintained at a regulated
temperature when the oil is heated. The crude product undergoes several
purifications, and the aniline blue is supplied in commerce in powder,
or dissolved in spirits of wine. It is insoluble in water, and this has
been an obstacle to its employment; but recently a similar substance has
been obtained in a soluble form, and is extensively used for dyeing
wool, under the name of “Nicholson’s blue.” Other blues have been
similarly prepared, and from the same two substances, magenta and
aniline, a colour known as “violet imperial” was formerly made in very
large quantities, but it has been superseded by the colours about to be
described. It may be well to mention that these blues and violets have
been found to contain bases formed of rosaniline, in which one, two, or
three atoms of hydrogen are replaced by the group C_{6}H_{5}. This group
of atoms will be noticed to belong to aniline, and chemists have named
it phenyl, and, therefore, bases of these coloured salts are
respectively named phenyl-rosaniline, di-phenyl-rosaniline,
tri-phenyl-rosaniline. But Dr. Hofman found that other groups of atoms
besides C_{6}H_{5} may be made to take the place of H in rosaniline. By
acting on rosaniline or its salts with iodides of ethyl, C_{2}H_{5}I, or
iodide of methyl, CH_{3}I, he obtained a beautiful series of violets, of
which many shades could be produced, varying from red-purple to blue.
These are the colours so well known as Hofman’s violets, and are
prepared on the large scale by heating a solution of magenta (chloride
of rosaniline) in alcohol or wood spirit, with the iodide of ethyl or
the iodide of methyl. The nature and proportions of the ingredients are
regulated according to the tint required. The vessels are hermetically
closed during the heating, which is accomplished by steam admitted into
a steam-jacket surrounding the vessel. The crude product has to be
separated from the substances with which it is mixed, and the colouring
matter is finally obtained, presenting in the solid state the peculiar
semi-metallic lustre so characteristic of these products. Like the other
colours, Hofman’s violets are salts of _colourless_ bases, which, as
indicated above, are substitution products of rosaniline. The tints they
produce incline to red, violet, or blue, according as one, two, or three
hydrogen atoms are replaced by the ethyl or methyl groups. Colours have
also been obtained from mauve and iodide of ethyl—for example, the dye
known in commerce as “dahlia.” Other colours are procured from magenta
by treating it with various compounds: one such is the “Britannia
violet,” discovered also by Mr. Perkin, who procures it from magenta and
a hydrocarbon-bromide derived from the action of bromine or common
turpentine. This is a very useful colour, and is largely used in dyeing
and printing violets, of which any shades may be obtained.
Another derivative of rosaniline is the aniline green. It is obtained by
dissolving the rosaniline salt in dilute sulphuric acid, adding crude
_aldehyde_ (a substance obtained by acting with oxidizing agents on
alcohol). The mixture is heated until a sample dissolves in acidulated
water with a blue tint; it is poured out into boiling water containing
in solution hyposulphite of sodium, boiled, the liquid filtered; and the
green dye, if required in the solid state, is precipitated by carbonate
of sodium. Aniline green dyes wool and silk, the latter especially, of a
magnificent green; perhaps as beautiful a colour as any of the coal-tar
series, and one which has the singular advantage among greens of looking
as beautiful in artificial light as in daylight. The manner in which
this dye was discovered is somewhat curious. It is related by Mr. Perkin
of a dyer, named Chirpin, that he was trying to render permanent a
_blue_ colouring matter, which had been found could be produced from
rosaniline by the action of aldehyde and sulphuric acid. After a number
of fruitless attempts at fixing it, he confided his perplexities to a
photographic friend, who evidently thought that if it was possible to
fix a photograph, anything else might be fixed in like manner, for he
recommended his confidant to try hyposulphite of sodium. On making the
experiment, however, the dyer did not succeed in fixing his blue, but
converted it into the splendid aldehyde green. Like other colouring
matters we have described, this is a salt of a colourless base
containing sulphur. Like rosaniline, the colourless base takes on the
characteristic colour of its salts by merely absorbing carbonic acid
from the air.
Again, by a modification of the process for producing the Hofman
violets, another green of an entirely different constitution may be
obtained. It is bluer in tint than the former, and is much used for
cotton and silks, under the name of “iodine green.”
In the manufacture of magenta there is formed a residuum or bye-product,
consisting of a resinous, feebly basic substance, from which Nicholson
obtained a dye, imparting to silk and wool a gorgeous golden yellow
colour. This dye cannot be obtained directly, but is always produced in
greater or less quantity when magenta is made on the large scale, and is
separated during the purification. By first dyeing the silk or wool with
magenta, and then with this dye, which is commercially known as
“phosphine,” brilliant scarlet tints are obtained. The yellow colours
have been found to be salts of a base termed chrysaniline, a sort of
chemical relative of rosaniline, as may be seen in comparing the formulæ
which represent their constitution, with which we place also the symbol
for another substance obtained by submitting rosaniline to the influence
of nascent hydrogen. This body, _leucaniline_, again yields rosaniline
very readily when the hydrogen is removed by oxidizing agents. It will
be noticed that the three bodies form a series the members of which
differ only by H_{2}, thus indicating their close relationship.
C_{20}H_{17}N_{3} Chrysaniline.
C_{20}H_{19}N_{3} Rosaniline.
C_{20}H_{21}N_{3} Leucaniline.
Some idea will have been obtained from the foregoing particulars of the
great colour-supplying capabilities of aniline; but we have not yet
exhausted the utility of this interesting substance. It is probable that
the letters on the page now under the reader’s eye owe their blackness
to an aniline product. For after all the salts furnishing the lovely
tints we have mentioned have been extracted, there is in their
manufacture a final residuum, and from this an intense black is
obtained, which is largely used in the manufacture of printing-ink.
We have mentioned _phenol_ as a substance yielding colours. Phenol is
the body now so well known as a disinfectant under the name of “carbolic
acid,” a name given to it by its discoverer, Runge, who prepared it from
coal-tar, in 1834. Phenol forms colourless crystals, which dissolve to
some extent in water, and very readily in alcohol. It is a powerful
antiseptic, that is, it arrests the process of putrefaction in animal or
vegetable bodies, and it is also highly poisonous. The constitution of
phenol is given by the formula C_{6}H_{5} OH, in which the reader will
recognize the same group of atoms already indicated as entering into the
aniline derivatives. From some of these phenol may in fact be obtained,
and although it cannot be formed _directly_ from benzol, phenol can be
made to furnish benzol. When crude phenol is treated with a sulphuric
acid and oxalic acid, a substance is obtained which presents itself as a
brittle resinous mass of a brown colour, with greenish metallic lustre.
This substance is called _rosolic acid_ by chemists, but in commerce it
is known as _aurine_, and is used for dyeing silk of an orange colour,
which, however, is not very permanent. But by heating rosolic acid with
liquid ammonia, a permanent red dye is procured which has been termed
_peonine_, and has been much used for woollen goods. But it lately had
the reputation of exerting a poisonous action, producing blistering and
sores when stockings or other articles dyed with it were worn in contact
with the skin. It is now, therefore, less extensively employed.
_Coralline_, another body identical with or very similar to the former,
is similarly prepared from rosolic acid by heating it with ammonia under
pressure.
Again, by heating coralline with aniline, a blue dye, known as
“azurine,” or “azuline,” was formerly made in large quantities; but it
has been supplanted by the aniline blues already described.
When phenol is acted upon by nitric acid new compounds are produced,
standing in the same relation to phenol as nitro-benzol does to benzol.
The final result of the action of nitric acid on phenol is _picric
acid_, called also “carbazotic acid,” and, more systematically,
“tri-nitro-phenol;” for it is regarded as phenol in which three of the
hydrogen atoms have been replaced by the group NO_{2} thus,
C_{6}H_{2}(NO_{2})_{3} OH. It forms bright yellow-coloured crystals, and
its solution readily imparts a bright pure yellow colour to wool, silk,
&c. It received the name of picric acid (πικρος, _bitter_) from the
exceedingly bitter taste of even an extremely diluted solution. It is
said that picric acid is employed as an adulterant in bitter ale instead
of hops. Now, the colouring power of picric acid is so great, that even
the minute quantity which could be used to impart bitterness to beer is
recognizable by dipping a piece of white wool into the beer, when, if
picric acid be present, the wool acquires a clear yellow tint. Besides
its employment as a yellow, it is useful for procuring green tints by
combination with the blues. Picric acid again furnishes, by treatment
with _cyanide of potassium_, a deep red colour, consisting of an acid
which, when combined with ammonia, furnishes a magnificent colouring
material—which is, in fact, _murexide_, a dye identical with the famous
Tyrian purple of the ancients, and formerly obtainable only from certain
kinds of shell-fish.
Naphthaline—another of the colour-yielding substances of coal-tar—is,
like benzol, a hydro-carbon, but one belonging to quite another chemical
series. Its formula is C_{10}H_{8}, and it has an interest to chemists
altogether apart from its industrial uses, from having been the subject
of the classic researches of the French chemist, Laurent—researches
which resulted in the introduction of new and fertile ideas into
chemical science, contributing largely to its rapid progress.
Naphthaline forms colourless crystals, which, like camphor, slowly
volatilize at ordinary temperatures, and are readily distilled in a
current of steam. It is thus sufficiently volatile to escape complete
deposition in the condensers of the gas-works, and to be partly carried
over into the mains, where its collection occasions some trouble. Nitric
acid acts upon naphthaline in a manner analogous to that in which it
acts on benzol, forming nitro-naphthaline, which, in its turn, submitted
to the action of iron filings and acetic acid, is transformed into a
base called “naphthylamine.” The salts of naphthylamine are coloured
products which, in some cases, have been found available as dyes. There
is a crimson colour, and a yellow largely used under the name of
“Manchester yellow,” for imparting to silk and wool a gorgeous golden
yellow colour. Another coloured derivative of naphthaline, called
“carminaphtha,” was discovered by Laurent in the course of his
researches.
It would be easy to fill this volume with descriptions of the
properties, and modes of preparing the numerous colouring matters that
have been obtained from coal-tar products. In order to give the reader
an idea of the extent to which the tar products have been made to
minister to our sense of the beautiful, a list is here given of the
principal colouring matters from these sources that have been employed
in the arts. The various names under which a product has been
commercially known are in most cases given. It must be understood that
the same name is frequently applied to products chemically distinct, and
some of the names which appear as synonyms may also in reality indicate
different substances.
LIST OF COAL-TAR COLOURS.
I. COLOURS DERIVED FROM ANILINE AND TOLUIDINE.
_Blues and Violets._
Mauve, aniline purple, Perkin’s violet, violine, mauve, rosaniline,
anodising, &c.
Aniline blue, rosaniline blue, Hofman’s blue, bleu de Paris, bleu de
Lyons, bleu de Mulhouse, bleu de Mexique, bleu de nuit, bleu lumière,
night blue.
Hofman’s blue.
Nicholson’s blue, soluble blue.
Hofman’s violet, rosaniline violet.
A long series of red and blue violets, bearing Hofman’s name and
distinguished in commerce by adding R or B, according to the redness or
the blueness of the tint, ranging from RRRR to BBBB.
Dahlia.
Toluidine blue.
Violet de Paris.
Mauvaniline.
Violaniline.
Regina blue, opal blue, bleu de Fayolle, violet de Mulhouse.
Britannia violet.
Violet imperial.
And many others.
_Reds._
Aniline red, new red, magenta, solferino, aniline, rougé, roseine,
azaline.
Rubine, rubine imperial.
Chrysaniline red.
(_The above are all salts of rosaniline_)
Xylidine, tar red, soluble red.
_Yellows._
Chrysaniline, phosphine, aniline yellow, yellow fuschine.
Chrysotoluidine.
Dinaline.
Field’s orange.
_Greens._
Aldehyde green, aniline green, viridine, emeraldine.
Iodine green, iodide of methyl green, iodide of ethyl green.
Perkin’s green.
_Browns._
Havanna brown.
Bismarck brown, aniline brown, Napoleon brown, aniline maroon.
_Greys and Blacks._
Aniline grey, argentine.
Argentine black.
II.—COLOURS DERIVED FROM PHENOL.
_Blues and Violets._
Isopurpuric acid, Grénat.
Azuline, azurine.
_Reds._
Picramic acid.
Coralline, peonine.
Red coralline.
_Yellows._
Picric acid, carbazotic acid.
Aurine, rosolic acid.
_Green._
Chloropicrin.
_Browns._
Picrate of ammonia.
Isopurpurate of potash.
Phenyl brown, phenicine.
III.—COLOURS DERIVED FROM NAPHTHALENE.
_Reds._
Pseudoalizarine, naphthalic red.
Roseonaphthaline, carminaphtha.
_Yellows._
Binitronaphthaline, naphthaline yellow, golden yellow, Manchester
yellow.
And others.
The introduction of aniline colours into dyeing and calico-printing has
caused quite a revolution in these arts, the processes having become
much more simple, and the facilities for obtaining every variety of tint
largely increased. The arts of lithography, type-printing,
paper-staining, &c., have also profited by the coal-tar colours. For
such purposes the colour is prepared by fixing it on alumina, a process
in which much difficulty was at first experienced, for the colours are
themselves almost all of a basic nature. The desired result is now
attained by fixing them on the alumina with tannic or benzoic acid.
These lakes produce brilliant printing-inks, which are extensively used.
The aniline colours are also employed for coloured writing-inks, tinted
soaps, imitations of bronzed surfaces, and for a variety of other
purposes.
Not many years ago coal-tar was a valueless substance: it was actually
given away by gas-makers to any one who chose to fetch it from the
works. It was then “matter in the wrong place;” but Mr. Perkin’s
discovery led to its being put in the right place, and it has become the
raw material of a manufacture creating an absolutely new industry, which
has developed with amazing rapidity. This industry dates from only 1856,
and in 1862 the annual value of its products was more than £400,000. Dr.
Hofman, in reporting on the coal-tar colours shown at the Paris
Exhibition of 1867, computed the value at that time at about £1,250,000,
although the products were much cheaper than before. Large manufactories
have been established in Great Britain, in France, Germany, Switzerland,
America, and other countries. The possibility of such an industry is an
interesting illustration of the manner in which the progress made in any
one branch of practical science may lead to unexpected developments in
other quarters. The quantity of aniline obtained from coal-tar is very
small compared to the amount of coal used, as may be seen from the
following table, in which the respective weights of the various products
required in the manufacture of _mauve_ are arranged as given by Mr.
Perkin for the produce of 100 lbs. of coal.
lbs. oz.
Coal 100 0
Coal-tar 10 12
Coal-tar naphtha 0 8½
Benzol 0 2¾
Nitro-benzol 0 4¼
Aniline 0 2¼
Mauve 0 0¼
From this we may perceive that had not the manufacture of gas been
greatly extended, so as to yield a large aggregate produce of tar, the
requisite supply for the manufacture of aniline would not have been
attainable; and the industrial application of the previously worthless
bye-product reacts upon gas manufacture by cheapening the price of that
commodity, thus tending still more to extend its use.