THE COAL-TAR COLOURS.
In a former chapter some slight reference has been made to those
bye-products of coal-tar which have proved so valuable in the production
of the aniline dyes. It is thought that the subject is of so interesting
a nature as to deserve more notice than it was possible to bestow upon it
in that place. With abstruse chemical formulae and complex chemical
equations it is proposed to have as little as possible to do, but even
the most unscientific treatment of the subject must occasionally
necessitate a scientific method of elucidation.
The dyeing industry has been radically changed during the last half
century by the introduction of what are known as the _artificial_ dyes,
whilst the _natural_ colouring matters which had previously been the sole
basis of the industry, and which had been obtained by very simple
chemical methods from some of the constituents of the animal kingdom, or
which were found in a natural state in the vegetable kingdom, have very
largely given place to those which have been obtained from coal-tar, a
product of the mineralised vegetation of the carboniferous age.
The development and discovery of the aniline colouring matters were not,
of course, possible until after the extensive adoption of
house-gas for illuminating purposes, and even then it was many years
before the waste products from the gas-works came to have an appreciable
value of their own. This, however, came with the increased utilitarianism
of the commerce of the present century, but although aniline was first
discovered in 1826 by Unverdorben, in the materials produced by the dry
distillation of indigo (Portuguese, _anil_, indigo), it was not until
thirty years afterwards, namely, in 1856, that the discovery of the
method of manufacture of the first aniline dye, mauveine, was announced,
the discovery being due to the persistent efforts of Perkin, to whom,
together with other chemists working in the same field, is due the great
advance which has been made in the chemical knowledge of the carbon,
hydrogen, and oxygen compounds. Scientists appeared to work along two
planes; there were those who discovered certain chemical compounds in the
resulting products of reactions in the treatment of _existing_
vegetation, and there were those who, studying the wonderful constituents
in coal-tar, the product of a _past_ age, immediately set to work to find
therein those compounds which their contemporaries had already
discovered. Generally, too, with signal success.
The discovery of benzene in 1825 by Faraday was followed in the course of
a few years by its discovery in coal-tar by Hofmann. Toluene, which was
discovered in 1837 by Pelletier, was recognised in the fractional
distillation of crude naphtha by Mansfield in 1848. Although the method
of production of mauveine on a large scale was not accomplished until
1856, yet it had been noticed in 1834, the actual year of its recognition
as a constituent of coal-tar, that, when brought into contact with
chloride of lime, it gave brilliant colours, but it required a
considerable cheapening of the process of aniline manufacture before the
dyes commenced to enter into competition with the old natural dyes.
The isolation of aniline from coal-tar is expensive, in consequence of
the small quantities in which it is there found, but it was discovered by
Mitscherlich that by acting upon benzene, one of the early distillates of
coal-tar, for the production of nitro-benzole, a compound was produced
from which aniline could be obtained in large quantities. There were thus
two methods of obtaining aniline from tar, the experimental and the
practical.
In producing nitrobenzole (nitrobenzene), chemically represented as
(C_{6}H_{5}NO_{2}), the nitric acid used as the reagent with benzene, is
mixed with a quantity of sulphuric acid, with the object of absorbing
water which is formed during the reaction, as this would tend to dilute
the efficiency of the nitric acid. The proportions are 100 parts of
purified benzene, with a mixture of 115 parts of concentrated nitric acid
(HNO_{3}) and 160 parts of concentrated sulphuric acid. The mixture is
gradually introduced into the large cast-iron cylinder into which the
benzene has been poured. The outside of the cylinder is supplied with an
arrangement by which fine jets of water can be made to play upon it in
the early stages of the reaction which follows, and at the end of from
eight to ten hours the contents are allowed to run off into a storage
reservoir. Here they arrange themselves into two layers, the top of which
consists of the nitrobenzene which has been produced, together with some
benzene which is still unacted upon. The mixture is then freed from the
latter by treatment with a current of steam. Nitrobenzene presents itself
as a yellowish oily liquid, with a peculiar taste as of bitter almonds.
It was formerly in great demand by perfumers, but its poisonous
properties render it a dangerous substance to deal with. In practice a
given quantity of benzene will yield about 150 per cent of nitrobenzene.
Stated chemically, the reaction is shown by the following equation:--
C_{6}H_{6} + HNO_{3} = C_{6}H_{5}NO_{2}, + H_{2}O
(Benzene) (Nitric acid) (Nitrobenzene) (Water).
The water which is thus formed in the process, by the freeing of one of
the atoms of hydrogen in the benzene, is absorbed by the sulphuric acid
present, although the latter takes no actual part in the reaction.
From the nitrobenzene thus obtained, the aniline which is now used so
extensively is prepared. The component atoms of a molecule of aniline are
shown in the formula C_{6}H_{5}NH_{2}. It is also known as phenylamine or
amido-benzole, or commercially as aniline oil. There are various methods
of reducing nitrobenzene for aniline, the object being to replace the
oxygen of the former by an equivalent number of atoms of hydrogen. The
process generally used is that known as Béchamp's, with slight
modifications. Equal volumes of nitrobenzene and acetic acid, together
with a quantity of iron-filings rather in excess of the weight of the
nitrobenzene, are placed in a capacious retort. A brisk effervescence
ensues, and to moderate the increase of temperature which is caused by
the reaction, it is found necessary to cool the retort. Instead of acetic
acid hydrochloric acid has been a good deal used, with, it is said,
certain advantageous results. From 60 to 65 per cent. of aniline on the
quantity of nitrobenzene used, is yielded by Béchamp's process.
Stated in a few words, the above is the process adopted on all hands for
the production of commercial aniline, or aniline oil. The details of the
distillation and rectification of the oil are, however, as varied as they
can well be, no two manufacturers adopting the same process. Many of the
aniline dyes depend entirely for their superiority, on the quality of the
oil used, and for this reason it is subject to one or more processes of
rectification. This is performed by distilling, the distillates at the
various temperatures being separately collected.
When pure, aniline is a colourless oily liquid, but on exposure rapidly
turns brown. It has strong refracting powers and an agreeable aromatic
smell. It is very poisonous when taken internally; its sulphate is,
however, sometimes used medicinally. It is by the action upon aniline of
certain oxidising agents, that the various colouring matters so well
known as aniline dyes are obtained.
Commercial aniline oil is not, as we have seen, the purest form of
rectified aniline. The aniline oils of commerce are very variable in
character, the principal constituents being pure aniline, para- and
meta-toluidine, xylidines, and cumidines. They are best known to the
colour manufacturer in four qualities--
(_a_) Aniline oil for blue and black.
(_b_) Aniline oil for magenta.
(_c_) Aniline oil for safranine.
(_d_) _Liquid toluidine.
From the first of these, which is almost pure aniline, aniline black is
derived, and a number of organic compounds which are further used for the
production of dyes. The hydrochloride of aniline is important and is
known commercially as "aniline salt."
The distillation and rectification of aniline oil is practised on a
similar principle to the fractional distillation which we have noticed as
being used for the distillation of the naphthas. First, light aniline
oils pass over, followed by others, and finally by the heavy oils, or
"aniline-tailings." It is a matter of great necessity to those engaged in
colour manufacture to apply that quality oil which is best for the
production of the colour required. This is not always an easy matter, and
there is great divergence of opinion and in practice on these points.
The so-called aniline colours are not all derived from aniline, such
colouring matters being in some cases derived from other coal-tar
products, such as benzene and toluene, phenol, naphthalene, and
anthracene, and it is remarkable that although the earlier dyes were
produced from the lighter and more easily distilled products of
coal-tar, yet now some of the heaviest and most stubborn of the
distillates are brought under requisition for colouring matters, those
which not many years ago were regarded as fit only to be used as
lubricants or to be regarded as waste.
It is scarcely necessary or advisable in a work of this kind to pursue
the many chemical reactions, which, from the various acids and bases,
result ultimately in the many shades and gradations of colour which are
to be seen in dress and other fabrics. Many of them, beautiful in the
extreme, are the outcome of much careful and well-planned study, and to
print here the complicated chemical formulae which show the great changes
taking place in compounds of complex molecules, or to mention even the
names of these many-syllabled compounds, would be to destroy the purpose
of this little book. The Rosanilines, the Indulines, and Safranines; the
Oxazines, the Thionines: the Phenol and Azo dyes are all substances which
are of greater interest to the chemical students and to the colour
manufacturer than to the ordinary reader. Many of the names of the bases
of various dyes are unknown outside the chemical dyeworks, although each
and all have complicated; reactions of their own. In the reds are
rosanilines, toluidine xylidine, &c.; in the blues--phenyl-rosanilines,
diphenylamine, toluidine, aldehyde, &c.; violets--rosaniline, mauve,
phenyl, ethyl, methyl, &c.; greens--iodine, aniline, leucaniline,
chrysotoluidine, aldehyde, toluidine, methyl-anilinine, &c.; yellows and
orange--leucaniline, phenylamine, &c.; browns--chrysotoluidine, &c.;
blacks--aniline, toluidine, &c.
To take the rosanilines as an instance of the rest.
Aniline red, magenta, azaleine, rubine, solferino, fuchsine, chryaline,
roseine, erythrobenzine, and others, are colouring matters in this group
which are salts of rosaniline, and which are all recognised in commerce.
The base rosaniline is known chemically by the formula C_{20}H_{l9}N_{3},
and is prepared by heating a mixture of magenta aniline, toluidine, and
pseudotoluidine, with arsenic acid and other oxidising agents. It is
important that water should be used in such quantities as to prevent the
solution of arsenic acid from depositing crystals on cooling. Unless
carefully crystallised rosaniline will contain a slight proportion of the
arseniate, and when articles of clothing are dyed with the salt, it is
likely to produce an inflammatory condition of skin, when worn. Some
years ago there was a great outcry against hose and other articles dyed
with aniline dyes, owing to the bad effects which were produced, and this
has no doubt proved very prejudicial to aniline dyes as a whole.
Again, the base known as mauve, or mauveine, has a composition shown by
the formula C_{27}H_{24}N_{4}. It is produced from the sulphate of
aniline by mixing it with a cold saturated solution of bichromate of
potash, and allowing the mixture to stand for ten or twelve hours. A
blue-black precipitate is then formed, which, after undergoing a process
of purification, is dissolved in alcohol and evaporated to dryness. A
metallic-looking powder is then obtained, which constitutes this
all-important base. Mauve forms with acids a series of well-defined salts
and is capable of expelling ammonia from its combinations. Mauve was the
first aniline dye which was produced on a large scale, this being
accomplished by Perkin in 1856.
The substance known as carbolic acid is so useful a product of a piece of
coal that a description of the method of its production must necessarily
have a place here. It is one of the most powerful antiseptic agents with
which we are acquainted, and has strong anaesthetic qualities. Some
useful dyes are also obtained from it. It is obtained in quantities from
coal-tar, that portion of the distillate known as the light oils being
its immediate source. The tar oil is mixed with a solution of caustic
soda, and the mixture is violently agitated. This results in the caustic
soda dissolving out the carbolic acid, whilst the undissolved oils
collect upon the surface, allowing the alkaline solution to be drawn from
beneath. The soda in the solution is then neutralised by the addition of
a suitable quantity of sulphuric acid, and the salt so formed sinks while
the carbolic acid rises to the surface.
Purification of the product is afterwards carried out by a process of
fractional distillation. There are various other methods of preparing
carbolic acid.
Carbolic acid is known chemically as C_{6}H_{5}(HO). When pure it appears
as colourless needle-like crystals, and is exceedingly poisonous. It has
been used with marked success in staying the course of disease, such as
cholera and cattle plague. It is of a very volatile nature, and its
efficacy lies in its power of destroying germs as they float in the
atmosphere. Modern science tells us that all diseases have their origin
in certain germs which are everywhere present and which seek only a
suitable _nidus_ in which to propagate and flourish. Unlike mere
deodorisers which simply remove noxious gases or odours; unlike
disinfectants which prevent the spread of infection, carbolic acid
strikes at the very root and origin of disease by oxidising and consuming
the germs which breed it. So powerful is it that one part in five
thousand parts of flour paste, blood, &c., will for months prevent
fermentation and putrefaction, whilst a little of its vapour in the
atmosphere will preserve meat, as well as prevent it from becoming
fly-blown. Although it has, in certain impure states, a slightly
disagreeable odour, this is never such as to be in any way harmful,
whilst on the other hand it is said to act as a tonic to those connected
with its preparation and use.
The new artificial colouring matters which are continually being brought
into the market, testify to the fact that, even with the many beautiful
tints and hues which have been discovered, finality and perfection have
not yet been reached. A good deal of popular prejudice has arisen against
certain aniline dyes on account of their inferiority to many of the old
dye-stuffs in respect to their fastness, but in recent years the
manufacture of many which were under this disadvantage of looseness of
dye, has entirely ceased, whilst others have been introduced which are
quite as fast, and sometimes even faster than the natural dyes.
It is convenient to express the constituents of coal-tar, and the
distillates of those constituents, in the form of a genealogical chart,
and thus, by way of conclusion, summarise the results which we have
noticed.
COAL.
|
.----------+-----------+----+-------------------+--------+----.
| | | | | |
Water House-gas Coal-tar Ammoniacal Coke |
| liquor |
.---------+-------+---------+---------. | Sulphur
| | | | | | (sulphurreted
First Second Heavy Anthracene Pitch | hydrogen:
light light oils (green | sulphurous
oils oils (creosote oils) | acid: oil
| (crude oils) | | of vitriol)
.----+----. naphtha) | Anthracene |
| | | | | |
Ammoniacal Benzene | | Alizarin or |
liquor toluene,| | dyer's madder |
&c. | | |
| | |
| | Sulphuric acid=Carbonate of=Hydrochloric
| | | ammonia acid
| | | (smelling
| | | salts)
| | |
| | Lime=Sulphate of Lime=Chloride of
| | | ammonia | ammonia (sal
| | | | ammoniac)
| | | |
| | .----+----. .----+----.
| | | | | |
| | Ammonia Sulphate Ammonia Chloride
| | of lime of lime.
| | (Plaster of Paris)
| |
| .--+-----+----------.
| | | |
| Crude Carbolic Naphthalin
| Creosote acid
|
.--------------+---+--+-------+--------+-----------.
| | | | |
Benzene=Nitric Acid Toluene Nylene Artificial Burning
| turpentine oils
Nitrobenzene= } Iron filings oil (solvent
(Essence de | } and acetic acid naphtha)
mirbane) |
|
Aniline=Various reagents
|
Aniline dyes
INDEX.
A.
Accidents, causes of mining
"Age of _Acrogens_"
_Alethopteris_
Alizarin
American coal-fields
Ammoniacal liquor
Aniline
Aniline dyes
Aniline oil, commercial
Aniline salt
Aniline "tailings"
Anthracene
Anthracite
Artificial turpentine oil
Asphalt
Australian coals
_Aviculopecten_
B.
Béchamp's process
Benzene
Bind
Bitumen in Trinidad
"Blower" a
Boghead coal
Bog-oak
Boring diamonds
Borrowdale graphite mine
Bovey Tracey lignite
British coal-fields
British North-American coal-measures
Briquettes
C.
_Calamites_, extinct horsetails
Carbolic acid
Carboniferous formation, the
_Cardiocarpum_, fossil fruit
Carelessness of miners
Causes of earth-movements
Changes of level
Charcoal as a disinfectant
Chemistry of a gas-flame
Chinese coals
Clanny's safety-lamp
Clayton's experiments with gas
Clay, regularity in deposition of
Club-mosses, great height of fossil
Coal-dust, danger from
Coal formed in large lakes or closed seas
Coal formation, geological position of
Coal formed by escape of gases
Coal-mine, the
Coal not the result of drifted vegetation
Coal-period, climate of
"Coal-pipes"
Coal-plants, classification of
Coal-seam, each, a forest growth
Coals of non-carboniferous age
Coal, vegetable origin of
Coke
"Cole"
"Condensers"
Cones of _Lepidodendra_
Conifers in coal-measures
Current-bedding in sandstone
D.
Davy-lamp
Dangers of benzene
Darwin on the Chonos Archipelago
Diamonds, how made artificially
Disintegration of vegetable substances
Disproportion in relative thickness of coal and coal-measures
E.
Early use of coal
Effects of an explosion
Encrinital limestone
_Equiseta_
"Essence de mirbane"
European coal-fields
Evelyn on the use of coal
Experiments illustrating fossilisation
F.
Filling retorts by machinery
Firedamp
Fire, mines on
First light oils
First record of an explosion
Flashing-point of oil
Flooding of pits
Fog and smoke
_Foraminifera_
Fossil ferns
Fructification on fossil-ferns
Furnace, ventilating
G.
Gas, coal
Gasholder, the
Gas, house, constituents of
_Glossopteris_
Graphite
"Green Grease"
H.
Hannay, of Glasgow
Heavy oils
Humboldt's safety-lamp
Hydraulic Main
I.
Impurities in house-gas
Indian coals
Insertion of rootlets of _stigmaria_
Insufficiency of modern forest growths
Ireland denuded of coal-beds
Iron, supplies of
L.
_Lepidodendra_
_Lepidostrobi_
Lignite
London lit by gas
M.
Mammoth trees
Marco Polo
Marsh gas
Medium oils
Metamorphism of coal by igneous agency
Methods of ventilation
Mountain limestone
Murdock's use of gas
Mussel beds
N.
Napthalin
_Neuropteris_
Newcastle, charters to
Nitro-benzole
O.
Objections to use of coal
Oils from coal and lignite
Oil-wells of America
Olefiant gas
_Orthoceras_
P.
Paraffins
Peat
_Pecopteris_
Pennsylvanian anthracite
Persian fire-worshippers
Pitch
Plumbago
_Polyzoa_
Prejudice against aniline dyes
Prohibitions of the use of coal
Proportions of explosive mixtures
_Psaronius_
"Purifiers"
Pyrites in coal
Q.
Quantity of coal raised in Great Britain
R.
Reptiles of the coal-era
Resemblance of American and British coal-_flora_
Retorts
Roman use of coal
Rosanilines, the
Royal Commission of 1866
S.
Sandstone, how formed
Shales
_Sigillaria_
South American coals
Spores of _lepidodrendron_
Spores, resinous matter in
Spores, inflammability of
Steel-mill
_Sternbergia_
_Stigmaria_
Subsidence throughout coal-era
Surturbrand at Brighton
Sussex iron-works
T.
Tar
Testing pits by the candle
Texas coal
Toluene, discovery of
Torbanehill mineral
Trappers
U.
Underclays
Uses to which coal is put
V.
Vaseline
Vegetation of the coal age
Ventilation of coal-pits
W.
"Washers"
Waste of fuel
Wealden lignite
Westphalian coal-field
Y.
Young's Paraffin Oil
Z.
Zoroastrians
End of Project Gutenberg's The Story of a Piece of Coal, by Edward A. Martin