The nature of the products obtained by the destructive distillation of
coal varies according to the temperature of distillation, and the age or
degree of carbonization of the coal. The watery liquor obtained by the dry
distillation of wood is acid, and contains among other things acetic acid,
which is sometimes prepared in this way, and from its origin is
occasionally spoken of as "wood vinegar." The older the wood, the more
complete its degree of conversion into coal, and the smaller the quantity
of oxygen it contains, the more alkaline does the watery liquid become.
Thus the gas-liquor is distinctly alkaline, and contains a considerable
quantity of ammonia, besides other volatile bases. The uses of ammonia are
manifold, and nearly our whole supply of this valuable substance is now
derived from gas-liquor. The presence of ammonia in this liquor is
accounted for when it is known that this compound is a gas composed of
nitrogen and hydrogen. It has already been explained that coal contains
from one to two per cent. of nitrogen, and during the process of
distillation about one-fifth of this nitrogen is converted into ammonia,
the remainder being converted partly into other bases, while a small
quantity remains in the coke.
Ammonia, the "volatile alkali" of the old chemists, and its salts are of
importance in pharmacy, but the chief use of this compound is to supply
nitrogen for the growth of plants. Plants must have nitrogen in some form
or another, and as they cannot assimilate it _directly_ from the
atmosphere where it exists in the free state, some suitable nitrogen
compound must be supplied to the soil. It is possible that certain
leguminous plants may derive their nitrogen from the atmosphere through
the intervention of micro-organisms, which appear capable of fixing free
nitrogen and of supplying it to the plant upon whose roots they flourish.
But this is second-hand nitrogen so far as concerns the plant. It is true
also that the atmosphere contains small traces of ammonia and acid oxides
of nitrogen, which are dissolved by rain and snow, and thus get washed
down into the soil. These are the natural sources of plant nitrogen. But
in agricultural operations, where large crops have to be raised as rapidly
as possible, some additional source of nitrogen must be supplied, and
this is the object of manuring the soil.
A manure, chemically considered, is a mixture of substances capable of
supplying the necessary nitrogenous and mineral food for the nourishment
of the growing plant. The ordinary farm or stable manure contains
decomposing nitrogenous organic matter, in which the nitrogen is given off
as ammonia, and thus furnishes the soil with which it is mixed with the
necessary fertilizer. But the supply of this manure is limited, and we
have to fall back upon gas-liquor and native nitrates to meet the existing
wants of the agriculturist. Important as is ammonia for the growth of
vegetation, it is not in this form that the majority of plants take up
their nitrogen. Soluble nitrates are, in most cases, more efficient
fertilizers than the salts of ammonia, and the ammonia which is supplied
to the soil is converted into nitrates therein before the plant can
assimilate the nitrogen. The oxidation of ammonia into nitric acid takes
place by virtue of a process called "nitrification," and there is very
good reason for believing that this transformation is the work of a
micro-organism present in the soil. The gas liquor thus supplies food to a
minute organism which converts the ammonia into a form available for the
higher plants. Some branches of agriculture--such as the cultivation of
the beet for sugar manufacture--are so largely dependent upon an
artificial source of nitrogen, that their very existence is bound up with
the supply of ammonia salts or other nitrogenous manures. The relationship
between the manufacture of beet-sugar and the distillation of coal for the
production of gas is thus closer than many readers will have imagined; for
while the supply of native guano or nitrate is uncertain, and its freight
costly on account of the distance from which it has to be shipped, the
sulphate of ammonia from gas-liquor is always at hand, and available for
the purposes of fertilization.
Then again, there are other products of industrial value which are
associated with ammonia, such, for example, as ammonia-alum and caustic
soda. This last is one of the most important chemical compounds
manufactured on a large scale, and is consumed in enormous quantities for
the manufacture of paper and soap, and other purposes. Salts of this
alkali are also essential for glass making. Of late years a method for the
production of caustic soda has been introduced which depends upon the use
of ammonia, and as this process is proving a formidable rival to the older
method of alkali manufacture, it may be said that such indispensable
articles as paper, soap, and glass are now to some extent dependent upon
gas-liquor, and may in course of time become still more intimately
connected with the manufacture of coal-gas.
But quantitative statements must be given in order to bring home to
general readers the actual value of the small percentage of nitrogen
present in coal. Thus it has been estimated, that one ton of coal gives
enough ammonia to furnish about 30 lbs. of the crude sulphate. The present
value of this salt is roughly about L12 per ton. The ten million tons of
coal distilled annually for gas making would thus give 133,929 tons of
sulphate, equal in money value to L1,607,148, supposing the whole of the
ammonia to be sold in this form. To this may be added the ammonia obtained
during the distillation of shale and the carbonization of coal for coke,
the former source furnishing about 22,000 tons, and the latter about 2500
tons annually. Small as is the legacy of nitrogen bequeathed to us from
the Carboniferous period, we see that it sums up to a considerable annual
addition to our industrial resources.
The three products resulting from the distillation of coal--viz. the gas,
ammoniacal-liquor, and coke--having now been made to furnish their tale,
we have next to deal with the tar. In the early days of gas manufacture
this black, viscid, unsavoury substance was in every sense a waste
product. No use had been found for it, and it was burnt, or otherwise
disposed of. No demand for the tar existed which could enable the gas
manufacturers to get rid of their ever-increasing accumulation. Wood-tar
had previously been used as a cheap paint for wood and metal-work, and it
was but a natural suggestion that coal-tar should be applied to the same
purposes. It was found that the quality of the tar was improved by getting
rid of the more volatile portions by boiling it in open pans; but this
waste--to say nothing of the danger of fire--was checked by a suggestion
made by Accum in 1815, who showed that by boiling down the tar in a still
instead of in open pans the volatile portions could be condensed and
collected, thus furnishing an oil which could be used by the varnish maker
as a substitute for turpentine. A few years later, in 1822, the
distillation of tar was carried on at Leith by Drs. Longstaff and Dalston,
the "spirit" being used by Mackintosh of Glasgow for dissolving
india-rubber for the preparation of that waterproof fabric which to this
day bears the name of the original manufacturer. The residue in the still
was burnt for lamp-black. Of such little value was the tar at this time
that Dr. Longstaff tells us that the gas company gave them the tar on
condition that they removed it at their own expense. It appears also that
tar was distilled on a large scale near Manchester in 1834, the "spirit"
being used for dissolving the residual pitch so as to make a black
varnish.
But the production of gas went on increasing at a greater rate than the
demand for tar for the above-mentioned purposes, and it was not till 1838
that a new branch of industry was inaugurated, which converted the
distillation of this material from an insignificant into an important
manufacture. In that year a patent was taken out by Bethell for preserving
timber by impregnating it with the heavy oil from coal-tar. The use of tar
for this purpose had been suggested by Lebon towards the end of the last
century, and a patent had been granted in this country in 1836 to Franz
Moll for this use of tar-products. But Bethell's process was put into a
working form by the great improvements in the apparatus introduced by
Breant and Burt, and to the latter is due the credit of having founded an
industry which is still carried on by Messrs. Burt, Bolton and Haywood on
a colossal scale. The "pickling" or "creosoting" of timber is effected in
an iron cylindrical boiler, into which the timber is run; the cylinder
being then closed the air is pumped out, and the air contained in the
pores of the wood thus escapes. The creosoting oil, slightly warmed, is
then allowed to flow into the boiler, and thus penetrates into the pores
of the wood, the complete saturation of which is insured by afterwards
pumping air into the cylinder and leaving the timber in the oil for some
hours under a pressure of 8 to 10 atmospheres.
All timber which is buried underground, or submerged in water, is
impregnated with this antiseptic creosote in order to prevent decay. It
will be evident that this application of tar-products must from the very
commencement have had an enormous influence upon the distillation of tar
as a branch of industry. Consider the miles of wooden sleepers over which
our railways are laid, and the network of telegraph wires carried all over
the country by wooden poles, of which the ends are buried in the earth.
Consider also the many subaqueous works which necessitate the use of
timber, and we shall gain an idea of the demand for heavy coal-tar oil
created by the introduction of Breant's process. Under the treatment
described a cubic foot of wood absorbs about a gallon of oil, and by far
the largest quantity of the tar oils is consumed in this way at the
present time. Now in the early days of timber-pickling the lighter oils of
the tar, which first come over on distillation, and which are too volatile
for the purpose of creosoting, were in much about the same industrial
position as the tar itself before its application as a timber
preservative. The light oil had a limited use as a solvent for
waterproofing and varnish making, and a certain quantity was burnt as
coal-tar naphtha in specially constructed lamps, the invention of the late
Read Holliday of Huddersfield, whose first patent was taken out in 1848
(see Fig. 4). Up to this time, be it remembered, that chemists had not
found out what this naphtha contained. But science soon laid hands on the
materials furnished by the tar-distiller, and the naphtha was one of the
first products which was made to reveal the secret of its hidden treasures
to the scientific investigator. From this period science and industry
became indissolubly united, and the researches of chemists were carried
on hand-in-hand with the technical developments of coal-tar products.
[Illustration: FIG. 4.--Read Holliday's lamp for burning light coal-tar
oils. The oil is contained in the cistern _c_, from whence it flows down
the pipe, when the stopcock is opened, into the burner _a_. Below the
burner is a little cup, in which some of the oil is kept burning, and the
heat from this flame volatilizes the oil as it flows down the pipe, the
vapour thus generated issuing from the jets in the burner and there
undergoing ignition. The burner and cup are shown on an enlarged scale at
_a_ in the lower figure.]
In 1825 Michael Faraday discovered a hydrocarbon in the oil produced by
the condensation of "oil gas"--an illuminating gas obtained by the
destructive distillation of oleaginous materials. This hydrocarbon was
analysed by its illustrious discoverer, and named in accordance with his
results "bicarburet of hydrogen." In 1834 the same hydrocarbon was
obtained by Mitscherlich by heating benzoic acid with lime, and by Peligot
by the dry distillation of calcium benzoate. For this reason the compound
was named "benzin" by Mitscherlich, which name was changed into "benzol"
by Liebig. In this country the hydrocarbon is known at the present time as
benzene. Twenty years after Faraday's discovery, viz. in 1845, Hofmann
proved the existence of benzene in the light oils from coal-tar, and in
1848 Hofmann's pupil, Mansfield, isolated considerable quantities of this
hydrocarbon from the said light oils by fractional distillation. At the
time of these investigations no great demand for benzene existed, but the
work of Hofmann and Mansfield prepared the way for its manufacture on a
large scale, when, a few years later, the first coal-tar colouring-matter
was discovered by our countryman, W. H. Perkin.
It is always of interest to trace the influence of scientific discovery
upon different branches of industry. As soon as it had been shown that
benzene could be obtained from coal-tar, the nitro-derivative of this
hydrocarbon--_i.e._ the oily compound produced by the action of nitric
acid upon benzene--was introduced as a substitute for bitter almond oil
under the name of "essence of mirbane." Nitrobenzene has an odour
resembling that of bitter almond oil, and it is still used for certain
purposes where the latter can be replaced by its cheaper substitute, such
as for the scenting of soap. Although the isolation of benzene from
coal-tar gave an impetus to the manufacture of nitrobenzene, no use
existed for the latter beyond its very limited application as "essence of
mirbane," and the production of this compound was at that time too
insignificant to take rank as an important branch of chemical industry.
The year 1856 marks an epoch in the history of the utilization of coal-tar
products with which the name of Perkin will ever be associated. In the
course of some experiments, having for their object the artificial
production of quinine, this investigator was led to try the action of
oxidizing agents upon a base known as aniline, and he thus obtained a
violet colouring matter--the first dye from coal-tar--which was
manufactured under a patent granted in 1858, and introduced into commerce
under the name of mauve. A brief sketch of the history of aniline will
serve to show how Perkin's discovery gave a new value to the light oils
from coal-tar and raised the manufacture of nitrobenzene into an important
branch of industry.
Thirty years before Perkin's experiments the Dutch chemist Unverdorben
obtained (1826) a liquid base by the distillation of indigo, which had the
property of forming beautifully crystalline salts, and which he named for
this reason "crystallin." In 1834 Runge discovered the same base in
coal-tar, although its identity was not known to him at the time, and
because it gave a bluish colour when acted upon by bleaching-powder, he
called it "kyanol." Again in 1840, by distilling a product obtained by the
action of caustic alkalies upon indigo, Fritzsche prepared the same base,
and gave it the name of aniline, from the Spanish designation of the
indigo plant, "anil," derived from the native Indian word, by which
name the base is known at the present time. That aniline could be obtained
by the reduction of nitrobenzene was shown by Zinin in 1842, who used
sulphide of ammonium for reducing the nitrobenzene, and named the
resulting base "benzidam." The following year Hofmann showed that
crystallin, kyanol, aniline, and benzidam were all one and the same base.
Thus when the discovery of mauve opened up a demand for aniline on the
large scale, the labours of chemists, from Unverdorben in 1826 to Hofmann
in 1843, had prepared the way for the manufacturer. It must be understood
that although Runge had discovered aniline in coal-tar, this is not the
source of our present supply, for the quantity is too small to make it
worth extracting. A mere trace of aniline is present in the tar ready
formed; from the time this base was wanted in large quantities it had to
be made by nitrating benzene, and then reducing the nitrobenzene.
The light oils of tar distillation rejected by the timber-pickling
industry now came to the front, imbued with new interest to the
technologist as a source of benzene for the manufacture of aniline. The
inauguration of this manufacture, like the introduction of steam
locomotion, is connected with a sad catastrophe. Mansfield, who first
showed manufacturers how to separate benzene and other hydrocarbons from
the light oils of coal-tar, and who devised for this purpose apparatus
similar in principle to that used on a large scale at the present time
(see Fig. 5), met with an accident which resulted in his death. In the
upper part of a house in Holborn in February 1856, this pioneer was
carrying on his experiments, when the contents of a still boiled over and
caught fire. In his endeavours to extinguish the flames he received the
injuries which terminated fatally. Applied science no less than pure
science has had its martyrs, and among these Mansfield must be ranked.
[Illustration: FIG. 5.--Mansfield's still. R the heating burner, A the
body of the still with stopcock, _i_, for running out the contents. B the
still-head kept in a cistern, C, of hot water or other liquid. The vapour
generated by the boiling of the liquid in A, partly condenses in B, from
whence the higher boiling-point portion flows back into the still. The
uncondensed vapour passes into the condensing-worm, D, which is kept cool
by a stream of water, and from thence flows into the receiver S. By
opening _m_ in the side-pipe any higher boiling-point oil condensing in
the delivery-pipe can be run back into the still.]
The operation of tar-distilling is about as unromantic a process as can be
imagined, but it must be briefly described before the subsequent
developments of the industry can be appreciated properly. It has already
been explained that the tar is a complex mixture of many different
substances. These various compounds boil at certain definite temperatures,
the boiling-point of a chemical compound being an inherent property. If a
mixture of substances boiling at different temperatures is heated in a
suitable vessel the compounds distil over, broadly speaking, in the order
of their boiling-points. The separation by this process is not absolute,
because compounds boiling at a certain temperature have a tendency to
bring over with them the vapours of other compounds which boil at a higher
temperature. But for practical purposes it will be sufficient to consider
that the general tendency is for the compounds of low boiling-point to
come over first, then the compounds of higher boiling-point, and finally
those of the highest boiling-point. This is the principle made use of by
the tar-distiller. The tar-still is a large iron pot provided with a
still-head from which the vapours boil out into a coil of iron pipe kept
cool in a vessel of water (see Fig. 6). The still is heated by a fire
beneath it, and the different portions which condense in the iron coil are
received in vessels which are changed as the different fractions of the
tar come over. The process is what chemists would call a rough fractional
distillation. The first fractions are liquid at ordinary temperatures, and
the water in the condenser is kept cold; then, as the boiling-point
rises, the fraction contains a hydrocarbon which solidifies on cooling,
and the water in the condenser is made hot to prevent the choking up of
the coil. Every one of these fractions of coal-tar, from the beginning to
the end of the process, has its story to tell--all the chief constituents
of the tar separated by this means have by chemical science been converted
into useful products.
[Illustration: FIG. 6.--Sectional diagram of tar-still with arched bottom.
The fireplace is at _i_; the hot gases pass over the bridge _k_ and
through _g_ into the flues _h_, _h_. The pipe at _c_ is to supply the
still with tar; _a_ is the exit pipe connected with the condenser, and _b_
a man-hole for cleaning out the still. The condenser and bottom pipe for
drawing off pitch have been omitted to avoid complication.]
It is customary at the present time to collect four distinct fractions
from the period when the tar begins to boil quietly, _i.e._ from the
point when the small quantity of watery liquor which is unavoidably
entangled with the tar has distilled over, by which time the temperature
in the still is about 110 deg. C. The small fraction that comes over up to
this temperature constitutes what the tar-distiller calls "first
runnings." From 110 deg. C. to 210 deg. C. there comes over a limpid inflammable
liquid known as "light oil," and this is succeeded by a fraction which
shows a tendency to solidify on cooling, owing to the separation of a
solid crystalline hydrocarbon known as naphthalene. This last fraction,
boiling between 210 deg. C. and 240 deg. C., is known as "carbolic oil," because
it contains, in addition to the naphthalene, the chief portion of the
carbolic acid present in the tar. From 240 deg. C. to 270 deg. C. there comes over
another fraction which shows but little tendency to solidify in the
condensing coil, and which is known as "heavy oil," or "creosote oil."
From 270 deg. C. up to the end of the distillation there distils a fraction
which is viscid in consistency, and has a tendency to solidify on cooling
owing to the separation of another crystalline hydrocarbon known as
anthracene, and which gives the name of "anthracene oil" to this last
fraction. When the latter has been collected there remains in the still
the black viscid substance known as pitch, which is obtained of any
desired consistency by leaving more or less of the anthracene oil mixed
with it, or by afterwards mixing it with the heavy oil from previous
distillations. The process carried out in the tar-still thus separates the
tar into--(1) First runnings, up to 110 deg. C. (2) Light oil, from 110 deg. to
210 deg. C. (3) Carbolic oil, from 210 deg. to 240 deg. C. (4) Creosote oil, from 240 deg.
to 270 deg. C. (5) Anthracene oil, from 270 deg. to pitch. (6) Pitch, left in
still.
It has already been said that coal-tar is a complex mixture of various
distinct chemical compounds. Included among the gases, ammoniacal liquor,
and tar, the compounds which are known to be formed by the destructive
distillation of coal already reach to nearly one hundred and fifty in
number. Of the substances present in the tar, about a dozen are utilized
as raw materials by the manufacturer, and these are contained in the
fractions described above. The first runnings and light oil contain a
series of important hydrocarbons, of which the three first members are
known to chemists as benzene, toluene, and xylene, the latter being
present in three different modifications. The carbolic oil furnishes
carbolic acid and naphthalene, and the anthracene oil the hydrocarbon
which gives its name to that fraction. Here we have only half a dozen
distinct chemical compounds to deal with, and if we confine our attention
to these for the present, we shall be enabled to gain a good general idea
of what chemistry has done with these raw materials. The products
separated during the processes which have to be resorted to for the
isolation of these raw materials have also their uses, which will be
pointed out incidentally.
Beginning with the first runnings and the light oil, from which the
hydrocarbons of the benzene series are separated, we have to make
ourselves acquainted with the treatment to which these fractions are
submitted by the tar-distiller. The light oil is first distilled from an
iron still, similar to a tar-still, and the first portions which come over
are added to the oily fraction brought over by the water of the first
runnings. The separation of the oil from the water in this last fraction
is a simple matter, because the hydrocarbons float as a distinct layer on
the water, and do not mix with it. We have at this stage, therefore, four
products to consider, viz. 1st, the oil from the first runnings; 2nd, the
first portions of the light oil; 3rd, the later portions of the light oil;
and 4th, the residue in the still. The first and second are mixed
together, and the third is washed alternately with alkali and acid to
remove acid and basic impurities, and can then be mixed with the first and
second products. The total product is then ready for the next operation.
The last portion of the light oil which remains in the still is useless as
a source of benzene hydrocarbons, and goes into the heavy oil of the later
tar fractions.
The process of purification is thus far one of fractional distillation
combined with chemical washing. In fact, all the processes of purification
to which these oils are submitted are essentially of the same character.
The principle of fractional distillation has already been explained
sufficiently for our present purpose. The process of washing a liquid may
appear mysterious to the uninitiated, but in principle it is extremely
simple. If we pour some water into a bottle, and then add some liquid
which does not mix with the water--say paraffin oil--the two liquids form
distinct layers, the one floating on the other. On shaking the bottle so
as to mix the contents, the two liquids form a homogeneous mixture at
first, but on standing for a short time separation into two layers again
takes place. Now if there was present in the paraffin oil some substance
soluble in the oil, but more soluble in water, such as alcohol, we should
by the operation described wash the alcohol out of the oil, and when the
liquids separated into layers after agitation the watery layer would
contain the alcohol. By drawing off the oil or the water the former would
then be obtained free from alcohol--it would have been "washed." This
operation is precisely what the manufacturer does on a large scale with
the coal-tar oils. These oils contain certain impurities of which some are
of an acid character and dissolve in alkalies, while others are basic and
dissolve in acid. The oil is therefore agitated in a suitable vessel
provided with mechanical stirring gear with an aqueous solution of caustic
soda, and after separation into layers the alkaline solution retaining the
acid impurities is drawn off. Then the oil may be washed with water in the
same way to remove the lingering traces of alkali, and then with
acid--sulphuric acid or oil of vitriol--which dissolves out basic
impurities and certain hydrocarbons not belonging to the benzene series
which it is desirable to get rid of. A final washing with water removes
any acid that may be retained by the oil.
The total product containing the benzene hydrocarbons is put through such
a series of washing operations as above described, and is then ready for
separation into its constituents by another and more perfect process of
fractional distillation. This final separation is effected in a piece of
apparatus somewhat complicated in structure, but simple in principle. It
is a development on a large scale of the apparatus used by Mansfield in
his early experiments. The details of construction are not essential to
the present treatment of the subject, but it will suffice to say that the
vapours of the boiling hydrocarbons ascend through upright columns, in
which the compounds of high boiling-point first condense and run back into
the still, while the lower boiling-point compounds do not condense in the
columns, but pass on into a separate condenser, where they liquefy and are
collected. But even with this rectification we do not get a perfect
separation--the hydrocarbons are not perfectly pure from a chemical point
of view, although they are pure enough for manufacturing purposes. Thus
the first fraction consists of benzene containing a small percentage of
toluene, then comes over a mixture containing a larger proportion of
toluene, then comes a purer toluene mixed with a small percentage of
xylene. The boiling-points of the three hydrocarbons are 81 deg. C., 111 deg. C.,
and 140 deg. C. respectively; but owing to the nature of fractional
distillation, a compound of a certain boiling-point always brings over
with it a certain quantity of the compound of higher boiling-point, and
that is why the rectifying column effects only a partial separation.
Of the hydrocarbons thus separated, benzene and toluene are by far the
most important; there is but a limited use for the xylenes at present, and
these and the hydrocarbons of higher boiling-point belonging to the same
series which distil over between 140 deg. and 150 deg. constitute what is known as
"solvent naphtha," because it is used for dissolving india-rubber for
waterproofing purposes. The hydrocarbons of still higher boiling-point
which remain in the still are used as burning naphtha for lamps. If
benzene of a higher degree of purity is required--as it is for the
manufacture of certain colouring-matters--the fraction containing this
hydrocarbon can be again distilled through the rectifying column, and a
large proportion of the toluene thus separated from it. Finally, pure
benzene can be obtained by submitting the rectified hydrocarbon to a
process of refrigeration in a mixture of ice and salt, when the benzene
solidifies to a white crystalline solid, while the toluene does not
solidify, and can be drained away from the benzene crystals which liquefy
at about 5 deg. C.
The account rendered by the technologist with respect to the light oils of
the tar is thus a pretty good one. Already we see that benzene, toluene,
solvent naphtha, and burning naphtha are separated from them. Even the
alkaline and acid washings may be made to surrender their contained
products, for the first of these contains a certain quantity of carbolic
acid, and the acid contains a strongly smelling base called pyridine, for
which there is at present no great demand, but which may one day become of
importance. The actual quantity of benzene in tar is a little over one per
cent. by weight, and of toluene there is somewhat less. The naphthas are
present to the extent of about 35 per cent.
Now let us consider some of the transformations which benzene and toluene
undergo in the hands of the manufacturing chemist. The production of
aniline from benzene by acting upon this hydrocarbon with nitric acid, and
then reducing the nitrobenzene, has already been referred to. For this
purpose we now heat the nitrobenzene with iron dust and a little
hydrochloric (muriatic) acid, and then distil over the aniline by means of
a current of steam blown through the still. By a similar process toluene
is converted into nitrotoluene, and the latter into toluidine.
The large quantity of aniline and toluidine now made has opened up a
channel for the use of the waste borings from cast-iron. These are ground
to a fine powder under heavy mill-stones, and constitute a most valuable
reducing agent, known technically as "iron swarf." The metallic iron
introduced in this form into the aniline still is converted into an oxide
of iron by the action of the nitrobenzene, and this oxide of iron is used
by the gas-maker for purifying the gas from sulphur as already described.
When the oxide of iron is exhausted, _i.e._ when it has taken up as much
sulphur as it can, it goes to the vitriol-maker to be burnt as a source of
this acid. Here we have a waste product of the aniline manufacture
utilized for the purification of coal-gas, and finally being made to give
up the sulphur, which it obtained primarily from the coal, for the
production of sulphuric acid, which is consumed in nearly every branch of
chemical industry.
Nitrotoluene and toluidine each exist in three distinct modifications, so
that it is more correct to speak of the nitrotoluenes and the toluidines;
but the explanation of these differences belongs to pure chemical theory,
and cannot now be attempted in detail. It must suffice to say that many
compounds having the same chemical composition differ in their properties,
and are said to be "isomeric," the isomerism being regarded as the result
of the different order of arrangement of the atoms within the molecule.
Consider a homely illustration. A child's box of bricks contains a certain
number of wooden blocks, by means of which different structures can be
built up. Supposing all the bricks to be employed for every structure
erected, the latter must in every case contain all the blocks, and yet the
result is different, because in each structure the blocks are arranged in
a different way. The bricks represent atoms, and the whole structure
represents a molecule; the structures all have the same ultimate
composition, and are therefore isomeric. This will serve as a rough
analogy, only it must not be understood that the different atoms of the
elements composing a molecule are of different sizes and shapes; on this
point we are as yet profoundly ignorant.
Now as long ago as 1856, at the time when Perkin began making mauve by
oxidizing aniline with bichromate of potash, it was observed by Natanson,
that when aniline was heated with a certain oxidizing agent a red
colouring-matter was produced. The same fact was observed in 1858 by
Hofmann, who used the tetrachloride of carbon as an oxidizing agent. These
chemists obtained the red colouring-matter as a by-product; it was formed
only in small quantity, and was regarded as an impurity. In the same year,
1858, two French manufacturers patented the production of a red dye formed
by the action of chromic acid and other oxidizing agents on aniline, the
colouring matter thus made being used for dying artificial flowers. Then,
a year later, the French chemist Verguin found that the best oxidizing
agent was the tetrachloride of tin, and this with many other oxidizing
substances was patented by Renard Freres and Franc, and under their patent
the manufacture of the aniline red was commenced on a small scale in
France. Finally, in 1860, an oxidizing agent was made use of almost
simultaneously by two English chemists, Medlock and Nicholson, which gave
a far better yield of the red than any of the other materials previously
in use, and put the manufacture of the colouring-matter on quite a new
basis. The oxidizing material patented by Medlock and Nicholson is arsenic
acid, and their process is carried on at the present time on an enormous
scale in all the chief colour factories in Europe, the colouring-matter
produced by this means being generally known as fuchsine or magenta.
In four years the accidental observation of Natanson and Hofmann, made, be
it remembered, in the course of abstract scientific investigation, had
thus developed into an important branch of manufacture. A demand for
aniline on an increased scale sprung up, and the light oils of coal-tar
became of still greater importance. The operations of the tar-distiller
had to undergo a corresponding increase in magnitude and refinement; the
production of nitrobenzene and necessarily of nitric acid had to be
increased, and a new branch of manufacture, that of arsenic acid from
arsenious acid and nitric acid, was called into existence. Perkin's mauve
prepared the way for the manufacture of aniline, and the discovery of a
good process for the production of magenta increased this branch of
manufacture to a remarkable extent. Still later in the history of the
magenta manufacture, attempts were made, with more or less success, to use
nitrobenzene itself as an oxidizing agent, and a process was perfected in
1869 by Coupier, which is now in use in many factories.
The introduction of magenta into commerce marks an epoch in the history of
the coal-tar colour industry--pure chemistry and chemical technology both
profited by the discovery. The brilliant red of this colouring-matter is
objected to by modern aestheticism, but the dye is still made in large
quantities, its value having been greatly increased by a discovery made
about the same time by John Holliday and the Baden Aniline and Soda
Company, and patented by the latter in 1877. Magenta is the salt of a base
now known as rosaniline, and it belongs therefore to the class of basic
colouring-matters. The dyes of this kind are as a group less fast, and
have a more limited application than those colouring-matters which possess
an acid character, so that the discovery above referred to--that magenta
could be converted into an acid without destroying its colouring power by
acting upon it with very strong sulphuric acid--opened up a new field for
the employment of the dye, and greatly extended its usefulness. In this
form the colouring-matter is met with under the name of "acid magenta."
It must be understood that the production of magenta from aniline by the
oxidizing action of arsenic acid or nitrobenzene is the result of chemical
change; the colouring-matter is no more present in the aniline than the
latter is contained in the benzene. And just in the same way that the
colourless aniline oil by chemical transformation gives rise to the
intensely colorific magenta, so the latter by further chemical change can
be made to give rise to whole series of different colouring-matters, each
consisting of definite chemical compounds as distinct in individuality as
magenta itself. Thus in 1860, about the time when the arsenic acid process
was inaugurated, two French chemists, Messrs. Girard and De Laire,
observed that by heating rosaniline for some time with aniline and an
aniline salt, blue and violet colouring-matters were produced. This
observation formed the starting-point of a new manufacture proceeding from
magenta as a raw material. The production of the new colouring-matters was
perfected by various investigators, and a magnificent blue was the final
result. But here also the dye was of a basic character, and being
insoluble in water had only a limited application, as a spirit bath had to
be used for dissolving the substance. In 1862, however, an English
technologist, the late E. C. Nicholson, found that by the action of strong
sulphuric acid the aniline blue could be rendered soluble in water or
alkali, and the value of the colouring-matter was enormously increased by
this discovery. The basic and slightly soluble spirit blue was by this
means converted into acid blues, which are now made in large quantities,
and sold under the names of Nicholson's blue, alkali blue, soluble blue,
and other trade designations. There is at the present time hardly any
other blue which for fastness, facility of dyeing, and beauty can compete
with this colouring-matter introduced by Nicholson as the outcome of the
work of Girard and De Laire.
Other transformations of rosaniline have yet to be chronicled. In 1862
Hofmann found that by acting upon this base--the base of magenta--with the
iodide of methyl, violet colouring-matters were produced, and these were
for some years extensively employed under the name of Hofmann's violets.
And still more remarkable, by the prolonged action of an excess of methyl
iodide upon rosaniline, Keisser found that a green colouring-matter was
formed. The latter was patented in 1866, and the dye was for some time in
use under the name of "iodine green." The statement that technology
profited by the introduction of magenta has therefore been justified.
It remains to add, that the tar obtained from one ton of Lancashire coal
furnishes an amount of aniline capable of giving a little over half a
pound of magenta. The colouring power of the latter will be inferred from
the fact, that this quantity would dye 375 square yards of white flannel
of a full red colour, and if converted into Hofmann violet by methylation,
would give enough colour to dye double this surface of flannel of a deep
violet shade. It should be stated also, that during the formation of
magenta by the arsenic acid process, there are formed small quantities of
other colouring-matters which are utilized by the manufacturer. Among
these by-products is a basic orange dye, which was isolated by Nicholson,
and investigated by Hofmann in 1862. Under the name of "phosphine" this
colouring-matter is still used, especially for the dyeing of leather. Even
the spent arsenic acid of the magenta-still has its use. The arsenious
acid resulting from the reduction of this arsenic acid is generally
obtained in the form of a lime salt after the removal of the magenta by
the purifying processes to which the crude product is submitted. From the
arsenical waste arsenious acid can be recovered, and converted back into
arsenic acid by the action of nitric acid. Quite recently the arsenical
residue has been used with considerable success in America as an
insecticide for the destruction of pests injurious to agricultural crops.
Concurrently with these technical developments of coal-tar products, the
scientific chemist was carrying on his investigations. The compounds which
science had given to commerce were made on a scale that enabled the
investigator to obtain his materials in quantities that appeared fabulous
in the early days when aniline was regarded as a laboratory curiosity, and
magenta had been seen by only a few chemists.
The fundamental problem which the modern chemist seeks to solve is in the
first place the composition of a compound, _i.e._ the number of the atoms
of the different elements which form the molecule, and in the next place
the way in which these atoms are combined in the molecule. Reverting to
our former analogy, the first thing to be found is how many different
blocks enter into the composition of the structure, and the next thing is
to ascertain how the blocks are arranged. When this is done, we are said
to know the "constitution" or "structure" of the molecule, and in many
cases when this is known we can build up or synthesise the compound by
combining its different groups of atoms by suitable methods. The coal-tar
industry abounds with such triumphs of chemical synthesis; a few of these
achievements will be brought to light in the course of the remaining
portions of this work.
The chemical investigation of magenta was commenced by Hofmann, whose name
is inseparably connected with the scientific development of the coal-tar
colour industry. In 1862 he showed that magenta was the salt of a base
which he isolated, analysed, and named rosaniline. He established the
composition of this base and of the violet and blue colouring-matters
obtained from it by the processes already described. In 1864 he made the
interesting discovery that magenta is not formed by the oxidation of
_pure_ aniline, but that a mixture of aniline and toluidine is essential
for the production of this colouring-matter. In fact, the aniline oil used
by the manufacturer had from the beginning consisted of a mixture of
aniline and toluidine, and at the present time "aniline for red" is made
by nitrating a mixture of benzene and toluene and reducing the
nitro-compounds.
From this work of Hofmann's suggestions naturally arose concerning the
"constitution" of rosaniline, and new and fruitful lines of work were
opened up. Large numbers of chemists of the greatest eminence pursued the
inquiry, but the details of their work, although of absorbing interest to
the chemist, cannot be discussed in the present volume. The final touch to
a long series of investigations was given by two German chemists, Emil and
Otto Fischer, who in 1878 proved the constitution of rosaniline by
obtaining from it a hydrocarbon, the parent hydrocarbon from which the
colouring-matter is derived. The purely scientific discovery of the
Fischers threw a flood of light on the chemistry of magenta, and enabled a
large number of colouring-matters related to the latter to be classed
under one group, having the parent hydrocarbon as a central type. This
hydrocarbon, it may be remarked, is known as triphenylmethane, as it is a
derivative of methane, or marsh gas. The blues and violets obtained from
rosaniline belong to this group, and so also do certain other
colouring-matters which had been manufactured before the Fischers'
discovery. In order to carry on the story of the utilisation of aniline,
it is necessary to know something about these other colouring-matters
which are obtained from it.
It has been explained that by the methylation of rosaniline Hofmann
obtained violet colouring-matters. Now as rosaniline is obtained by the
oxidation of a mixture of aniline and toluidine, it seems but natural that
if these bases were methylated first and then oxidized a violet dye would
be produced. The French chemist Lauth first obtained a violet
colouring-matter by this method in 1861. In 1866 this violet dye was
manufactured in France by Poirrier, and it is still made in large
quantities, being known under the name of "methyl violet." This
colouring-matter, and a bluer derivative of it discovered in 1868,
gradually displaced the Hofmann violets, chiefly owing to their greater
cheapness of production. We are thus introduced to methylated aniline as a
source of colouring-matters, and as the compound in question has many
different uses in the coal-tar industry, a few words must be devoted to
its technology.
Aniline, toluidine, and similar bases can be methylated by the action of
methyl iodide, but the cost of iodine is too great to enable this process
to be used by the manufacturer. Methyl chloride, however, answers equally
well, and this compound, which is a liquid of very low boiling point (-23 deg.
C.), is prepared on a large scale from the waste material of another
industry, viz. the beet-sugar manufacture. It is interesting to see how
distinct industries by chemical skill are made to act and react upon one
another. Thus the cultivation of the beet, as already explained, is
largely dependent on the supply of ammonia from gas-liquor. During the
refining of the beet-sugar, a large quantity of uncrystallisable treacle
is separated, and this is fermented for the manufacture of alcohol. When
the latter is distilled off there remains a spent liquor containing among
other things potassium salts and nitrogenous compounds. This waste liquor,
called "vinasse," is evaporated down and ignited in order to recover the
potash, and during the ignition, ammonia, tar, gas, and other volatile
products are given off. Among the volatile products is a base called
trimethylamine, which is a derivative of ammonia; the salt formed by
combining trimethylamine with hydrochloric acid when heated gives off
methyl chloride as a gas which can be condensed by pressure.
Here we have a very pretty cycle of chemical transmigration. The nitrogen
of the coal plants, stored up in the earth for ages, is restored in the
form of ammonia to the crops of growing beet; the nitrogen is made to
enter into the composition of the latter plant by the chemico-physiological
process going on, and the nitrogenous compounds removed from the plant and
heated to the point of decomposition in presence of the potash (which also
entered into the composition of the plant), give back their nitrogen
partly in the form of a base from which methyl chloride can be obtained.
The latter is then made to methylate a product, aniline, derived indirectly
from coal-tar. The utilisation of the "vinasse" for this purpose was made
known by Camille Vincent of Paris in 1878.
The methylation of aniline can obviously be carried out by the foregoing
process only when beet-sugar residues are available. There is another
method which is more generally used, and which is interesting as bringing
in a distinct branch of industry. The same result can, in fact, be arrived
at by heating dry aniline hydrochloride, _i.e._ the hydrochloric acid salt
of aniline, with methyl alcohol or wood-spirit in strong metallic boilers
under great pressure. This is the process carried on in most factories,
and it involves the use of pure methyl alcohol, a branch of manufacture
which has been called into existence to meet the requirements of the
coal-tar colour maker.[4] This alcohol or wood-spirit is obtained by the
destructive distillation of wood, and is purified by a series of
operations which do not at present concern us. It must be mentioned that
the product of the methylation of aniline, which it is the object of the
manufacturer to obtain, is an oily liquid called dimethylaniline, which,
by virtue of the chemical transformation, is quite different in its
properties to the aniline from which it is derived. By a similar
operation, using ethyl alcohol, or spirit of wine, diethylaniline can be
obtained, and by heating dry aniline hydrochloride with aniline under
similar conditions a crystalline base called diphenylamine is also
prepared.
Now these products--dimethylaniline, diethylaniline, and
diphenylamine--are derived from aniline, and they are all sources of
colouring-matters. Methyl-violet is obtained by the oxidation of
dimethylaniline by means of a gentle oxidizer; a mixture of bases is not
necessary as in the case of the magenta formation. Then in 1866
diphenylamine was shown by Girard and De Laire to be capable of yielding a
fine blue by heating it with oxalic acid, and this blue, on account of the
purity of its shade, is still an article of commerce. It can be made
soluble by the action of sulphuric acid in just the same way as the other
aniline blue. Furthermore, by acting with excess of methyl chloride on
methyl violet, a brilliant green colouring-matter was manufactured in
1878, which was obviously analogous to the iodine green already mentioned,
and which for some years held its own as the only good coal-tar green.
These are the dyes--methyl violet and green, and diphenylamine blue--which
were in commerce before the discovery of the Fischers, and which this
discovery enabled chemists to class with magenta, aniline blue, and
Hofmann violet in the triphenylmethane group.
Later developments bring us into contact with other dyes of the same
class, and with the industrial evolution of the purely scientific idea
concerning the constitution of the colouring-matters of this group.
Benzene and toluene again form the points of departure. By the action of
chlorine upon the vapour of boiling toluene there are obtained, according
to the extent of the action of the chlorine, three liquids of use to the
colour manufacturer. The first of these is benzyl chloride, the second
benzal chloride, and the third benzotrichloride or phenyl chloroform.
Benzyl chloride, it may be remarked in passing, plays the same part in
organic chemistry as methyl chloride, and enables certain compounds to be
benzylated, just in the same way that they can be methylated. The bluer
shade of methyl violet, introduced in 1868, and still manufactured, is a
benzylated derivative. By the action of benzotrichloride on
dimethylaniline in the presence of dry zinc chloride, Oscar Doebner
obtained in 1878 a brilliant green colouring-matter which was manufactured
under the name of "malachite green." It will be remembered that this was
about the time when the Fischers were engaged with their investigations.
These last chemists, by virtue of their scientific results, were enabled
to show that Doebner's green was a member of the triphenylmethane group,
and they prepared the same compound by another method which has enabled
the manufacturer to dispense with the use of the somewhat expensive and
disagreeable benzotrichloride. The Fischers' method consists in heating
dimethylaniline with bitter-almond oil and oxidizing the product thus
formed, when the green colouring-matter is at once produced. This method
brings the technologist into competition with Nature, and we shall see the
result.
Benzoic aldehyde or bitter-almond oil is one of the oldest known products
of the vegetable kingdom, and has from time to time been made the subject
of investigation by chemists since the beginning of the century. It arises
from the fermentation of a nitrogenous compound found in the almond, and
known as amygdalin, the nature of the fermentative change undergone by
this substance having been brought to light by Woehler and Liebig. The
discovery of a green dye, requiring for its preparation a vegetable
product which was very costly, compelled the manufacturer to seek another
source of the oil. Pure chemistry again steps in, and solves the problem.
In 1863 it was known to Cahours that benzal chloride, on being heated with
water or alkali, gave benzoic aldehyde, and in 1867 Lauth and Grimaux
showed that the same compound could be formed by oxidizing benzyl chloride
in the presence of water. It was but a step from the laboratory into the
factory in this case, and at the present time the aldehyde is made on a
large scale by chlorinating boiling toluene beyond the stage of benzyl
chloride, and heating the mixture of benzal chloride and benzotrichloride
with lime and water under pressure. By this means the first compound is
transformed into benzoic aldehyde, and the second into benzoic acid. This
last substance is also required by the colour-maker, as it is used in the
manufacture of blue by the action of aniline on rosaniline; without some
such organic acid the transformation of rosaniline into the blue is very
imperfect.
Benzoic acid, like the aldehyde, is a natural product which has long been
known. It was obtained from gum benzoin at the beginning of the
seventeenth century, and its preparation from this source was described by
Scheele in 1755. The same chemist afterwards found it in urine, and from
these two sources, the one vegetable and the other animal, the acid was
formerly prepared. Its relationship to benzene has already been alluded to
in connection with the history of that hydrocarbon. It will be remembered
that by heating this acid with lime Mitscherlich obtained benzene in
1834. In one operation, therefore, setting out from toluene, we make these
two natural products, the aldehyde and acid, which are easily separable by
technical processes. The wants of the technologist have been met, and he
has been enabled to compete successfully with Nature, for he can
manufacture these products much more cheaply than when he had to depend
upon bitter almonds or gum benzoin. The synthetical bitter-almond oil is
chemically identical with that from the plant. Besides its use for the
manufacture of colouring-matters, it is employed for flavouring purposes
and in perfumery, this being the first instance of a coal-tar perfume
which we have had occasion to mention. The odour in this case, it must be
remembered, is that of the actual compound which imparts the
characteristic taste and smell to the almond; it is not the result of
substituting a substance which has a particular odour for another having a
similar odour, as is the case with nitrobenzene, which, as already
mentioned, is used in large quantities under the name of "essence of
mirbane," for imparting an almond-like smell to soap.
The introduction of malachite green marks another epoch in the history of
the technology of the triphenylmethane colours. The action between benzoic
aldehyde and other bases analogous to dimethylaniline was found to be
quite general, and the principle was extended to diethylaniline and
similarly constituted bases. Various green dyes--some of them acids formed
by the action of sulphuric acid on the colour base--are now manufactured,
and many other colouring-matters of the same group are synthesised by the
benzoic aldehyde process.
One other development of this branch of manufacture has yet to be
recorded. The new departure was made in 1883 by Caro and Kern, who
patented a process for the synthesis of colouring-matters of this group.
In this synthesis a gas called phosgene is used, the said gas having been
discovered by John Davy in 1811, who gave it its name because it is formed
by the direct union of chlorine and carbon monoxide under the influence of
sunlight. Caro and Kern's process is the first technical application of
Davy's compound. By the action of phosgene on dimethylaniline and
analogous bases in the presence of certain compounds which promote the
chemical interaction, a number of basic colouring-matters of brilliant
shades of violet ("crystal violet") and blue ("Victoria blue," "night
blue") are produced, these being all members of the triphenylmethane
group. One of these dyes is a fine basic yellow known as "auramine," which
is a derivative of diphenylmethane.
TAR.
(Light oil)
| |
| |
---------------- --------
| | / Benzyl chloride
Benzene Toluene { Benzal chloride }
| | | \ Benzotrichloride}<-
| | ------------ |
Aniline-> Magenta (and Phosphine)<-Toluidine | |
/ \ | \ | |
/ \ | \ ---Benzoic acid and<-
Dimethylaniline Diphenylamine | \ | Benzoic aldehyde
| | | \ | /
| | | Aniline Blues: /
| | Hofmann Violet Nicholson and /
| | and Iodine Green Soluble /
| Diph. Blue /
| /
------------------------------------------------ /
| | | /
Crystal Violet, Methyl Violet | /
Auramine, & and Green Malachite
other Phosgene dyes Green series
From benzene and toluene alone about forty distinct colouring-matters of
the rosaniline group are sent into commerce. The relationship of those
compounds to each other and to their generating substances is not easy to
grasp by those to whom the facts are presented for the first time. The
scheme on page 107 shows these relationships at a glance.
The colouring-matters derived from these two hydrocarbons are far from
being exhausted. During the oxidation of aniline for the production of
mauve--which colouring-matter, it may be mentioned, is no longer made--a
red compound is formed as a by-product. This was isolated by Perkin in
1861, and studied scientifically by Hofmann and Geyger, who established
its composition in 1872, the dye being at that time manufactured under the
name of "saffranine." It appears to have been first introduced about 1868.
The conditions of formation of this dye were at first imperfectly
understood, but the problem was attacked by chemists and technologists,
and the first point of importance resulting from their work was that
saffranine was derived from one of the toluidines present in the
commercial aniline. To record the various steps in this chapter of
industrial chemistry would take us beyond the scope of the present work.
In addition to the chemists named, Caro, Bindschedler, and others
contributed to the technology, while the scientific side of the matter was
first taken up by Nietzki in 1877, by Otto Witt in 1878, and by Bernthsen
in 1886. It is to the work of these chemists, and especially to that of
Witt, that we owe our present knowledge of the constitution of this and
allied colouring-matters. Space will not admit of our traversing the
ground, although to chemists it is a line of investigation full of
interest; it will be sufficient to say that by 1886 these investigators
had accomplished for these colouring-matters what the Fischers had done
for the rosaniline group--they established their constitution, and showed
that they were derivatives of diphenylamine, containing two nitrogen atoms
joined together in a particular way. The parent-substance from which these
compounds are derived is known at the present time as "azine" (French,
azote = nitrogen), and the dyes belong accordingly to the azine group. The
first coal-tar colouring-matter, Perkin's mauve, is a member of this
class.
The azine dyes are basic, and mostly of a red or pink shade; they are
somewhat fugitive when exposed to light, but possess a certain value on
account of their affinity for cotton, and the readiness with which they
can be used in admixture with other colouring-matters. Some of the best
known are made by oxidizing certain derivatives of aniline or toluidine,
in the presence of these or analogous bases. To make this intelligible a
little more chemistry is necessary. Aniline is a derivative of benzene in
which one atom of hydrogen is replaced by the residue of ammonia. Ammonia
is composed of one atom of nitrogen and three atoms of hydrogen; benzene
is composed of six atoms of carbon and six of hydrogen. If one atom of
hydrogen is supposed to be withdrawn from ammonia, there remains a residue
called the amido-group, and if we imagine this group to be substituted for
one of the hydrogen atoms in benzene, we have an amido-derivative, _i.e._
amidobenzene or aniline. Similarly, the toluidines are amidotoluenes. If
two hydrogen atoms in benzene or toluene are replaced by two amido-groups,
we have diamidobenzenes and diamidotoluenes, which are strongly basic
substances, capable of existing in several isomeric modifications. Certain
of these diamido-compounds when oxidized in the presence of a further
quantity of aniline, toluidine, and such amido-compounds, give rise to
unstable blue products, which readily become transformed into red dyes of
the azine group.
Some azine dyes are produced by another method, which is instructive
because it brings us into contact with a derivative of dimethylaniline
which figures largely in the coal-tar colour industry. By the action of
nitrous acid on this base, there is produced a compound known as
nitrosodimethylaniline, which was discovered by Baeyer and Caro in 1874,
and which contains the residue of nitrous acid in place of one atom of
hydrogen. The residue of nitric acid which replaces hydrogen in benzene is
the nitro-group, and the compound is nitrobenzene. The analogy with
nitrous acid will therefore be sufficiently understood--the residue of
this acid is the nitroso-group, and compounds containing this group are
nitroso-derivatives. In 1879, Otto Witt found that the nitroso-group in
nitrosodimethylaniline acted as an oxidizing group, and enabled this
compound to act upon certain diamido-derivatives of benzene and toluene,
with the formation of unstable blue compounds, which on heating the
solution changed into red colouring-matters of the azine group. This
process soon bore fruit industrially, and azines of a red, violet, and
blue shade were introduced under the names of neutral red, violet, and
blue, Basle blue, &c., some of these surviving at the present time.
We have now to turn to another chapter in the history of dimethylaniline.
In 1876, Lauth discovered a new colour test for one of the
diamidobenzenes. By heating this base with sulphur, and oxidizing the
product, a violet colouring-matter was formed, and the same compound was
produced by oxidizing the base in an aqueous solution in the presence of
sulphuretted hydrogen. Lauth's violet was never manufactured in quantity
because the yield is small; but in the hands of Dr. Caro the work of Lauth
bore fruit in another direction. Instead of using the diamidobenzene, Caro
used its dimethyl-derivative, and by this means obtained a splendid blue
dye, which was introduced under the name of "methylene blue." Here again
we find scientific research reacting on technology. A few words of
chemical explanation will make this manufacture intelligible. By the
action of reducing agents on nitro and nitroso-compounds, the nitro and
nitroso-group become converted into the amido-group. Thus when
nitrobenzene is reduced by iron and an acid we get aniline; similarly when
nitrosodimethylaniline is reduced by zinc and an acid we get
amidodimethylaniline, and this is the base used in the preparation of
methylene blue. By oxidizing this base in the presence of sulphuretted
hydrogen, the colouring-matter is formed. Other methods of arriving at the
same result were discovered and patented in due course, but the various
processes cannot be discussed here.
Lauth's violet and methylene blue became the subjects of scientific
investigation in 1879 by Koch, and in 1883 a series of brilliant
researches were commenced by Bernthsen which extended over several years,
and which established the constitution of these compounds. It was shown
that they are derivatives of diphenylamine containing sulphur as an
essential constituent. The parent-compound is diphenylamine in which
sulphur replaces hydrogen, and is therefore known as thiodiphenylamine. It
can be prepared by heating diphenylamine with sulphur, and is sometimes
called thiazine, because it is somewhat analogous in type to azine. We
must therefore credit dimethylaniline with being the industrial generator
of the thiazines. The blue is largely used for cotton dyeing, producing on
this fibre when properly mordanted an indigo shade. By the action of
nitrous acid the blue is converted into a green known as "methylene
green."
Although the scope of this work admits of our dealing with only a few of
the more important groups of colouring-matters, it will already be evident
that the chemist has turned benzene and toluene to good account. But great
as is the demand for these hydrocarbons for the foregoing purposes, there
are other branches of the coal-tar industry which are dependent upon them.
It will serve as an answer to those who are continually raising the cry of
brilliancy as an offence to aesthetic taste if we consider in the next
place a most valuable and important black obtained from aniline. All
chemists who studied the action of oxidizing agents, such as chromic acid,
on aniline, from Runge in 1834 to Perkin in 1856, observed the formation
of greenish or bluish-black compounds. After many attempts to utilize
these as colouring-matters, success was achieved by John Lightfoot of
Accrington near Manchester in 1863. By using as an oxidizing agent a
mixture of potassium chlorate and a copper salt, Lightfoot devised a
method for printing and dyeing cotton fabrics, the use of which spread
rapidly and created an increased demand for the hydrochloride of aniline,
this salt being now manufactured in enormous quantities under the
technical designation of "aniline salt." Lightfoot's process was improved
for printing purposes by Lauth in 1864, and many different oxidizing
mixtures have been subsequently introduced, notably the salts of vanadium,
which are far more effective than the salts of copper, and which were
first employed by Lightfoot in 1872. In 1875-76 Coquillion and
Goppelsroeder showed that aniline black is produced when an electric
current is made to decompose a solution of an aniline salt, the oxidizing
agent here being the nascent oxygen resulting from the electrolysis. In
these days when the generation of electricity is so economically effected,
this process may become more generally used, and the coal-tar industry
may thus be brought into relationship with another branch of applied
science. Aniline black is seldom used as a direct colouring-matter; it is
generally produced in the fibre by printing on the mixture of aniline salt
and oxidizing compounds thickened with starch, &c., and then allowing the
oxidation to take place spontaneously in a moist and slightly heated
atmosphere. By a similar process, using a dye-bath containing the aniline
salt and oxidizing mixture, cotton fibre is easily dyed. The black cannot
be used for silk or wool, as the oxidizing materials attack these fibres,
but for cotton dyeing and calico printing this colouring-matter has come
seriously into competition with the black dyes obtained from logwood and
madder. The use of aniline for this purpose, first rendered practicable by
Lightfoot, is among the most important of the many wonderful applications
of coal-tar products in the tinctorial industry.
The year 1863 witnessed the introduction of the first of a new series of
colouring-matters which have had an enormous influence both on the art of
the dyer as well as in the utilization of tar-products which were formerly
of but little value. We can consider the history of some of these colours
now, because the earliest of them was produced from aniline. The formation
of a yellow compound when nitrous acid acts upon aniline was observed by
several chemists prior to the date mentioned. In 1863 the firm of Simpson,
Maule and Nicholson manufactured a yellow dye by passing nitrous gas into
a solution of aniline in alcohol, and this had a limited application under
the name of "aniline yellow." Soon afterwards, viz. in 1866, the firm of
Roberts, Dale & Co. of Manchester introduced a brown dye under the name of
"Manchester brown"--this compound, which was discovered by Dr. Martius in
1865, having been produced by the action of nitrous acid on one of the
diamidobenzenes. Ten years later Caro and Witt discovered an orange
colouring-matter belonging to the same class, and the latter introduced
the compound into commerce as "chrysoidine." These three compounds are
basic, and the first of them is no longer used as a direct dye because it
is fugitive. Chrysoidine is still used to a large extent, and the
brown--now known as "Bismarck brown"--is one of the staple products of the
colour manufacturer at the present time. From this fragment of
technological history let us now turn to chemical science.
The chemist whose name will always be associated with the compounds of
this group is the late Dr. Peter Griess of Burton-on-Trent. He commenced
his study of the action of nitrous acid on organic bases in 1858, and
from that time till the period of his death in 1888, he was constantly
contributing to our knowledge of the resulting compounds. In 1866, he and
Dr. Martius established the composition of aniline yellow, and the
following year Caro and Griess did the same thing for the Manchester
brown. In 1877 Hofmann and Witt established the constitution of
chrysoidine, the final outcome of all this work being to show that the
three colouring-matters belonged to the same group. The further
development of these discoveries has been one of the most prolific sources
of new colouring-matters. A brief summary of our present position with
respect to this group must now be attempted.
When nitrous acid acts upon an amido-derivative of a benzenoid hydrocarbon
in the presence of a mineral acid, there is formed a compound in which the
amido-group is replaced by a pair of nitrogen atoms joined together in a
certain way, which is different to the mode of combination in the azines.
This pair of nitrogen atoms is combined on the one hand with the
hydrocarbon residue, and on the other with the residue of the mineral
acid. The resulting compound is very unstable; its solution decomposes
very readily, and generally has to be kept cool by ice. Freezing machines
turning out large quantities of ice are kept constantly at work in
factories where these produces are made. The latter are known as
"diazo-compounds"--Griess's compounds _par excellence_--and they are
prepared on a large scale by dissolving a salt of the amido-base,
generally the hydrochloride, in water with ice, and adding sodium nitrite.
The result is a diazo-salt; aniline, for example, giving diazobenzene
chloride, and toluidine diazotoluene chloride. Similarly all
amido-derivatives of a benzenoid character can be "diazotised." The
importance of this discovery will be seen more fully in the next chapter.
At present we are more especially concerned with aniline.
The extreme instability of the diazo-salts enables them to combine with
the greatest ease with amido-derivatives and with other compounds. The
very property which in the early days rendered their investigation so
difficult, and which taxed the ingenuity of chemists to the utmost, has
now placed these compounds in the front rank as colour generators. When a
diazo-salt acts on an amido-derivative there is formed a compound which is
more or less unstable, but which readily undergoes transformation under
suitable conditions into a stable substance in which two hydrocarbon
residues are joined together by the pair of nitrogen atoms. These products
are dye-stuffs, known as "azo-colours," and aniline yellow, Bismarck
brown, and chrysoidine are the oldest known technical compounds belonging
to the group. The parent substance is "azobenzene," and these three
colouring-matters are mono-, di- and triamido-azobenzene respectively.
A new phase in the technology of tar-products was entered upon when Witt
caused a diazo-salt to act upon diamidobenzene. This was the first
industrial application of Griess's discovery. Azobenzene, which was
discovered by Mitscherlich in 1834, and azotoluene are now manufactured by
reducing nitrobenzene and nitrotoluene with mild reducing agents. These
parent compounds are not in themselves colouring-matters, but they are
transformed into bases which give rise to a splendid series of azo-dyes,
as will be described subsequently. Let it be recorded here that these two
compounds are to be added to the list of valuable products obtained from
benzene and toluene. And it must also be remembered that the introduction
of these azo-colours has necessitated the manufacture on a large scale of
sodium nitrite as a source of nitrous acid. Without entering into
unnecessary detail it may be stated broadly that this salt is made by
fusing Chili saltpetre, which is the nitrate of sodium, with metallic
lead, litharge or oxide of lead being obtained as a secondary product.
Then again, the manufacture of Bismarck brown requires dinitrobenzene,
this being made by the nitration of benzene beyond the stage of
nitrobenzene. The brown is made by reducing the dinitrobenzene to
diamidobenzene, and then treating a solution of the latter with sodium
nitrite and an acid. The azo-colour is formed at once, and no special
refrigeration is required in this particular case.
It has already been stated that the old aniline yellow of 1863 is no
longer used on account of its fugitive character. In 1878 Graessler found
that by the action of very strong sulphuric acid this azo-compound could
be converted into a sulpho-acid in just the same way that magenta can be
converted into acid magenta. Under the name of "acid yellow" this
sulpho-acid is now used, not only as a direct yellow colouring-matter, but
as a starting-point in the manufacture of other azo-dyes. The use of acid
yellow for this last purpose will be dealt with again in the next chapter.
There is one other use for aniline yellow which dates from the year of its
discovery, when Dale and Caro found that by adding sodium nitrite to
aniline hydrochloride and heating the mixture, a blue colouring-matter is
produced. The latter was introduced in 1864 under the name of "induline."
It was shown subsequently by the scientific researches of several chemists
that the blue produced by Dale and Caro's method results from the action
of the aniline salt on the aniline yellow, which is formed by the action
of the nitrous acid on the aniline and aniline salt. This explanation was
proved to be correct in 1872 by Hofmann and Geyger, who prepared the
colouring-matter by heating aniline yellow and aniline salt with alcohol
as a solvent. These chemists established the composition and gave it the
name of "azodiphenyl blue." Later, viz. in 1883, the manufacture was
improved by Otto Witt and E. Thomas, and the dye, under the old name of
"induline," is now largely manufactured by first preparing aniline yellow
and then heating this with aniline and aniline salt. The colouring-matter
as formed by this method is basic and insoluble in water; it is made acid
and soluble by treatment with sulphuric acid, which converts it into a
sulpho-acid. Induline belongs to the sober-tinted colours, and produces a
shade somewhat resembling indigo. Closely related thereto is a bluish-grey
called "nigrosine," obtained by heating nitrobenzene with aniline, as well
as a certain bluish by-product obtained during the formation of magenta,
and known as "violaniline."
It will be convenient here to pause and reflect upon the great industrial
importance of the two coal-tar hydrocarbons upon which we have thus far
concentrated our attention. Their uses are by no means exhausted as yet,
but they have already been made to account for such a number of valuable
products that the reader may find it useful to have the results presented
in a collected form. This is given below as a chronological summary--
1856. Mauve discovered by Perkin; leading to manufacture of aniline.
1860. Arsenic acid process for magenta discovered; leading to
manufacture of arsenic acid.
1860. Aniline blue discovered; leading in 1862 to soluble and Nicholson
blues.
1861. Methyl violet discovered; manufactured in 1866; leading to a new
use of copper salts as oxidizing agents, and to the manufacture of
dimethylaniline.
1862. Hofmann violets discovered; leading to manufacture of methyl
iodide from iodine, phosphorus, and wood spirit.
1862. Phosphine (chrysaniline) discovered in crude magenta.
1863. Aniline black introduced; leading to a new use for potassium
chlorate and copper salts, and to the manufacture of aniline salt.
1863. Aniline yellow introduced, the first azo-colour.
1864. Induline discovered; leading to new use for aniline yellow.
1866. Manchester brown introduced, the second azo-colour; leading to the
manufacture of sodium nitrite, and of dinitrobenzene.
1866. Iodine green introduced; leading to further use for methyl iodide.
1866. Diphenylamine blue introduced; leading to manufacture of
diphenylamine.
1868. Blue shade of methyl violet introduced; leading to manufacture of
benzyl chloride.
1868. Saffranine introduced.
1869. Nitrobenzene process for magenta discovered.
1876. Chrysoidine introduced, the third azo-colour.
1876. Methylene blue introduced; leading to manufacture of
nitrosodimethylaniline.
1877. Acid magenta discovered.
1878. Methyl green introduced; leading to utilization of waste from
beet-sugar manufacture.
1878. Malachite green discovered; leading to manufacture of benzoic
aldehyde.
1878. Acid yellow discovered; leading to new use for aniline yellow.
1879. Neutral red and allied azines introduced; leading to a new use for
nitrosodimethylaniline.
1883. Phosgene colours of rosaniline group introduced; leading to
manufacture of phosgene.