Read Ebook: Coal and What We Get from It by Meldola Raphael

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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 Br?ant'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 . 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.

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 P?ligot 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.

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 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 , 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.

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 . 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.

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 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? C., 111? C., and 140? 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? and 150? 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? 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.

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 Fr?res 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.

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 , 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 , 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.

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.

Benzoic acid, like the aldehyde, is a natural product which has long been known. It was obtained from gum benzo?n 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 benzo?n. 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.

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" , 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 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 "chryso?dine." These three compounds are basic, and the first of them is no longer used as a direct dye because it is fugitive. Chryso?dine 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 chryso?dine, 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.

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 chryso?dine 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 Gr?ssler 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--

Imagine the industrial revolution which would be caused by the discovery of a process for obtaining indigo synthetically from a coal-tar hydrocarbon, at a price which would compare favourably with that of the natural product. This has not actually been done as yet, but chemists have attempted to compete with Nature in this direction, and the present state of the competition is that the natural product can be cultivated and made more cheaply. Nevertheless the dye can be synthesised from a coal-tar hydrocarbon, and this is one of the greatest achievements of modern chemistry in connection with the tar-products. For more than half a century indigo had been undergoing investigation by chemists, and at length the work culminated in the discovery of a method for producing it artificially. This discovery was the outcome of the labour of Adolf v. Baeyer, who commenced his researches upon the derivatives of indigo in 1866, and who in 1880 secured the first patents for the manufacture of the colouring-matter. It is to the laborious and brilliant investigations of this chemist that we owe nearly all that is at present known about the chemistry of indigo and allied compounds.

The light oil of the tar-distiller has now been sufficiently dealt with so far as regards colouring-matters; let us pass on to the next fraction of the tar, the carbolic oil. The important constituents of this portion are carbolic acid and naphthalene. The carbolic oil is in the first place separated into two distinct portions by washing with an alkaline solution. Carbolic acid or phenol belongs to a class of compounds derived from hydrocarbons of the benzene and related series by the substitution of the residue of water for hydrogen. This water-residue is known to chemists as "hydroxyl"--it is water less one atom of hydrogen. Carbolic acid or phenol is hydroxybenzene; and all analogous compounds are spoken of as "phenols." It will be understood in future that a phenol is a hydroxy-derivative of a benzenoid hydrocarbon. Now these phenols are all more or less acid in character by virtue of the hydroxyl-group which they contain. For this reason they dissolve in aqueous alkaline solutions, and are precipitated therefrom by acids. This will enable us to understand the purification of the carbolic oil.

The two layers into which this oil separates after washing with alkali are the aqueous alkaline solution of the carbolic acid and other phenols, and the undissolved naphthalene contaminated with oily hydrocarbons and other impurities. Each of these portions has its industrial history. The alkaline solution, on being drawn off and made acid, yields its mixture of phenols in the form of a dark oil from which carbolic acid is separated by a laborious series of fractional distillations. The undissolved hydrocarbon is similarly purified by fractional distillation, and furnishes the solid crystalline naphthalene. The tar from one ton of Lancashire coal yields about 1-1/2 lbs. of carbolic acid, equal to about 1 per cent. by weight of the tar, and about 6-1/4lbs. of naphthalene, so that this last hydrocarbon is one of the chief constituents of the tar, of which it forms from 8 to 10 per cent. by weight.

The crude carbolic acid as separated from the alkaline solution is a mixture of several phenolic compounds, and all of these but the carbolic acid itself are gradually removed during the process of purification. Among the compounds associated with the carbolic acid are certain phenols of higher boiling-point, which bear the same relationship to carbolic acid that toluene bears to benzene. That is to say, that while phenol itself is hydroxybenzene, these other compounds, which are called "cresols," are hydroxytoluenes. The cresols form an oily liquid largely used for disinfecting purposes under the designation of "liquid carbolic acid," or "cresylic acid." Carbolic acid is a white crystalline solid possessing strongly antiseptic properties, and is therefore of immense value in all cases where putrefaction or decay has to be arrested. It was discovered in coal-tar by Runge in 1834, and was obtained pure by Laurent in 1840.

The gradual establishment of the germ-theory of disease, chiefly due to the labours of Pasteur, has led to a most important application of carbolic acid. Once again we find the coal-tar industry brought into contact with another department of science. Arguing from the view that putrefactive change is brought about by the presence of the germs of micro-organisms ever present in the atmosphere, Sir Joseph Lister proposed that during surgical operations the incised part should be kept under a spray of the germicidal carbolic acid to prevent subsequent mortification. No operation upon portions of the body exposed to the air is at present conducted without this precaution, and many a human life must have been saved by Lister's treatment. To this result the chemist and technologist have contributed, not only by the discovery of the carbolic acid in the tar, but also by the development of the necessary processes for its purification. It should be added that the phenol used must be of the greatest possible purity, and the requirements of the surgeon have been met by chemical and technological skill.

From surgery back to colouring-matters, and from these to pharmaceutical preparations and perfumes, are we led in following up the cycles of chemical transformation which these tar-products have undergone in the hands of the technologist, guided by the researches of the chemist. It was observed by Runge in 1834 that crude carbolic acid, on treatment with lime, gave a red, acid colouring-matter which he separated and named "rosolic acid." The observation was followed up, and many other chemists obtained red colouring-matters by the oxidation of crude phenol. In 1859, the colour-giving property of carbolic acid acquired industrial importance from a discovery made by Kolbe and Schmitt in Germany, and by Persoz in France. These chemists found that a good yield of the colouring-matter was obtained by heating phenol with oxalic and sulphuric acids. Under the names of "corallin" and "aurin" the dye-stuff was introduced into commerce, and it is still used for certain purposes, especially for the preparation of coloured lakes for paper-staining.

In the fragrant blossom of the meadowsweet there is contained an acid which is found also as an ether in the oil of wintergreen . This is salicylic acid, a white crystalline compound which has been known to chemists since 1839. In 1860 Kolbe prepared the sodium salt of this acid by passing carbon dioxide gas into phenol in which metallic sodium had been dissolved. It was found subsequently that the same transformation was brought about by heating the dry sodium salt of carbolic acid in an atmosphere of carbon dioxide. This process of Kolbe's is now worked on a manufacturing scale for the preparation of artificial salicylic acid. The acid and its salts and ethers find numerous applications as antiseptics, for the preservation of food, and in pharmacy.

Salicylic acid is employed also for the manufacture of certain azo-dyes in a way that it will be very instructive to consider, because the process used may be taken as typical of the general method of preparing such compounds. Solutions of diazo-salts act not only upon amido- and diamido-compounds, as we have seen in the case of aniline yellow and chryso?dine, but also upon phenols, forming acid azo-colours. This important fact was made known in 1870 by the German chemists Kekul? and Hidegh, but more than six years elapsed before this discovery was taken advantage of by the technologist. Large numbers of these acid azo-dyes are now made from various diazotised amido-compounds combined with different phenols and phenolic acids. The mode of procedure is to diazotise the amido-compound by sodium nitrite and hydrochloric acid in the manner already described, and then add the diazo-salt solution to the phenolic compound dissolved in alkali. The colouring-matter is at once formed. Salicylic acid possesses the characters both of an acid and a phenol. It combines readily with diazo-salts under the circumstances described, and gives rise to azo-dyes, some of which are of technical value.

Some other uses of carbolic acid must next be considered. Of the colouring-matters derived from coal-tar, none is more widely known than the oldest artificial yellow dye, picric acid. This is a phenol derivative, and was first obtained as long ago as 1771 by Woulfe, by acting upon indigo with nitric acid. Laurent in 1842 was the first to obtain this dye from carbolic acid, from which compound it is still manufactured by acting upon the sulpho-acid with nitric acid. Chemically considered, it is trinitrophenol. It has a very wide application as a dye, and has been used as an explosive agent. A similar colouring-matter was made from cresol in 1869, and introduced under the name of "Victoria yellow," which is dinitro-cresol. Other dyes derived directly or indirectly from phenol will take us back once again to toluene.

A new diazotisable diamido-compound was obtained from this last hydrocarbon, and introduced in 1886 by Leonhardt & Co. One of the three isomeric nitrotoluenes furnishes a sulpho-acid which, on treatment with alkali, gives a compound derived from a hydrocarbon known as stilbene, and this, on reduction, is converted into the diamido-compound referred to. The latter, which is a disulpho-acid as well as a diamido-compound, can be diazotised and combined with phenols, &c. The stilbene azo-dyes thus prepared from phenol and salicylic acid, like the chrysamines, are yellow colouring-matters, containing two azo-groups. It is a valuable characteristic of these secondary azo-dyes that they all possess a special affinity for vegetable fibre, and their introduction has exerted a great influence upon the art of cotton-dyeing. We shall have to return to these cotton-dyes again shortly.

Before leaving this branch of the subject, the following scheme is presented to show the relationships and inter-relationships of the products thus far dealt with in the present chapter--

The existence of naphthalene in coal-tar was made known in 1820 by Garden, who gave it this name because the oils obtained from the tar by distillation went under the general designation of naphtha. The greater portion of the hydrocarbon is contained in the carbolic oil, and is separated and purified in the manner described. A further quantity of impure naphthalene separates out from the next fraction--the creosote oil, and this is similarly washed and purified by distillation. The large quantity of naphthalene existing in tar has already been referred to, but although it is such an important constituent, it was only late in the history of the colour industry that it found any extensive application. In early times it was regarded as a nuisance, and was burnt as fuel, or for the production of a dense soot, which was condensed to form lampblack. It will be remembered that the first of the coal-tar colours made required only the light oils. There are at present only a few direct uses for naphthalene, but one of its applications is sufficiently important to be mentioned.

The hydrocarbon is a white crystalline solid melting at 80? C., and boiling at 217? C. Although it has a high boiling-point, it passes readily into vapour at lower temperatures, and the vapour on condensation forms beautiful silvery crystalline scales. This product is "sublimed naphthalene." The vapour of naphthalene burns with a highly luminous flame, and if mixed with coal-gas, it considerably increases the luminosity of the flame. Advantage is taken-of this in the so-called "albo-carbon light," which is the flame of burning coal-gas saturated with naphthalene vapour. The burner is constructed so that the gas passes through a reservoir filled with melted naphthalene kept hot by the flame itself .

To appreciate properly the value of those discoveries which have enabled manufacturers to utilize this hydrocarbon, it is only necessary to recall to mind the actual quantity produced in this country. Supposing that ten million tons of coal are used annually for gas-making, and that the 500,000 tons of tar resulting therefrom contain only eight per cent. of naphthalene, there would be available about 40,000 tons of this hydrocarbon annually. Great as have been the recent advancements in the utilization of naphthalene derivatives, there is still a larger quantity of this hydrocarbon produced than is necessary to supply the wants of the colour-manufacturer. From this last statement it will be inferred that naphthalene is now a source of colouring-matters. Let us consider how this has been brought about.

The phenols of naphthalene are called naphthols--they bear the same relationship to naphthalene that carbolic acid bears to benzene. Owing to the structure of the naphthalene molecule there are two isomeric naphthols, whereas there is only one phenol. The naphthols--known as alpha- and beta-naphthol, respectively--are now made on a large scale from naphthalene, by heating the latter with sulphuric acid; at a low temperature the alpha sulpho-acid is produced, and at a higher temperature the beta sulpho-acid, and these acids on fusion with caustic soda furnish the corresponding naphthols. Similarly there are two amidonaphthalenes, known as alpha- and beta-naphthylamine respectively. As aniline is to benzene, so are the naphthylamines to naphthalene. The alpha-compound is made in precisely the same way as aniline, viz. by acting upon naphthalene with nitric acid so as to form nitronaphthalene, and then reducing the latter with iron dust and acid. Beta-napthylamine cannot be made in this way; it is prepared from beta-naphthol by heating the latter in presence of ammonia, when the hydroxyl becomes replaced by the amido-group in accordance with a process patented in 1880 by the Baden Aniline Company. The principle thus utilized is the outcome of the scientific work of two Austrian chemists, Merz and Weith. Setting out from the naphthols and naphthylamines we shall be led into industrial developments of the greatest importance.

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