v. Baeyer were taken up by various chemists, whose work together with that
of the original discoverer has led to the elucidation of the constitution of these colouring-matters. The phthaleins are members of the triphenylmethane group, and are therefore related to magenta, corallin, malachite green, methyl violet, and the phosgene dyes.
It has been said that the phenolic and amidic derivatives of naphthalene, _i.e._ the naphthols and naphthylamines, are of the greatest importance to the colour industry. One of the first uses of alpha-naphthylamine has already been mentioned, viz. for the production of the Manchester yellow, which was afterwards made more advantageously from alpha-naphthol. A red colouring-matter possessing a beautiful fluorescence was afterwards (1869) made from this naphthylamine and introduced as "Magdala red." The latter was discovered by Schiendl of Vienna in 1867. It was prepared in precisely the same way as induline was prepared from aniline yellow. The latter, which is amido-azobenzene, and which is prepared, broadly speaking, by the action of nitrous acid on aniline, has its analogue in amido-azonaphthalene, which is similarly prepared by the action of nitrous acid on naphthylamine. Just as aniline yellow when heated with aniline and an aniline salt gives induline, so amido-azonaphthalene when heated with naphthylamine and a salt of this base gives Magdala red. The latter is, therefore, a naphthalene analogue of induline, as was shown by Hofmann in 1869, and the knowledge of the constitution of the azines which has been gained of late years, enables us to relegate the colouring-matter to this group. This knowledge has also enabled the manufacture to be conducted on more rational principles, viz. by the method employed for the production of the saffranines, as previously sketched.
The introduction of azo-dyes, formed by the action of a diazotised amido-compound on a phenol or another amido-compound, marks the period from which the naphthols and naphthylamines rose to the first rank of importance as raw materials for the colour manufacturer. The introduction of chrysoidine in 1876 was immediately followed by the manufacture of acid azo-dyes obtained by combining diazotised amido-sulpho-acids with phenols of various kinds, or with bases, such as dimethylaniline and diphenylamine. From what has been said in the foregoing portion of this volume, it is evident that all such azo-compounds result from the combination of two things, viz. (1) a diazotised amido-compound, and (2) a phenolic or amidic compound. Either (1) or (2) or both may be a sulpho-acid, and the resulting dye will then also be a sulpho-acid.
The first of these colouring-matters derived from the naphthols, was introduced in 1876-77 by Roussin and Poirrier, and by O. N. Witt. They were prepared by converting aniline into a sulpho-acid (sulphanilic acid), diazotising this and combining the diazo-compound with alpha- or beta-naphthol. The compounds formed are brilliant orange dyes, the latter being still largely consumed as "naphthol orange." Other dye-stuffs of a similar nature were introduced by Caro about the same time, and were prepared from the diazotised sulpho-acid of alpha-naphthylamine combined with the naphthols. By this means alpha-naphthol gives what is known as "acid brown," or "fast brown," and beta-naphthol a fine crimson, known as "fast red," or "roccellin." Diazotised compounds combine also with this same sulpho-acid of alpha-naphthylamine (known as naphthionic acid), the first colouring-matter formed in this way having been introduced by Roussin and Poirrier in 1878. It was prepared by diazotising a nitro-derivative of aniline, and acting with the diazo-salt on napthionic acid, and this dye is still used to some extent under the name of "archil substitute." In 1878, the firm of Meister, Lucius & Bruening of Hoechst-on-the-Main gave a further impetus to the utilization of naphthalene by discovering two isomeric disulpho-acids of beta-naphthol formed by heating that phenol with sulphuric acid. By combining various diazotised bases with these sulpho-acids, a splendid series of acid azo-dyes ranging in shade from bright orange to claret-red, and to scarlets rivalling cochineal in brilliancy were given to the tinctorial industry.
The colouring-matters introduced in 1878 by the Hoechst factory under the names of "Ponceaux" of various brands, and "Bordeaux," although to some extent superseded by later discoveries, still occupy an important position. Their discovery not only increased the consumption of beta-naphthol, but also that of the bases which were used for diazotising. These bases are alpha-naphthylamine and those of the aniline series. The intimate relationship which exists between chemical science and technology--a relationship which appears so constantly in the foregoing portions of this work--is well brought out by the discovery under consideration. A little more chemistry will enable this statement to be appreciated.
Going back for a moment to the hydrocarbons obtained from light oil, it will be remembered that benzene and toluene have thus far been considered as the only ones of importance to the colour-maker. Until the discovery embodied in the patent specification of 1878, the portions of the light oil boiling above toluene were of no value in the colour industry. Benzene and toluene are related to each other in a way which chemists describe by saying that they are "homologous." This means that they are members of a regularly graduated series, the successive terms of which differ by the same number of atoms of carbon and hydrogen. Thus toluene contains one atom of carbon and two atoms of hydrogen more than benzene. Above toluene are higher homologues, viz. xylene, cumene, &c., which occur in the light oil, the former being related to toluene in the same way that toluene is related to benzene, while cumene again contains one atom of carbon and two atoms of hydrogen more than xylene. This relationship between the members of homologous series is expressed in other terms by saying that the weight of the molecule increases by a constant quantity as we ascend the series.
The homology existing among the hydrocarbons extends to all their derivatives. Thus phenol is the lower homologue of the cresols. Also we have the homologous series--
Benzene. Nitrobenzene. Aniline. Toluene. Nitrotoluene. Toluidine. Xylene. Nitroxylene. Xylidine. Cumene. Nitrocumene. Cumidine.
The bases of the third column when diazotised and combined with the disulpho-acids of beta-naphthol give a graduated series of dyes beginning with orange and ending with bluish scarlet. Thus it was observed that the toluidine colour was redder than the aniline colour, and it was a natural inference that the xylidine colour would be still redder. At the time of this discovery no azo-colour of a true scarlet shade had been manufactured successfully. A demand for the higher homologues of benzene was thus created, and the higher boiling-point fractions of the light oil, which had been formerly used as solvent naphtha, became of value as sources of colouring-matters. The isolation of coal-tar xylene (which is a mixture of three isomeric hydrocarbons) is easily effected by fractional distillation with a rectifying column, and by nitration and reduction, in the same way as in the manufacture of aniline, xylidine is placed at the disposal of the colour-maker.
Xylidine scarlet, although at the time of its introduction the only true azo-scarlet likely to come into competition with cochineal, was still somewhat on the orange side. The cumidine dye would obviously be nearer the desired shade. To meet this want, cumidine had to be made on a large scale, but here practical difficulties interposed themselves. The quantity of cumene in the light oil is but small, and it is associated with other hydrocarbons which are impurities from the present point of view, and from which it is separated only with difficulty. A new source of the base had therefore to be sought, and here again we find chemical science ministering to the wants of the technologist.
It was explained in the last chapter that aniline and similar bases can be methylated by heating their dry salts with methyl alcohol under pressure. In this way dimethylaniline is made, and dimethyltoluidine or dimethylxylidine can similarly be prepared. Now it was shown by Hofmann in 1871, that if this operation is conducted at a very high temperature, and under very great pressure, the methyl-alcohol residue, _i.e._ the methyl-group, does not replace the amidic hydrogen or the hydrogen of the ammonia residue, but the methylation takes place in another way, resulting in the formation of a higher homologue of the base started with. For example, by heating aniline salt and pure wood-spirit to a temperature considerably above that necessary for producing dimethylaniline, toluidine is formed. In a similar way, by heating xylidine hydrochloride and methyl alcohol for some time in a closed vessel at about 300 deg. C. cumidine is produced. Hofmann's discovery was thus utilized in 1882, and by its means the base was manufactured, and cumidine scarlet, very similar in shade to cochineal, became an article of commerce.
While the development of this branch of the colour industry was taking place by means of the new naphthol disulpho-acids, the cultivation of the fertile field of the azo-dyes was being carried on in other directions. It came to be realized that the fundamental discovery of Griess was capable of being extended to all kinds of amido-compounds. The azo-dyes hitherto introduced had all been derived from amido-compounds containing only one amido-group, and they accordingly contained only one azo-group; they were _primary_ azo-compounds. It was soon found that aniline yellow, which already contains one azo-group as well as an amido-group, could be again diazotised and combined with phenols so as to produce compounds containing two azo-groups, _i.e._ _secondary_ azo-compounds. The sulpho-acid of aniline yellow--Graessler's "acid yellow"--was the first source of azo-dyes of this class. By diazotising this amidoazo-sulpho-acid, and combining it with beta-naphthol, a fine scarlet dye was discovered by Nietzki in 1879, and introduced under the name of "Biebrich scarlet." Two years later a new sulpho-acid of beta-naphthol was discovered by Bayer & Co. of Elberfeld, and this gave rise, when combined with diazotised acid yellow and analogous compounds, to another series of brilliant dyes introduced as "Crocein scarlets."
From these beginnings the development of the azo-dyes has been steadily carried on to the present time--year by year new diazotisable amido-compounds or new sulpho-acids of the naphthols and naphthylamines are being discovered, and this branch of the colour industry has already assumed colossal dimensions. An important departure was made in 1884 by Boettiger, who introduced the first secondary azo-colours derived from benzidine. As already explained in connection with salicylic acid, this base and its homologue tolidine form tetrazo-salts, which combine with phenols and amines or their sulpho-acids. One of the first colouring-matters of this group was obtained by combining diazotised benzidine with the sulpho-acid of alpha-naphthylamine (naphthionic acid), and was introduced under the name of "Congo red." Then came the discovery (Pfaff, 1885), that the tetrazo-salts of benzidine and tolidine combine with phenols, amines, &c., in two stages, one of the diazo-groups first combining with one-half of the whole quantity of phenol to form an intermediate compound, which then combines with the other half of the phenol to form the secondary azo-dye. In the hands of the "Actiengesellschaft fuer Anilinfabrikation" of Berlin this discovery has been utilized for the production of a number of such azo-colours containing two distinct phenols, or amines, or sulpho-acids. Tolidine has been found to give better colouring-matters in most cases than benzidine, and it is scarcely necessary to point out that an increased demand for the nitrotoluene from which this base is made is the necessary consequence of this discovery.
It is impossible to attempt to specify by name any of these recent benzidine and tolidine dyes. Their introduction has been the means of finding new uses for the naphthylamines and naphthols and their sulpho-acids, and has thus contributed largely to the utilization of naphthalene. An impetus has been given to the investigation of these sulpho-acids, and chemical science has profited largely thereby. The process by which beta-naphthylamine is prepared from beta-naphthol, already referred to, viz. by heating with ammonia under pressure, has been extended to the sulpho-acids of beta-naphthol, and by this means new beta-naphthylamine sulpho-acids have been prepared, and figure largely in the production of these secondary azo-colours. The latter, as previously stated, possess the most valuable property of dyeing cotton fibre directly, and by their means the art of cotton dyeing has been greatly simplified. The shades given by these colours vary from yellow through orange to bright scarlet, violet, or purple.
In addition to benzidine and tolidine, other diazotisable amido-compounds have of late years been pressed into the service of the colour-manufacturer. The derivative of stilbene, already mentioned as being prepared from a sulpho-acid of one of the nitrotoluenes, forms tetrazo-salts, which can be combined with similar or dissimilar phenols, amines, or sulpho-acids, as in the case of benzidine and tolidine. Various shades of red and purple are thus obtained from the diazotised compound, when the latter is combined with the naphthylamines, naphthols, or their sulpho-acids. These, again, are all cotton dyes. The nitro-derivatives of the ethers of phenol and cresol, when reduced in the same way that nitrobenzene and nitrotoluene are reduced to azobenzene and azotoluene, also furnish azo-compounds which, on further reduction, give bases analogous to benzidine and tolidine. Secondary azo-colours derived from these bases and the usual naphthalene derivatives are also manufactured. It is among the secondary azo-dyes that we meet with the first direct dyeing blacks, the importance of which will be realized when it is remembered that the ordinary aniline-black is not adapted for wool dyeing. The azo-blacks are obtained by combining diazotised sulpho-acids of amidoazo-compounds of the benzene or naphthalene series with naphthol sulpho-acids or other naphthalene derivatives.
One other series of azo-compounds must be briefly referred to. It has long been known that aniline and toluidine when heated with sulphur evolve sulphuretted hydrogen and give rise to thio-bases, that is, aniline or toluidine in which the hydrogen is partly replaced by sulphur. One of the toluidines treated in this way is transformed into a thiotoluidine which, when diazotised and combined with one of the disulpho-acids of beta-naphthol, forms a red azo-dye, introduced by Dahl & Co. in 1885 as "thiorubin." By modifying the conditions of reaction between the sulphur and the base, it was found in 1887 by Arthur Green, that a complicated thio-derivative of toluidine could be produced which possessed very remarkable properties. The sulpho-acid of the thio-base is a yellow dye, which was named by its discoverer "primuline." Not only is primuline a dye, but it contains an amido-group which can be diazotised. If therefore the fabric dyed with primuline is passed through a nitrite bath, a diazo-salt is formed in the fibre, and on immersing the latter in a second bath containing naphthol or other phenol or an amine, an azo-dye is precipitated in the fibre. By this means there are produced valuable "ingrain colours" of various shades of red, orange, purple, &c.
So much for the azo-dyes, one of the most prolific fields of industrial enterprise connected with coal-tar technology. From the introduction of aniline yellow in 1863 to the present time, about 150 distinct compounds of this group have been given to the tinctorial industry. Of these over thirty are cotton dyes containing two azo-groups. Sombre shades, rivalling logwood black, bright yellows, orange-reds, browns, violets, and brilliant scarlets equalling cochineal, have been evolved from the refuse of the gas-works. The artificial colouring-matters have in this last case once again threatened a natural product, and with greater success than the indigo synthesis, for the introduction of the azo-scarlets has caused a marked decline in the cochineal culture.
In addition to the azo-colours, there are certain other products which claim naphthalene as a raw material. In 1879 it was found that one of the sulpho-acids of beta-naphthol when treated with nitrous acid readily gave a nitroso-sulpho-acid. A salt of this last acid, containing sodium and iron as metallic bases, was introduced in 1884, under the name of "naphthol green." It is used both as a dye for wool and as a pigment. It may be mentioned here that other nitroso-derivatives of phenols, such as those of resorcinol and the naphthols, under the name of "gambines," are largely used for dyeing purposes, owing to the facility with which they combine with metallic mordants to form coloured salts in the fibre. In this same year, 1879, it was found that by heating nitrosodimethylaniline with beta-naphthol in an appropriate solvent, a violet colouring-matter was formed. This is now manufactured under the name of "new blue," or other designations, and is largely used for producing an indigo-blue shade on cotton prepared with a suitable mordant. The discovery of this colouring-matter gave an impetus to further discoveries in the same direction. It was found that nitrosodimethylaniline reacted in a similar way with other phenolic or with amidic compounds. In 1881 Koechlin introduced an analogous dye-stuff prepared by the action of the same nitroso-compound on gallic acid. Gallocyanin, as it is called, imparts a violet blue shade to mordanted cotton. Other colouring-matters of the same group are in use; some of them, like "new blue," being derivatives of naphthalene. These compounds all belong to a series of which the parent substance is constructed on a type similar to azine; it contains a nitrogen and oxygen atom linking together the hydrocarbon residues, and is therefore known as "oxazine." The researches of Nietzki in 1888 first established the true constitution of the oxazines.
Closely related to this group is a colouring-matter introduced by Koechlin and Witt in 1881 under the name of "indophenol." It is prepared in the same way as the azines of the "neutral red" group; viz. by the action of nitrosodimethylaniline on alpha-naphthol, or by oxidizing amidodimethylaniline in the presence of alpha-naphthol. Indophenol belongs to that group of blue compounds formed as intermediate products in the manufacture of azines, as mentioned in connection with "neutral red." But while these intermediate blues resulting from the oxidation of a diamine in the presence of another amine are unstable, and pass readily into red azines, indophenol is stable, and can be used for dyeing and printing in the same way as indigo. The shades which it produces are very similar to this last dye, but for certain practical reasons it has not been able to compete with the natural dye-stuff.
The story of naphthalene is summarized in the schemes on pp. 164, 165.
Light Oil:
Benzene->Disulpho-acid-->Resorcinol.[6] \ \-->Nitrobenzene} } Sulphanilic acid.[7] } Aniline} Amidoazobenzene and sulpho-acid.[7] } } Nitrosodimethylaniline[8] and } } amidodimethylaniline.[8] } Sulpho-acid-->Amidosulpho-acid-->Amidophenol } and ethers.[6] } Azobenzene-->Benzidine.[7]
}Toluidines[7] Sulpho-acids.[7] } Amidoazotoluene[7] } and sulpho-acid.[7] } Thiotoluidine and primuline.[7] Toluene-->Nitrotoluenes} Azotoluene-->Tolidine.[7] } Sulpho-acid and Stilbene-derivative.[7]
} Sulpho-acids.[7] Xylene->Nitroxylenes->Xylidines[7]} Amidoazoxylene[7] and sulpho-acid.[7] } Cumidine.[7]
Carbolic Oil:
Phenol-->Nitrophenol and ethers } Azophenol ethers-->Diamidic bases[9] } analogous to benzidine. } Amidophenol[9] and ethers.[9]
Cresols-->Nitrocresols and ethers-->Amidocresols[9] and ethers.[9]
|>Nitronaphthalene | | | |>[Greek: a]-Naphthylamine[9][10]{Naphthionic acid.[9][10] | {Amidoazonaphthalene | | | {Magdala red. | {Sulpho-acid.[9] | Naphthalene>|>Tetrachloride-->Phthalic acid and anhydride. | | } {Manchester yellow. | }[Greek: a]-Naphthol {Acid naphthol yellow. | } {Sulpho-acids.[10] |>Sulpho-acids} | } {[Greek: b]-Naphthylamine | } { and sulpho-acids.[9][10] | }[Greek: b]-Naphthol{Sulpho-acids & | } { [Greek: b]-naphthylamine | } { sulpho-acids.[9][10] | } {Nitrososulpho-acid and { naphthol green.
The fraction of coal-tar succeeding the carbolic oil, viz. the creosote oil, does not at present supply the colour manufacturer with any raw materials beyond the small proportion of naphthalene which separates from it in a very impure condition as "creosote salts." This oil consists of a mixture of the higher homologues of phenol with various hydrocarbons and basic compounds. It is the oil used for creosoting timber in the manner already described; and among its other applications may be mentioned its use as an illuminating agent and as a source of lampblack. In order to burn the oil effectively as a source of light, a specially-constructed burner is used, which is fed by a stream of oil raised from a reservoir at its foot by means of compressed air, which also aids the combustion of the oil. There is produced by this means a great body of lurid flame, which is very serviceable where building or other operations have to be carried on at night (see Fig. 10). For lampblack the oil is simply burnt in iron pans set in ovens, and the sooty smoke conducted into condensing chambers. The creosote oil constitutes more than 30 per cent. by weight of the tar--the time may come when this fraction, like the light oil and carbolic oil, may be found to contain compounds of value to the colour-maker or to other branches of chemical manufacture.
The utilization of the next fraction, anthracene oil, is one of the greatest triumphs which applied chemical science can lay claim to since the foundation of the coal-tar colour industry. This discovery dates from 1868, when it was shown by two German chemists, Graebe and Liebermann, that the colouring-matter of madder was derived from the hydrocarbon anthracene. Like indigo, madder may be regarded as one of the most ancient of natural dye-stuffs. It consists of the powdered roots of certain plants of the genus _Rubia_, such as _R. tinctoria_ (see Fig. 11), _R. peregrina_, and _R. munjista_, which were at one time cultivated on an enormous scale in various parts of Europe and Asia. It is estimated that at the time of Graebe and Liebermann's discovery, 70,000 tons of madder were produced annually in the madder-growing countries of the world. At that time we were importing madder into this country at the rate of 15,000 to 16,000 tons per annum, at a cost of L50 per ton. In ten years the importation had fallen to about 1600 tons, and the price to L18 per ton. At the present time the cultivation of madder is practically extinct.
There is no better gauge of the practical utility of a scientific discovery than the financial effect. In addition to madder, a more concentrated extract containing the colouring-matter itself was largely used by dyers and cotton printers under the name of "garancin." In 1868 we were importing, in addition to the 15,000 to 16,000 tons of madder, about 2000 tons of this extract annually, at a cost of L150 per ton. By 1878 the importation of garancin had sunk to about 140 tons, and the price had been lowered to L65 per ton. The total value of the imports of madder and garancin in 1868 was over one million pounds sterling; in ten years the value of these same imports had been reduced to about L38,000. Concurrently with this falling off in the demand for the natural colouring-matter, the cultivation of the madder plant had to be abandoned, and the vast tracts of land devoted to this purpose became available for other crops. A change amounting to a revolution was produced in an agricultural industry by a discovery in chemistry.
In the persons of two Frenchmen, Messrs. Robiquet and Colin, science laid hands on the colouring-matter of the _Rubia_ in 1826. These chemists isolated two compounds which they named alizarin and purpurin. It is now known that there are at least six distinct colouring-matters in the madder root, all of these being anthracene derivatives. It is known also that the colouring-matters do not exist in the free state in the plant, but in the form of compounds known as glucosides, _i.e._ compounds consisting of the colouring-matter combined with the sugar known as glucose. It may be mentioned incidentally that the colouring-matter of the indigo plant also exists as a glucoside in the plant. During a period of more than forty years from the date of its isolation, alizarin was from time to time submitted to examination by chemists, but its composition was not completely established till 1868, when Graebe and Liebermann, by heating it with zinc-dust, obtained anthracene. This was the discovery which gave the death-blow to the madder culture, and converted the last fraction of the tar-oil from a waste product into a material of the greatest value.
The large quantity of madder consumed for tinctorial purposes is indicative of the value of this dye-stuff. It produces shades of red, purple, violet, black, or deep brown, according to the mordant with which the fabric is impregnated. The colours obtained by the use of madder are among the fastest of dyes, the brilliant "Turkey red" being one of the most familiar shades. The discovery of the parent hydrocarbon of this colouring-matter which had been in use for so many ages--a colouring-matter capable of furnishing both in dyeing and printing many distinct shades, all possessed of great fastness--was obviously a step towards the realization of an industrial triumph, viz. the chemical synthesis of alizarin. Within a year of their original observation, this had been accomplished by Graebe and Liebermann, and almost simultaneously by W. H. Perkin in this country. From that time the anthracene, which had previously been burnt or used as lubricating grease, rose in value to an extraordinary extent. In two years a material which could have been bought for a few shillings the ton, rose at the touch of chemical magic to more than two hundred times its former value.
Anthracene is a white crystalline hydrocarbon, having a bluish fluorescence, melting at 213 deg. C. and boiling above 360 deg. C. It was discovered in coal-tar by Dumas and Laurent in 1832, and its composition was determined by Fritzsche in 1857. It separates in the form of crystals from the anthracene oil on cooling, and is removed by filtration. The adhering oil is got rid of by submitting the crystals to great pressure in hydraulic presses. Further purification is effected by powdering the crude anthracene cake and washing with solvent naphtha, _i.e._ the mixture of the higher homologues of benzene left after the rectification of the light oil. Another coal-tar product, viz. the pyridine base referred to in the last chapter, has been recently employed for washing anthracene with great success. It is used either by itself or mixed with the solvent naphtha. The anthracene by washing with these solvents is freed from more soluble impurities, and may then contain from 30 to 80 per cent. of the pure hydrocarbon. The washing liquid, which is recovered by distillation, contains, among other impurities dissolved out of the crude anthracene, a hydrocarbon isomeric with the latter, and known as phenanthrene, for which there is at present but little use, but which may one day be turned to good account. The actual amount of anthracene contained in coal-tar corresponds to about 1/2 lb. per ton of coal distilled, _i.e._ from 1/4 to 1/2 per cent. by weight of the tar. Owing to the great value of alizarin and the large quantity of this colouring-matter annually consumed, anthracene is now by far the most important of the coal-tar hydrocarbons.
Alizarin, purpurin, and the other colouring-matters of madder are hydroxyl derivatives of a compound derived from anthracene by the replacement of two atoms of hydrogen by two atoms of oxygen. These oxygen derivatives of benzenoid hydrocarbons form a special group of compounds known as quinones. Thus there is quinone itself, or benzoquinone, which is benzene with two atoms of oxygen replacing two atoms of hydrogen. There are also isomeric quinones of the naphthalene series known as naphthaquinones. A dihydroxyl derivative of one of the latter is in use under the somewhat misappropriate name of "alizarin black." With this exception no other quinone derivative is used in the colour industry till we come to the hydrocarbons of the anthracene oil. Phenanthrene forms a quinone which has been utilized as a source of colouring-matters, but these are comparatively unimportant. The quinone with which we are at present concerned is anthraquinone.
The latter is prepared by oxidizing the anthracene--previously reduced by sublimation to the condition of a very finely-divided crystalline powder--with sulphuric acid and potassium dichromate. The quinone is purified, converted into a sulpho-acid, and the sodium salt of the latter on fusion with alkali gives alizarin, which is dihydroxy-anthraquinone. It is of interest to note that in this case a monosulpho-acid gives a dihydroxy-derivative. During the process of fusion potassium chlorate is added, by which means the yield of alizarin is considerably increased. In the original process of Graebe and Liebermann, dibromanthraquinone was fused with alkali; but this method was soon improved upon by the discovery of the sulpho-acid by Caro and Perkin in 1869, and from this period the manufacture of artificial alizarin became commercially successful.
In addition to alizarin, other anthracene derivatives are of industrial importance. The purpurin, discovered among the colouring-matters of madder in 1826, is a trihydroxy-anthraquinone; it can be prepared by the oxidation of alizarin, as shown by De Lalande in 1874. Isomeric compounds known as "flavopurpurin" and "anthrapurpurin" are also made from the disulpho-acids of anthraquinone by fusion with alkali and potassium chlorate. These two disulpho-acids are obtained simultaneously with the monosulpho-acid by the action of fuming sulphuric acid on the quinone, and are separated by the fractional crystallization of their sodium salts from the monosulpho-acid (which gives alizarin) and from each other. The purpurins give somewhat yellower shades than alizarin. Another trihydroxy-anthraquinone, although not obtained directly from anthracene, must be claimed as a tar-product. It is prepared by heating gallic acid with benzoic and sulphuric acids, or with phthalic anhydride and zinc chloride, and is a brown dye known as "anthragallol" or "anthracene-brown." The anthracene derivative is in this process built up synthetically. A sulpho-acid of alizarin has been introduced for wool dyeing under the name of alizarin carmine, and a nitro-alizarin under the name of alizarin orange. The latter on heating with glycerin and sulphuric acid is transformed into a remarkably fast colouring-matter known as alizarin blue, which is used for dyeing and printing. By heating alizarin blue with strong sulphuric acid, it is converted into alizarin green.
The great industry arising out of the laboratory work of two German chemists has influenced other branches of chemical manufacture, and has reacted upon the coal-tar colour industry itself. A new application for caustic soda and potassium chlorate necessitated an increased production of these materials. The first demand for fuming sulphuric acid on a large scale was created by the alizarin manufacture in 1873, when it was found that an acid of this strength gave better results in the preparation of sulpho-acids from anthraquinone. The introduction of this acid into commerce no doubt exerted a marked influence on the production of other valuable sulpho-acids, such as acid magenta in 1877, acid yellow in 1878, and acid naphthol yellow in 1879. The introduction of artificial alizarin has also simplified the art of colour printing on cotton fabrics to such an extent that other colouring-matters, also derived from coal-tar, are largely used in combination with the alizarin to produce parti-coloured designs. The manufacture of one coal-tar colouring-matter has thus assisted in the consumption of others.
Artificial alizarin is used in the form of a paste, which consists of the colouring-matter precipitated from its alkaline solution by acid, and mixed with water so as to form a mixture containing from 10 to 20 per cent. of alizarin. The magnitude of the industry will be gathered from the estimate that the whole quantity of anthracene annually made into alizarin corresponds to a daily production of about 65 tons of 10 per cent. paste, of which only about one-eighth is made in this country, the remainder being manufactured on the Continent. The total production of alizarin corresponds in money value to about L2,000,000 per annum. One pound of dry alizarin has the tinctorial power of 90 pounds of madder. Seeing therefore that the raw material anthracene was at one time a waste product, and that the quantity of alizarin produced in the factory corresponds to nearly five pounds of 20 per cent. paste for one pound of anthracene, it is not surprising that the artificial has been enabled to compete successfully with the natural product.
The industrial history of anthracene is thus summarized. (See opposite.)
Anthracene | Anthraquinone | | } Sulpho-acid (Alizarin carmine) |->Monosulpho-acid-->Alizarin----------} Purpurin | } Nitro-alizarin-->Alizarin blue |->[Greek: a]-Disulpho-acid-->Flavopurpurin | | | |->[Greek: b]-Disulpho-acid-->Anthrapurpurin | Alizarin green.
The black, viscid residue left in the tar-still after the removal of the anthracene oil is the substance known familiarly as pitch. From the latter, after removal of all the volatile constituents, there is prepared asphalte, which is a solution of the pitchy residue in the heavy tar-oils from which all the materials used in the colour industry have been removed. Asphalte is used for varnish-making, in the construction of hard pavements, and for other purposes. A considerable quantity of pitch is used in an industry which originated in France in 1832, and which is still carried out on a large scale in that country, and to a smaller extent in this and other tar-producing countries. The industry in question is the manufacture of fuel from coal-dust by moulding the latter in suitable machines with pitch so as to form the cakes known as "briquettes" or "patent fuel." By this means two waste materials are disposed of in a useful way--the pitch and the finely-divided coal, which could not conveniently be used as fuel by itself. From two to three million tons of this artificial fuel are being made annually here and on the Continent.
The various constituents of coal-tar have now been made to tell their story, so far as relates to the colouring-matters which they furnish. If the descriptive details are devoid of romance to the general reader, the results achieved in the short period of thirty-five years, dating from the discovery of mauve by Perkin, will assuredly be regarded as falling but little short of the marvellous. Although the most striking developments are naturally connected with the colouring-matters, whose history has been sketched in the foregoing pages, and whose introduction has revolutionized the whole art of dyeing, there are other directions in which the coal-tar industry has in recent times been undergoing extension. Certain tar-products are now rendering good service in pharmacy. Salicylic acid and its salts have long been used in medicine. By distilling a mixture of the dry lime salts of benzoic and acetic acids there is obtained a compound known to chemists as acetophenone, which is used for inducing sleep under the name of hypnone. The acetyl-derivative of aniline and of methylaniline are febrifuges known as "antifebrine" and "exalgine." Ethers of salicylic acid and its homologues, prepared from these acids and phenol, naphthols, &c., are in use as antiseptics under the general designation of "salols."
In 1881 there was introduced into medicine the first of a group of antipyretics derived from coal-tar bases of the pyridine series. It has already been explained that this base is removed from the light oil by washing with acid. Chemically considered, it is benzene containing one atom of nitrogen in place of a group consisting of an atom of carbon and an atom of hydrogen. The quantity of pyridine present in coal-tar is very small, and no use has as yet been found for it excepting as a solvent for washing anthracene or for rendering the alcohol used for manufacturing purposes undrinkable, as is done in this country by mixing in crude wood-spirit so as to form methylated spirit. The salts of pyridine were shown by McKendrick and Dewar to act as febrifuges in 1881, but they have not hitherto found their way into pharmacy. The chief interest of the base for us centres in the fact that it is the type of a group of bases related to each other in the same way as the coal-tar hydrocarbons. Thus in coal-tar, in addition to pyridine, there is another base known as quinoline, which is related to pyridine in the same way that naphthalene is related to benzene. Similarly there is a coal-tar base known as acridine, which is found associated with the anthracene, and which is related to quinoline in the same way that anthracene is related to naphthalene. The three hydrocarbons are comparable with the three bases, which may be regarded as derived from them in the same manner that pyridine is derived from benzene--
Benzene ... ... ... Pyridine Naphthalene ... ... Quinoline Anthracene ... ... ... Acridine
Some of these bases and their homologues are found in the evil-smelling oil produced by the destructive distillation of bones (Dippel's oil, or bone oil), and the group is frequently spoken of as the pyridine group. The colouring-matter described as phosphine, obtained as a by-product in the manufacture of magenta (p. 94), is a derivative of acridine, and a yellow colouring-matter discovered by Rudolph in 1881, and obtained by heating the acetyl derivative of aniline with zinc chloride, is a derivative of a homologue of quinoline. This dye-stuff, known as "flavaniline," is no longer made; but it is interesting as having led to the discovery of the constitution of phosphine by O. Fischer and Koerner in 1884.
The antipyretic medicines which we have first to consider are derivatives of quinoline. This base was discovered in coal-tar by Runge in 1834, and was obtained by Gerhardt in 1842 by distilling cinchonine, one of the cinchona alkaloids, with alkali. Now it is of interest to note that the quinoline of coal-tar is of no more use to the technologist than the aniline; these bases are not contained in the tar in sufficient quantity to enable them to be separated and purified with economical advantage. If the colour industry had to depend upon this source of aniline only, its development would have been impossible. But as chemistry enabled the manufacturer to obtain aniline in quantity from benzene, so science has placed quinoline at his disposal. This important discovery was made in 1880 by the Dutch chemist Skraup, who found that by heating aniline with sulphuric acid and glycerin in the presence of nitrobenzene, quinoline is produced. The nitrobenzene acts only as an oxidizing agent; the amido-group of the aniline is converted into a group containing carbon, hydrogen, and nitrogen, _i.e._ the pyridine group. The discovery of Skraup's method formed the starting-point of a series of syntheses, which resulted in the formation of many products of technical value. In all these syntheses the fundamental change is the same, viz. the conversion of an amidic into a pyridine group. We may speak of the amido-group as being "pyridised" in such processes. Thus alizarin blue, which is formed by heating nitro-alizarin with glycerin and sulphuric acid, results from the pyridisation of the nitro-group. By an analogous method Doebner and v. Miller prepared a homologue of quinoline (quinaldine) in 1881, by the action of sulphuric acid and a certain modification of aldehyde known as paraldehyde on aniline.
Quinoline and its homologue quinaldine have been utilized as sources of colouring-matters. A green dye-stuff, known as quinoline green, was formerly made by the same method as that employed for producing the phosgene colours by Caro and Kern's process (p. 106). The phthalein of quinaldine was introduced by E. Jacobsen in 1882 under the name of quinoline yellow, a colouring-matter which forms a soluble sulpho-acid by the action of sulphuric acid.
To return to coal-tar pharmaceutical preparations. At the present time seven distinct derivatives of quinoline, all formed by pyridising the amido-group in aniline, amido-phenols, &c., are known in medicine under such names as kairine, kairoline, thalline, and thermifugine. The mode of preparation of these compounds cannot be entered into here, Kairine, the first of the artificial alkaloids, is a derivative of hydroxy-quinoline, which was discovered in 1881 by Otto Fischer. All these quinoline derivatives have the property of lowering the temperature of the body in certain kinds of fevers, and may therefore be considered as the first artificial products coming into competition with the natural alkaloid, quinine. There is reason for believing that the latter alkaloid, the most valuable of all febrifuges, is related to the quinoline bases, so that if its synthesis is accomplished--as may certainly be anticipated--we shall have to look to coal-tar as a source of the raw materials.
Another valuable artificial alkaloid, discovered in 1883 by Ludwig Knorr, claims aniline as a point of departure. When aniline and analogous bases are diazotised, and the diazo-salts reduced in the cold with a very gentle reducing agent, such as stannous chloride, there are formed certain basic compounds, containing one atom of nitrogen and one atom of hydrogen more than the original base. These bases were discovered in 1876 by Emil Fischer, and they are known as hydrazines, the particular compound thus obtained from aniline being phenylhydrazine. By the action of this base on a certain compound ether derived from acetic acid, which is known as aceto-acetic ether, there is formed a product termed "pyrazole," and this on methylation gives the alkaloid in question, which is now well known in pharmacy under the name of "antipyrine."
While dealing with this first industrial application of a hydrazine, it must be mentioned that the original process by which Fischer prepared these bases was improved upon by Victor Meyer and Lecco in 1883, who discovered the use of a cold solution of stannous chloride for reducing the diazo-chloride to the hydrazine. By this method the manufacture of phenylhydrazine and other hydrazines is effected on a large scale--all kinds of amido-compounds and their sulpho-acids can be diazotised and reduced to their hydrazines. Out of this discovery has arisen the manufacture of a new class of colouring-matters related to the azo-dyes. The hydrazines combine with quinones and analogous compounds with the elimination of water, the oxygen coming from the quinone, and the hydrogen from the hydrazine. The resulting products are coloured compounds very similar in properties to the azo-dyes, and one of these was introduced in 1885 by Ziegler, under the name of "tartrazine." The latter is obtained by the action of a sulpho-acid of phenylhydrazine on dioxytartaric acid, and is a yellow dye, which is of special interest on account of its extraordinary fastness towards light.
Another direction in which coal-tar products have been utilized is in the formation of certain aromatic compounds which occur in the vegetable kingdom. Thus the artificial production of bitter-almond oil from toluene has already been explained. By heating phenol with caustic alkali and chloroform, the aldehyde of salicylic acid, _i.e._ salicylic aldehyde, is formed, and this, on heating with dry sodium acetate and acetic anhydride, passes into _coumarin_, the fragrant crystalline substance which is contained in the Tonka bean and the sweet-scented woodruff. Furthermore, the familiar flavour and scent of the vanilla bean, which is due to a crystalline substance known as vanillin, can be obtained from coal-tar without the use of the plant. The researches of Tiemann and Haarman having shown that vanillin is a derivative of benzene containing the aldehyde group, one hydroxyl- and one methoxy-group, the synthesis of this compound soon followed (Ulrich, 1884). The starting-point in this synthesis is nitrobenzoic aldehyde, so that here again we begin with toluene as a raw material. A mixture of vanillin and benzoic aldehyde when attenuated to a state of extreme dilution in a spirituous solvent, gives the perfume known as "heliotrope."
Not the least romantic chapter of coal-tar chemistry is this production of fragrant perfumes from the evil-smelling tar. Be it remembered that these products--which Nature elaborates by obscure physiological processes in the living plant--are no more contained in the tar than are the hundreds of colouring-matters which have been prepared from this same source. It is by chemical skill that these compounds have been built up from their elemental groups; and the artificial products, as in the case of indigo and alizarin, are chemically identical with those obtained from the plant. Among the late achievements in the synthesis of vegetable products from coal-tar compounds is that of juglone, a crystalline substance found in walnut-shell. It was shown by Bernthsen in 1884 that this compound was hydroxy-naphthaquinone, and in 1887 its synthesis from naphthalene was accomplished by this same chemist in conjunction with Dr. Semper.
Another recent development in the present branch of chemistry brings a coal-tar product into competition with sugar. In 1879 Dr. Fahlberg discovered a certain derivative of toluene which possessed an intensely sweet taste. By 1884 the manufacture of this product had been improved to a sufficient extent to enable it to be introduced into commerce as a flavouring material in cases where sweetness is wanted without the use of sugar, such as in the food of diabetic patients. Under the name of "saccharin," Fahlberg thus gave to commerce a substance having more than three hundred times the sweetening power of cane-sugar--a substance not only possessed of an intense taste, but not acted upon by ferments, and possessing distinctly antiseptic properties. The future of coal-tar saccharin has yet to be developed; but its advantages are so numerous that it cannot fail to become sooner or later one of the most important of coal-tar products. In cases where sweetening is required without the possibility of the subsequent formation of alcohol by fermentation, saccharin has been used with great success, especially in the manufacture of aerated waters. Its value in medicine has been recognized by its recent admission into the Pharmacopoeia.
The remarkable achievements of modern chemistry in connection with coal-tar products do not end with the formation of colouring-matters, medicines, and perfumes. The introduction of the beautiful dyes has had an influence in other directions, and has led to results quite unsuspected until the restless spirit of investigation opened out new fields for their application. A few of these secondary uses are sufficiently important to be chronicled here. In sanitary engineering, for example, the intense colouring power of fluorescein is frequently made use of to test the soundness of drains, or to find out whether a well receives drainage from insanitary sources. In photography also coal-tar colouring-matters are playing an important part by virtue of a certain property which some of these compounds possess.
The ordinary photographic plate is, as is well known, much more sensitive to blue and violet than to yellow or red, so that in photographing coloured objects the picture gives a false impression of colour intensity, the violets and blues impressing themselves too strongly, and the yellows and reds too feebly. It was discovered by Dr. H. W. Vogel in 1873 that if the sensitive film is slightly tinted with certain colouring-matters, the sensitiveness for yellow and red can be much increased, so that the picture is a more natural representation of the object. Plates thus dyed are said to be "isochromatic" or "orthochromatic," and by their use paintings or other coloured objects can be photographed with much better results than by the use of ordinary plates. The boon thus conferred upon photographic art is therefore to be attributed to coal-tar chemistry. Among the numerous colouring-matters which have been experimented with, the most effective special sensitizers are erythrosin, one of the phthaleins, quinoline red, a compound related to the same group, and cyanin, a fugitive blue colouring-matter obtained from quinoline in 1860 by Greville Williams.
In yet another way has photography become indebted to the tar chemist. Two important developers now in common use are coal-tar products, viz. hydroquinone and eikonogen. The history of these compounds is worthy of narration as showing how a product when once given by chemistry to the world may become applicable in quite unexpected directions. Chloroform is a case in point. This compound was discovered by Liebig in 1831, but its use as an anaesthetic did not come about till seventeen years after its discovery. It was Sir James Simpson who in 1848 first showed the value of chloroform in surgical operations. A similar story can be told with respect to these photographic developers.
Towards the middle of the last century a French chemist, the Count de la Garaye, noticed a crystalline substance deposited from the extract of Peruvian bark, then, as now, used in medicine. This substance was the lime salt of an acid to which Vauquelin in 1806 gave the name of quinic acid (_acide quinique_). In 1838 Woskresensky, by oxidizing quinic acid with sulphuric acid and oxide of manganese, obtained a crystalline substance which he called quinoyl. The name was changed to quinone by Woehler, and, as we have already seen (p. 172), the term has now become generic, indicating a group of similarly constituted oxygen derivatives of hydrocarbons. Hydroquinone was obtained by Caventou and Pelletier by heating quinic acid, but these chemists did not recognize its true nature. It was the illustrious Woehler who in 1844 first prepared the compound in a state of purity, and established its relationship to quinone. This relationship, as the name given by Woehler indicates, is that of the nature of a hydrogenised quinone. The compound is readily prepared by the action of sulphurous acid or any other reducing agent on the quinone.
It has long been known in photography, that a developer must be of the nature of a reducing agent, either inorganic or organic, and many hydroxylic and amidic derivatives of hydrocarbons come under this category. Thus, pyrogallol, which has already been referred to as a trihydroxybenzene (p. 146), when dissolved in alkali rapidly absorbs oxygen--it is a strong reducing agent, and is thus of value as a developer. But although pyrogallol is a benzene derivative, and could if necessary be prepared synthetically, it can hardly be claimed as a tar product, as it is generally made from gallic acid. Now hydroquinone when dissolved in alkali also acts as a reducing agent, and in this we have the first application of a true coal-tar product as a photographic developer. Its use for this purpose was suggested by Captain Abney in 1880, and it was found to possess certain advantages which caused it to become generally adopted.
As soon as a practical use is found for a chemical product its manufacture follows as a matter of course. In the case of hydroquinone, the original source, quinic acid, was obviously out of question, for economical reasons. In 1877, however, Nietzki worked out a very good process for the preparation of quinone from aniline by oxidation with sulphuric acid and bichromate of soda in the cold. This placed the production of quinone on a manufacturing basis, so that when a demand for hydroquinone sprung up, the wants of the photographer were met by the technologist. Eikonogen is another organic reducing agent, discovered by the writer in 1880, and introduced as a developer by Dr. Andresen in 1889. It is an amido-derivative of a sulpho-acid of beta-naphthol, so that naphthalene is the generating hydrocarbon of this substance.
The thio-derivative of toluidine described as "primuline" (p. 160), has recently been found by its discoverer to possess a most remarkable property which enables this compound to be used for the photographic reproduction of designs in azo-colours. Diazotised primuline, as already explained, combines in the usual way with amines and phenols to form azo-dyes. Under the influence of light, however, the diazotised primuline is decomposed with the loss of nitrogen, and the formation of a product which does not possess the properties of a diazo-compound. The product of photochemical decomposition no longer forms azo-colours with amines or phenols. If, therefore, a fabric is dyed with primuline, then diazotised by immersion in a nitrite bath, and exposed under a photographic negative, those portions of the surface to which the light penetrates lose the power of giving a colour with amines or phenols. The design can thus be developed by dipping the fabric into a solution of naphthol, naphthylamine, &c. By this discovery another point of contact has been established between photography and coal-tar products. Nor is this the only instance of its kind, for it has also been observed that a diazo-sulpho-acid of one of the xylenes does not combine with phenols to form azo-dyes excepting under the influence of light. A fabric can therefore be impregnated with the mixture of diazo-sulpho-acid and naphthol, and exposed under a design, when the azo-colour is developed only on those portions of the surface which are acted upon by light.
The last indirect application of coal-tar colouring-matters to which attention must be called is one of great importance in biology. The use of these dyes as stains for sections of animal and vegetable tissue has long been familiar to microscopists. Owing to the different affinities of the various components of the tissue for the different colouring-matters, these components are capable of being differentiated and distinguished by microscopical analysis. Furthermore, the almost invisible organisms which in recent times have been shown to play such an important part in diseases, have in many cases a special affinity for particular colouring-matters, and their presence has been revealed by this means. The micro-organism of tubercle, for example, was in this way found by Koch to be readily stained by methylene blue, and its detection was thus rendered possible with certainty. Many of the dyes referred to in the previous pages have rendered service in a similar way. To the pure utilitarian such an application of coal-tar products will no doubt compensate for any defects which they may be supposed to possess from the aesthetic point of view.[11]
From a small beginning there has thus developed in a period of five-and-thirty years an enormous industry, the actual value of which at the present time it is very difficult to estimate. We shall not be far out if we put down the value of the coal-tar colouring-matters produced annually in this country and on the Continent at L5,000,000 sterling. The products which half a century or so ago were made in the laboratory with great difficulty, and only in very small quantities, are now turned out by the hundredweight and the ton.[12] To achieve these results the most profound chemical knowledge has been combined with the highest technological skill. The outcome has been to place at the service of man, from the waste products of the gas-manufacturer, a series of colouring-matters which can compete with the natural dyes, and which in many cases have displaced the latter. From this source we have also been provided with explosives such as picric acid; with perfumes and flavouring materials like bitter-almond oil and vanillin; with a sweetening principle like saccharin--compared with which the product of the sugar-cane is but feeble; with dyes which tint the photographic film, and enable the most delicate gradations of shade to be reproduced; with developers such as hydroquinone and eikonogen; with disinfectants which contribute to the healthiness of our towns; with potent medicines which rival the natural alkaloids; and with stains which reveal the innermost structure of the tissues of living things, or which bring to light the hidden source of disease. Surely if ever a romance was woven out of prosaic material it has been this industrial development of modern chemistry.
But although the results are striking enough when thus summed up, and although the industrial importance of all this work will be conceded by those who have the welfare of the country in mind, the paths which the pioneers have had to beat out can unfortunately be followed but by the few. It is not given to our science to strike the public mind at once with the magnitude of its achievements, as is the case with the great works of the engineer. Nevertheless the scientific skill which enables a Forth Bridge to be constructed for the use of the travelling public of this age--marvellous as it may appear to the uninstructed--is equalled, if not surpassed, by the mastery of the intricate atomic groupings which has enabled the chemist to build up the colouring-matters of the madder and indigo plants.
A great industry needs no excuse for its existence provided that it supplies something of use to man, and finds employment for many hands. The coal-tar industry fulfils these conditions, as will be gathered from the foregoing pages. If any further justification is required from a more exalted standpoint than that of pure utilitarianism it can be supplied. It is well known to all who have traced the results of applying any scientific discovery to industrial purposes, that the practical application invariably reacts upon the pure science to the lasting benefit of both. In no department of applied science is this truth more forcibly illustrated than in the branch of technology of which I have here attempted to give a popular account. The pure theory of chemical structure--the guiding spirit of the modern science--has been advanced enormously by means of the materials supplied by and resulting from the coal-tar industry. The fundamental notion of the structure of the benzene molecule marks an epoch in the history of chemical theory of which the importance cannot be too highly estimated. This idea occurred, as by inspiration, to August Kekule of Bonn in the year 1865, and its introduction has been marked by a quarter century of activity in research such as the science of chemistry has never experienced at any previous period of its history. The theory of the atomic structure of the benzene molecule has been extended and applied to all analogous compounds, and it is in coal-tar that we have the most prolific source of the compounds of this class.
It was scarcely to be wondered at that an idea which has been so prolific as a stimulator of original investigation should have exerted a marked influence on the manufacture of tar-products. All the brilliant syntheses of colouring-matters effected of late years are living witnesses of the fertility of Kekule's conception. In the spring of 1890 there was held in Berlin a jubilee meeting commemorating the twenty-fifth anniversary of the benzene theory. At that meeting the representative of the German coal-tar colour industry publicly declared that the prosperity of Germany in this branch of manufacture was primarily due to this theoretical notion. But if the development of the industry has been thus advanced by the theory, it is no less true that the latter has been helped forward by the industry.
The verification of a chemical theory necessitates investigations for which supplies of the requisite materials must be forthcoming. Inasmuch as the very materials wanted were separated from coal-tar and purified on a large scale for manufacturing purposes, the science was not long kept waiting. The laborious series of operations which the chemist working on a laboratory scale had to go through in order to obtain raw materials, could be dispensed with when products which were at one time regarded as rare curiosities became available by the hundredweight. It is perhaps not too much to say that the advancement of chemical theory in the direction started by Kekule has been accelerated by a century owing to the circumstance that coal-tar products have become the property of the technologist. In other words, we might have had to wait till 1965 to reach our present state of knowledge concerning the theory of benzenoid compounds if the coal-tar industry had not been in existence. And this is not the only way in which the industry has helped the science, for in the course of manufacture many new compounds and many new chemical transformations have been incidentally discovered, which have thrown great light on chemical theory. From the higher standpoint of pure science, the industry has therefore deservedly won a most exalted position.
With respect to the value of the coal-tar dyes as tinctorial agents, there is a certain amount of misconception which it is desirable to remove. There is a widely-spread idea that these colours are fugitive--that they rub off, that they fade on exposure to light, that they wash out, and, in short, that they are in every way inferior to the old wood or vegetable dyes. These charges are unfounded. One of the best refutations is, that two of the oldest and fastest of natural colouring-matters, viz. alizarin and indigo, are coal-tar products. There are some coal-tar dyes which are not fast to light, and there are many vegetable dyes which are equally fugitive. If there are natural colouring-matters which are fast and which are aesthetically orthodox, these are rivalled by tar-products which fulfil the same conditions. Such dyes as aniline black, alizarin blue, anthracene brown, tartrazine, some of the azo-reds and naphthol green resist the influence of light as well as, if not better than, any natural colouring-matter. The artificial yellow dyes are as a whole faster than the natural yellows. There are at the present time some three hundred coal-tar colouring-matters made, and about one-tenth of that number of natural dyes are in use. Of the latter only ten--let us say 33 per cent.--are really fast. Of the artificial dyes, thirty are extremely fast, and thirty fast enough for all practical requirements, so that the fast natural colours have been largely outnumbered by the artificial ones. If Nature has been beaten, however, this has been rendered possible only by taking advantage of Nature's own resources--by studying the chemical properties of atoms, and giving scope to the play of the internal forces which they inherently possess--
"Yet Nature is made better by no mean, But Nature makes that mean: so, o'er that art, Which, you say, adds to Nature, is an art That Nature makes."
The story told in this chapter is chronologically summarized below--
1820. Naphthalene discovered in coal-tar by Garden.
1832. Anthracene discovered in coal-tar by Dumas and Laurent.
1834. Phenol discovered in coal-tar by Runge.
1842. Picric acid prepared from phenol by Laurent; manufactured in Manchester in 1862.
1845. Benzidine discovered by Zinin.
1859. Corallin and aurin discovered by Kolbe and Schmitt and by Persoz; leading to manufacture from oxalic acid and phenol.
1860. Synthesis of salicylic acid by Kolbe.
1864. Manchester yellow discovered by Martius, leading to manufacture of alpha-naphthylamine and then to alpha-naphthol.
1867. Magdala red discovered by Schiendl.
1868. Synthesis of alizarin by Graebe and Liebermann, leading to the utilization of anthracene, caustic soda, potassium chlorate and bichromate, and calling into existence the manufacture of fuming sulphuric acid.
1870. Gallein, the first of the phthaleins, discovered by A. v. Baeyer, followed in 1871 by coerulein, and in 1874 by the eosin dyes (Caro). These discoveries necessitated the manufacture of phthalic acid and resorcinol.
1873. Orthochromatic photography discovered by Vogel.
1876. Azo-dyes from the naphthols introduced by Roussin and Poirrier and Witt, leading to the manufacture of the naphthols, sulphanilic acid, &c.
1877. Preparation of quinone from aniline by Nietzki, utilized in photography in 1880 for manufacture of hydroquinone.
1878. Disulpho-acids of beta-naphthol introduced by Meister, Lucius, and Bruening, leading to azo-dyes from aniline, toluidine, xylidine, and cumidine.
1879. Acid naphthol yellow introduced by Caro.
" Biebrich scarlet, the first secondary azo-colour, introduced by Nietzki.
" Nitroso-sulpho acid of beta-naphthol discovered by the writer; followed in 1883 by naphthol green (O. Hoffmann), and in 1889 by eikonogen (Andresen).
" Beta-naphthol violet, the first of the oxazines, discovered by the writer; followed in 1881 by gallocyanin.
" Coal-tar saccharin discovered by Fahlberg; manufacture made practicable in 1884.
1880. Synthesis of indigo by A. v. Baeyer.
" Quinoline synthesised by Skraup's process.
1881. Kairine introduced by O. Fischer, the first artificial febrifuge.
" Indophenol discovered by Koechlin and Witt.
" Azo-dyes from new sulpho-acid of beta-naphthol introduced by Bayer & Co.
1883. Antipyrine introduced by L. Knorr, leading to manufacture of phenylhydrazine.
1884. Congo red, the first secondary azo-colour from benzidine, introduced by Boettiger. Beginning of manufacture of cotton azo-dyes, and leading to the production of benzidine and tolidine on a large scale.
1885. Secondary azo-dyes from benzidine and tolidine containing two dissimilar amines, phenols, &c., introduced by Pfaff.
" Tartrazine discovered by Ziegler; manufacture of sulpho-acid of phenylhydrazine and of dioxytartaric acid.
1885. Thiorubin introduced by Dahl & Co., leading to manufacture of thiotoluidine; followed by primuline, discovered by A. G. Green in 1887.
1886. Secondary azo-dyes of stilbene series introduced by Leonhardt & Co.
ADDENDUM.
By passing steam over red-hot carbon, a mixture of carbon monoxide and hydrogen is formed. This mixture of inflammable gases is known as "water-gas," and in the preparation of the gas on a large scale, coke is used as a source of carbon. If, therefore, water-gas became generally used, another use for coke would be added to those already referred to (p. 47).
With reference to the consumption of coal in London (p. 46), it appears from the Report of a Committee of the Corporation of London, issued at the end of 1890, that the present rate of consumption in the Metropolis is 9,709,000 tons per annum. This corresponds to 26,600 tons per diem. It has been proved by experiment, that when coal is burnt in an open grate, from one to three per cent. of the coal escapes in the form of unburnt solid particles, or "soot," and about 10 per cent. is lost in the form of volatile compounds of carbon. It has been estimated that the total amount of coal annually wasted by imperfect combustion in this country is 45,000,000 tons, corresponding to about L12,000,000, taking the value of coal at the pit's mouth. Taking the unconsumed solid particles at the very lowest estimate of 1 per cent., it will be seen that, in London alone, we are sending forth carbonaceous and tarry matter into the atmosphere at the rate of about 266 tons daily; and volatile carbon compounds at the daily rate of 2660 tons (see p. 32). At the price of coal in London this means that, in solid combustibles alone, we are absolutely squandering about L10,000 annually, to say nothing of the damage caused by the presence of this sooty pall. Such facts as these require no comment; they speak for themselves in sombre gloom, and in the sickliness of our town vegetation--they give a new meaning to the term "in darkest London," and they plead eloquently for science and legislation to show us "the way out."
INDEX.
Accum, condensation of tar-oils, 69
Acetic acid, 64
Acetophenone, 178
Acid brown, 151
Acid greens, 106
Acid magenta, 92
Acid naphthol yellow, 143
Acid yellow, 120
Acridine, 180
Air, composition of, 24
Albo-carbon light, 140
Alizarin black, 172
Alizarin blue, 174
Alizarin carmine, 174
Alizarin green, 174
Alizarin orange, 174
Alkali blue, 93
Amidodimethylaniline, 112
Ammonia in gas-liquor, 64
Ammonia, origin in gas-liquor, 65
Analyses of coal, 23
Aniline black, 114
Aniline, history, 75
Aniline, manufacture, 87
Aniline yellow, 116
Annual production of ammonia, 68
Anthracene, 171
Anthracene brown, 174
Anthracene oil, 81, 167
Anthragallol, 174
Anthrapurpurin, 174
Anthraquinone, 173
Antifebrine, 179
Antipyrine, 183
Archil substitute, 151
Arctic coal, 12
Arsenic acid process, 90
Arsenic acid, recovery, 94
Artificial alizarin, 173
Artificial purpurin, 173
Asphalte, 176
Auramine, 106
Aurin, 132
Azines, 109
Azo-blacks, 159
Azo-colours, 118
Azo-dyes for cotton, 158
Azobenzene, 119
Azo-dyes from salicylic acid, 135
Azotoluene, 119
Baeyer, A. v., indigo, 127
Baeyer, A. v., phthaleins, 146
Basle blue, 111
Becher, early experiments, 34
Beet-sugar cultivation, 67
Benzal chloride, 102
Benzaldehyde, 103
Benzene, discovery, 73
Benzene, final purification, 86
Benzene in tar-oil, 73
Benzene theory, 196
Benzidam, 75
Benzidine, 136
Benzoic acid, manufacture, 104
Benzotrichloride, 102
Benzyl chloride, 102
Bernthsen, methylene blue, 113
Bethell, timber preserving, 70
Biebrich scarlet, 156
Biology, dyes used in, 192
Bismarck brown, 116
Bitter-almond oil, 103
Bone oil, 180
Boettiger, Congo-red, 157
Breant, timber pickling, 70
Briquettes, 178
Burning naphtha, 86
Calorific value of carbon, 25
Calorific value of coal, 20
Carbolic acid, 129
Carbolic oil, 23, 80, 129
Carbon dioxide, 24
Carboniferous period, 11
Caro and Kern, phosgene dyes, 106
Caro and Wanklyn, rosolic acid, 133
Caro, fluorescein and eosin, 147
Caro, methylene blue, 112
Chemical washing, 83
Chlorophyll, 29
Chrysamines, 136
Chrysoidine, 116
Cinnamic acid, 128
Clayton, Dean, distils coal, 35
Clegg, Samuel, gas-engineer, 40
Coal, amount raised, 59
Coal-fields of United Kingdom, 60
Coal-gas, composition, 56
Coal-gas, manufacture, 42
Coal-mining, history, 57
Coal, origin, 9
Coal, supply, 59
Coerulein, 147
Coke, composition of, 48
Coke from gas-retorts, 45
Coke-oven tar, 49
Coke, uses of, 47
Combustion, 23
Composition of coal, 23
Congo red, 157
Conservation of energy, 18
Constitution of molecules, 95
Corallin, 132
Cotton dyes, 137
Coumarin, 185
Coupier's process, 91
Creosote oil, 163
Creosoting of timber, 70
Cresols, 131
Cresylic acid, 131
Cretaceous coal, 12
Crocein scarlets, 156
Crystal violet, 106
Cumidine, 155
Cyanin, 188
Dale and Caro, induline, 120
Destructive distillation, 33
Diazo-compounds, 116
Diazotype, 191
Dimethylaniline, 100
Dinitrobenzene, 120
Diphenylamine, 101
Diphenylamine blue, 101
Distillation, fractional, 78
Doebner, malachite green, 102
Dover, coal under, 60
Dumas and Laurent, anthracene, 171
Dundonald, Earl, early experiments, 39
Eikonogen, 189
Electricity as an illuminating agent, 62
Eocene coal, 12
Eosin, 147
Erythrosin, 188
Essence of mirbane, 74
Exalgine, 179
Fahlberg, saccharin, 186
Faraday, discovers benzene, 73
Fast red, 151
Fertilization by ammonia, 66
Fire-damp, 32
First runnings, 80
Fischer, E. and O., rosaniline, 97
Fischer, hydrazines, 183
Fischer, malachite green, 103
Flavaniline, 180
Flavopurpurin, 174
Fluorescein, 147
Foot-pound, 19
Fractional distillation, 78
Fritzsche, aniline, 75
Gallein, 146
Gallic acid, 146
Gallocyanin, 162
Gambines, 161
Garancin, 168
Garden, naphthalene, 139
Gas producers, 57
Gas, quantity obtained from coal, 45
Girard and De Laire, rosaniline blues, 92
Glucosides, 169
Goethe, visit to coke-burner, 48
Graebe and Liebermann, alizarin, 170
Graphite, 12
Graessler, acid yellow, 120
Green, A. G., primuline, 160
Griess, diazo-compounds, 116
Hales, Rev. Stephen, distils coal, 37
Hofmann, benzene in tar-oil, 73
Hofmann, red from aniline, 89
Hofmann's violets, 93
Homologous series, 152
Horse-power, 22
Hull, Prof., coal supply, 59
Hydrazines, 183
Hydrocarbons of benzene series, 82
Hydrogen, calorific value, 25
Hydroquinone, 189
Hypnone, 179
Indigo plants, 124
Indigo, syntheses, 126
Indophenol, 162
Indulines, 121
Ingrain colours, 160
Iodine green, 94
Iron smelting, 16
Iron swarf, 87
Isochromatic plates, 188
Isomerism, 88
Joule, mechanical equivalent of heat, 19
Juglone, 186
Jurassic coal, 12
Kairine, 182
Kekule, benzene theory, 196
Kekule and Hidegh, azo-dyes, 135
Koch, tubercle, 193
Koechlin, gallocyanin, 162
Koechlin and Witt, indophenol, 162
Kolbe, salicylic acid, 134
Kolbe and Schmitt, phenol dye, 132
Kyanol, 75
Lampblack, 139, 166
Laurent, phenol, 131
Laurent, phthalic acid, 143
Laurent, picric acid, 136
Lauth, methyl violet, 98
Lauth's violet, 111
Leonhardt & Co., stilbene dyes, 137
Lightfoot, aniline black, 114
Light oil, 80
Light oils, early uses, 71
Lister, antiseptic surgery, 131
London, coal introduced, 58
London, illuminated by gas, 40
Lucigen burner, 163
Madder, 168
Magdala red, 149
Magenta, history, 89
Malachite green, 102
Manchester brown, 116
Manchester yellow, 142
Mansfield, isolation of benzene, 73
Mansfield's still, 77
Manures, 66
Marsh gas, 32
Mauve, discovery, 74
Mechanical value of coal, 22
Medlock, magenta process, 90
Methyl chloride, 99
Methylene green, 113
Methyl green, 101
Methyl violet, 98
Mirbane, essence, 74
Murdoch, introduces coal-gas, 40
Naphthalene, annual production, 141
Naphthalene in carbolic oil, 130
Naphthionic acid, 151
Naphthol green, 161
Naphthol orange, 151
Naphthols, 141
Naphthylamines, 142
Natanson, aniline red, 89
Neutral red, 111
Neutral violet, 111
New blue, 161
Nicholson blue, 93
Nicholson, magenta process, 90
Nietzki, azines, 109
Nietzki, Biebrich scarlet, 156
Nietzki, quinone, 191
Night blue, 106
Nigrosine, 121
Nitrification, 66
Nitrobenzene process, 91
Nitrosodimethylaniline, 111
Non-Carboniferous coal, 11
Number of compounds in tar, 81
Old Red Sandstone, coal in, 12
Oligocene brown coal, 12
Orthochromatic plates, 188
Oxazines, 162
Oxide of iron for gas purifying, 44
Paraffin oil and wax, 50
Patent fuel, 178
Perfumes, 185
Perkin, alizarin, 170
Perkin, discovers mauve, 74
Permanence of dyes, 198
Permian coal, 12
Persoz, phenol dye, 132
Pharmaceutical preparations, 178
Phenanthrene, 172
Phenols, 129
Phosgene dyes, 106
Phosphine, 94, 180
Photographic developers, 189
Phthaleins, 146
Phthalic acid, 143
Picric acid, 136
Pitch, 81, 176
Plants, growth of, 26
Ponceaux, 151
Primary azo-dyes, 156
Primuline, 160
Propiolic acid, 128
Purpurin, 169
Pyrazole, 183
Pyridine, 87
Pyridine bases, 179
Pyrogallol, 146
Quinaldine, 182
Quinic acid, 189
Quinoline, 180
Quinoline green, 182
Quinoline red, 188
Quinoline yellow, 182
Quinones, 172
Read Holliday's lamp, 72
Rectification of hydrocarbons, 84
Resorcinol, 145
Rhodamines, 148
Roccellin, 151
Roman coal-mining, 57
Rosaniline blues, 92
Rosaniline from rosolic acid, 133
Rosolic acid, 132
Runge, kyanol, 75
Runge, phenol in tar, 131
Saccharin, 186
Saffranine, 108
Salicylic acid, 134
Salicylic aldehyde, 185
Salols, 179
Schiendl, Magdala red, 149
Scotland, shale-oil industry, 53
Sea coal, 58
Secondary azo-dyes, 156
Shale, nature of, 51
Shale-oil industry, 50
Skraup, quinoline, 181
Sodium nitrite, 119
Solar energy, 28
Soluble blue, 93
Solvent naphtha, 86
Steam-engines, 20
Stilbene azo-dyes, 137
Structure of coal, 15
Sulphanilic acid, 150
Sulphuric acid, 45
Sunlight, source of energy in coal, 30
Surgery, antiseptic, 131
Tar distilling, 77
Tar, first utilization, 69
Tar from coke-ovens, 49
Tar, quantity obtained from coal, 45
Tartrazine, 184
Tertiary coal, 12
Thalline, 182
Thermifugine, 182
Thiazines, 113
Thiodiphenylamine, 113
Thiorubin, 160
Timber pickling, 70
Tolidine, 136
Toluene from tar, 81
Torbane Hill coal, 52
Transformation of wood into coal, 31
Triassic coal, 12
Trimethylamine, 99
Trinitrophenol, 136
Triphenylmethane, 97
Tubercle bacillus, 193
Turkey red, 170
Underclay, 13
Unverdorben, crystallin, 75
Uses of coal, 16
Vanillin, 185
Vegetable deposits, recent, 13
Verguin, red from aniline, 90
Victoria blue, 106
Victoria yellow, 137
Vinasse, 99
Vincent, methyl chloride, 100
Violaniline, 121
Wasteful use of coal, 32, 203
Water, composition of, 25
Water gas, 203
Watson, Bishop, coke-oven tar, 50
Watson, Bishop, distils coal, 37
Wealden iron industry, 17
Whitaker, W., coal in S.-E. England, 60
Winsor, promotes introduction of gas, 41
Witt, azines, 109
Witt, chrysoidine, 116
Witt, naphthol orange, 150
Wood vinegar, 64
Woody fibre, 26
Woulfe, picric acid, 136
Xylenes from tar, 81
Xylidine scarlet, 153
Young, James, burning-oil, 51
Ziegler, tartrazine, 184
Zinin, benzidam, 75
THE END.
_Richard Clay & Sons, Limited, London & Bungay._
Footnotes:
[1] "An experiment concerning the Spirit of Coals," _Phil. Trans._ (abridged), vol. viii. p. 295.
[2] _Report of the Coal Commissioners_ (1866-71), vol. i.
[3] In a paper read before the Royal Statistical Society by Mr. Price-Williams in 1889, this author points out that, owing to the introduction of the Bessemer process and other economical improvements, the amount of coal used in the iron and steel manufacture had fallen in 1867 to about sixteen and a half per cent. of the total quantity raised.
[4] This remark does not apply to Great Britain; our Excise regulations have practically killed those branches of manufacture requiring the use of pure wood-spirit.
[5] Since the above was written, new synthetical processes for the production of indigo have been made known in Germany by Karl Heumann. Of the commercial aspect of these discoveries it is of course impossible at present to form an opinion.
[6] With phthalic anhydride gives fluorescein and dyes of eosin and rhodamin series.
[7] Diazotised and combined with naphthylamines, naphthols, or their sulpho-acids give azo-dyes.
[8] With naphthols give oxazines and indophenol.
[9] Diazotised and combined with phenols, amines, or sulpho-acids to form azo-dyes.
[10] Combined with diazotised amido-compounds to form azo-dyes.
[11] Since the above was written the continuation of Koch's researches upon the tubercle bacillus has culminated in the discovery of his now world-renowned lymph for the inoculation of patients suffering from tubercular disease.
[12] In one large factory in Yorkshire there is a set of stills kept constantly at work making pure aniline at the rate of two hundred tons per month. The monthly consumption of coal in this factory is two thousand tons, equal to twenty-four thousand tons per annum.
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