Chapter III (2)

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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