Coal, and What We Get from It

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.

Fig. 11.—The Madder Plant
(Rubia tinctoria).

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 £50 per ton. In ten years the importation had fallen to about 1600 tons, and the price to £18 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 £150 per ton. By 1878 the importation of garancin had sunk to about 140 tons, and the price had been lowered to £65 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 £38,000. 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° C. and boiling above 360° 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 ½ lb. per ton of coal distilled, i.e. from ¼ to ½ 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 £2,000,000 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.)

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—

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 Körner 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 alkaloïds, 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 phthaleïn 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 alkaloïds, 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 alkaloïd, quinine. There is reason for believing that the latter alkaloïd, 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 alkaloïd, 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 alkaloïd 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 aërated waters. Its value in medicine has been recognized by its recent admission into the Pharmacopœia.

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 fluoresceïn 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 phthaleïns, 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 anæsthetic 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 Wöhler, 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 Wöhler 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 Wöhler 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 æsthetic point of view.[6]

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 £5,000,000 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.[7] 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 alkaloïds; 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 Kekulé 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 Kekulé’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 Kekulé 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 æsthetically 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—

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 £12,000,000, 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 £10,000 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.