Coal Part 7

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 a.n.a.lysis. 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 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 quant.i.ties, 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 alkalods; 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 surpa.s.sed, 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 ill.u.s.trated 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 a.n.a.logous compounds, and it is in coal-tar that we have the most prolific source of the compounds of this cla.s.s.

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 circ.u.mstance 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, pota.s.sium chlorate and b.i.+.c.hromate, and calling into existence the manufacture of fuming sulphuric acid.

1870. Gallen, the first of the phthalens, discovered by A. v. Baeyer, followed in 1871 by coerulen, 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 Bruning, leading to azo-dyes from aniline, toluidine, xylidine, and c.u.midine.

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 Kochlin 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 Bottiger. 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 pa.s.sing 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, c.o.ke is used as a source of carbon. If, therefore, water-gas became generally used, another use for c.o.ke 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."

THE END.