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—