Anthracene was discovered in coal-tar by Laurent in 1832, and its properties were investigated by Anderson in 1862. It may be remarked that such investigations were not conducted with a view to any industrial uses of anthracene, but merely for the sake of chemistry as a science. Certainly no one could have supposed at that time that the slightest relation existed between anthracene and madder. Anthracene is a white solid hydro-carbon, which comes over only in the last stages of the distillation of coal-tar, accompanied by naphthaline, from which it is easily separated by means of spirits of wine, by which the naphthaline is readily dissolved, but the anthracene scarcely. Anderson, in 1861, discovered, among other results, that anthracene, C14H10, by treatment with nitric acid became changed into oxy-anthracene, C14H8O2; and this reaction we shall see is a step in the process of procuring alizarine from anthracene. Phenol, as already mentioned, can be made to yield benzol, by a process of deoxidization. With a view to similarly obtaining a hydro-carbon from alizarine, Graebe and Liebermann passed its vapours over heated zinc filings, and thus produced anthracene from alizarine. It now remained to find a means of reversing this process, that is, so to act on anthracene as to produce alizarine, and this was effected by treating anthracene with bromine, forming a substance which, on fusing with caustic potash, yielded alizarate of potash, from which pure alizarine resulted by treatment with hydrochloric acid. A much cheaper method was, however, necessary for manufacturing purposes, and it was found in a process by which oxy-anthracene, C14O8H2, is treated at a high temperature with strong sulphuric acid, and the product so formed heated with a strong solution of potash, yielding alizarate of potassium as before. Many other interesting substances appear to be formed in the reactions, but the nature of these bodies has as yet been imperfectly investigated. No doubt whatever can be entertained of the identity of natural with artificial alizarine; and the production of this substance, the first instance of a natural colouring matter made artificially, may be regarded as a great triumph of chemical science. It was not long ago supposed that the chemical bodies found in plants or animals, or produced by vital actions, could not possibly be formed by any artificial process from their elements. The laws which presided at their formation were, it was conceived, wholly different from those which governed the chemicals of the laboratory, for they were held to act exclusively under the influence of a mysterious agent, namely, “vital force.” It was supposed, for example, that from pure carbon, oxygen, and hydrogen, no chemist would ever be able to produce such a compound as acetic acid. Accordingly the domain of chemical science, previous to the end of the first quarter of the present century, was divided by an impassable barrier into the two regions of organic and inorganic chemistry. Now, however, the chemist is able to build up in his laboratory from their very elements a great number of the so-called organic bodies. And it is quite possible to do this in the case of alizarine; that is, a chemist having in his laboratory the elements, hydrogen, carbon, oxygen, &c., could actually build up the substance which gives its value to madder.
The quantity of anthracene procurable from coal-tar is, unfortunately, comparatively small, for it is found that from the distillation of 2,000 tons of coal only one ton of anthracene can be obtained. The use of artificial alizarine would doubtless entirely supplant the employment of madder-root if anthracene could be obtained in larger quantities; and the change would be highly advantageous to this country, for as no madder is grown in Great Britain, and we consume nearly half the whole annual growth, it follows that every year a million pounds sterling go out of the country for this commodity. When anthracene is produced from coal in sufficient abundance, this sum will be available for the support of our own population. In the meantime, the manufacture of artificial alizarine is restricted only by the supply of its raw material.
The foregoing paragraphs of the present article, which were written for the first edition of this work, not long after the introduction of artificial alizarine, require some supplementary reference to the subsequent progress of discovery and to the increased importance of the manufacture of the coal-tar colours on the large scale. Since the first introduction of alizarine as a commercial product, the substance has received much attention from chemists. The constitution of the body called above oxy-anthracene is now better understood, and its chemical relationship is more clearly indicated by the systematic name of anthraquinone, which it now bears. The process of the manufacture of alizarine has received some advantageous modifications, and the artificial product may now be said to have entirely displaced the madder-root in dyeing. But, more than this, chemists have found means of preparing a number of “derivatives” of alizarine, many of which are either colouring matters or are easily converted into such. Nearly thirty of these substances have been described, and several of them have found extensive industrial applications. We may mention alizarine blue, C17 H9 NO4, and another substance, produced by combining that with sodium bi-sulphite, and having the formula C17 H9 NO4 2Na H SO3. This last, manufactured largely, and sold under the name of “alizarine blue S.,” is remarkable for being one of the most permanent of all colouring matters. It is said to be a faster colour than even indigo blue, which, indeed, it is rapidly replacing in dyeing, where it is applicable to cotton with a chromium mordant and to silk with one of alumina. Two other colouring matters have also been derived from anthracene, and are much used in dyeing; one is commercially named anthracene purple, the other is anthracene green, which supplies the calico printer with very fast shades of olive-green.
Several of the substances enumerated in the list of coal-tar colours, in pages [689] and [690], are now but little used, or altogether abandoned in dyeing and calico printing, because either their beautiful hues prove too fugitive, or other bodies of the same class can be produced at a much cheaper rate. The range of choice is now of the amplest, for chemical discovery has been wonderfully active, but in many cases the real nature and relationship of the artificial colouring matters enumerated above have only quite recently been made out. Mauve (now called rosaline), for example, the oldest of all the colour-tar colours, and one which, as we have seen, was manufactured on an extensive scale many years ago, is now scarcely made at all, because much cheaper violets have taken its place. The science of the tinctorial substances has lately taken a much more distinct form, and this knowledge has borne fruit for industrial purposes. It would be out of place here to review what has been done in this way, but a few facts will show the richness of the field. It was only in 1886 that the true chemical constitution of a class of coal-tar derivatives, called azines, was first made out. They present themselves as pale yellow or orange coloured crystallized solids, which melt at a comparatively high temperature and may be distilled without decomposition. Although highly coloured substances themselves, before they are converted into fast dyes they require further treatment, which introduces into their molecules another group of atoms. An almost indefinite number of such compounds are theoretically possible, but from only a very few of them many useful dye stuffs are now prepared on the large scale. Amongst the most important of these are “neutral red,” “neutral violet,” and two other violet colouring matters, “red dyestuff,” “fuchsia,” “giroflé,” “Magdala red,” “indazine” and “Basle blue.”
Among the colouring matters before enumerated are “aniline yellow” and “Bismarck brown.” Their real nature was not understood until a few years ago; and though the use of the aniline yellow itself has been abandoned on account of its fugacity, the substance has been found a most prolific parent, which has supplied dye stuffs of the most diverse and brilliant hues. These form what chemists term the azo colours, and they have been manufactured in great variety and on a very large scale. In 1876, the class of them called chrysoidines was introduced, and again, in 1878, tropœolines. Great numbers of different azo colours have been sent into commerce under various names, such as “butter yellow,” “crocein scarlet,” “Biebrich scarlet,” “Congo red,” “Bordeaux G.,” “fast red,” &c., &c. About 140 of these azo dyes have been described, and the commercial importance of this one class of compounds alone may be inferred from the fact of no fewer than 200 patents having been taken out for processes relating to their manufacture in the eleven years from 1878 to 1888.
It would not be difficult to fill this book with instances of the way in which the resources of modern life have been increased by chemistry alone, a science almost entirely the creation of the present century. Many of the processes of manufacture in which chemistry is applied to the production of articles of every-day use have been so often described, that they may be assumed to be already so well known as to offer few elements of novelty to the general reader, whose interest would also be likely to flag if he were carried over a long range of even the brilliant discoveries that are so delightful and instructive for the special students of this science. There is no parallel to the rapidity of the progress made by the younger branch of the science which concerns itself with the chemistry of one element—namely, carbon and its various combinations, and it is from these carbon compounds that our examples have been drawn. In the explosives, we have some of these compounds supplying resistless forces for rending rocks, and furnishing in warfare the most dreadful powers of destruction. In anæsthetics, we see beneficent applications of others in alleviating suffering and annulling pain; and again we have just shown how richly another set of them can minister to our sense of beauty. The discussion of these topics has afforded an opportunity for bringing before the reader some of the laws or summarized statements of experimental facts, and also some of those symbolical conceptions of the constitution of compounds, which together furnish the clues that guide the chemist through the vast labyrinth of the endless transformations of matter. The results attained show that the notions expressed by such words as atom, molecule, compound radical, structural formula, etc., have a true representative correspondence with something in the actual constitution of bodies.
Fig. 357.—James Prescott Joule, F.R.S.
THE GREATEST DISCOVERY OF THE AGE.
The indulgent reader who may have followed the course of the foregoing pages, will perhaps peruse the title of this article with some little bewilderment. His attention has been drawn to one after another of a series of remarkable and important discoveries, and he will naturally wonder what can be the discovery which is greater than any of these. Now, a discovery is great in proportion to the extent and importance of the results that flow from it. These results may be immediate and practical, as in the case of vaccination; or they may be scientific and intellectual, as in Newton’s discovery of the identity of the force which draws a stone to the ground with that which holds the planets in their orbits. Such discoveries as most enlarge our knowledge of the world in which we live, by embracing in simple laws a vast field of phenomena, are precisely those which are most prolific in useful applications. If we admit, as we must, the truth of Bacon’s aphorism, which declares that “Man, as the minister and interpreter of nature, is limited in act and understanding by his observation of the order of nature; neither his understanding nor his power extends farther,”[[19]] then it would be easy to show that the discovery of which we have to treat, more than any other, must be of immense practical service to mankind in every one of the ways in which a knowledge of the order of nature can be of use, viz.:—“First, In showing in how to avoid attempting impossibilities. Second, In securing us from important mistakes in attempting what is, in itself, possible, by means either inadequate or actually opposed to the end in view. Third, In enabling us to accomplish our ends in the easiest, shortest, most economical, and most effectual manner. Fourth, In inducing us to attempt, and enabling us to accomplish, objects which, but for such knowledge, we should never have thought of undertaking.”[[20]]