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able meathen, by lettin khe bitter er be formed, il
ments, principally, it became known that cold water dissolves out of madder-root, besides other matters, the colouring matter alizarin, a yellow bitter substance now known by the name of rubian after that of the genus rubia, and a highly nitrogenised matter acting as a ferment. The brown aqueous solution thus obtained is at first transparent and of a sweetish bitter taste, but in a short time it becomes turbid and for some hours continues to deposit an orange-coloured, flocculent powder, the liquid at the same time losing its bitterness. The explanation of these phenomena is that the yellow substance, rubian, undergoes fermentation by the influence of the nitrogenised and highly putrescible substance which is present at the same time in the freshly-prepared aqueous liquid.
Higgins showed that the colouring matter alizarin already existing in the fresh liquid might be removed from it by suitable means, so as to deprive the latter of active dyeing powers, and that then, by letting this stand in a warm place, fermentation would proceed, all the bitter matter rubian disappear, and a fresh quantity of colouring matter be formed in the liquid. On the other hand, he showed that, by using boiling water to make the extract of the madder-root, the ferment was coagulated and deprived of its activity, the liquid remained permanently bitter, and only that quantity of colouring matter present at the time in the root could be obtained. He also established that, as in the ordinary fermentation of grapejuice, the presence of air was non-essential to the continuance of the change in the infusion of madder made with cold water when once it had been started, there only remained to be made the legitimate assumption that, in accordance with Decaisne's observations upon the juice within the cells of the plant, even the dyeing matter always found in the most freshly-prepared infusion of the root is derived from the same source, the bitter matter rubian.
Schunck soon afterwards observed that rubian also yielded alizarin when treated with the mineral acids. Much of the madder used by the dyer is indeed first subjected to the action of sulphuric acid, by which it is converted into what is called in the arts garancin. By this treatment not only does the quantity of alizarin appear to be increased, but colouring and other matters which affect the purity of the desired tints are some of them removed and some so far modified as to become inactive..
Alizarin is one of those dyes which require the use of mordants, that is, of substances serving to fix it to the fibres of cotton or woollen fabrics. These mordants also serve to develop the colours which madder (or alizarin) imparts. Thus prepared with aluminum hydrate, cloth takes a Turkey-red colour
of air was ordinary sebe obtainedring matters
when boiled with madder, while cloth impregnated with iron hydrate acquires the purple colour so common in cotton prints.
The composition of alizarin has now to be considered, for it was this that first suggested that it might be made artificially. Alizarin has been repeatedly analysed by different chemists, but with conflicting results. The variations in the results were no doubt due in part to the alizarin examined being of different degrees of purity, but partly to the fact that it is difficult to determine by unaided analysis the ratio of the hydrogen to the carbon in organic substances of complex constitution with sufficient accuracy to enable it to be expressed by numbers of atoms. Schunck gave alizarin one chemical formula, and Strecker gave it another inconsistent with this; and now that the analogy of alizarin in properties to certain other bodies has led to its being again examined with the powerful aid that such analogy always furnishes, chemists have come to learn its true composition, and that both Schunck's and Strecker's formulæ are incorrect. This brings us to the consideration of the way in which this valuable substance can be obtained independently of the producing powers of the rubiaceæ.
After carrying out some investigations on a different subject altogether, Graebe and Liebermann recognised the analogy just referred to of alizarin to the members of a class of bodies they had been studying, and were in consequence led to try the effect of heating alizarin with powdered zinc. This they found to be the production of a body already well known to chemists, called anthracene.
Anthracene is a soft, white lamellated body without taste or smell, which can be obtained from coal or wood. Most persons are aware that when coal is heated in a distilling vessel, besides illuminating gas and other matters, a large quantity of the black, offensive-smelling, viscid liquid known as tar comes over. This tar is a mixture of many different substances, and these, by redistilling the tar, can be partly separated from each other. On account of the value of some of these substances the distillation of tar constitutes an important branch of industry. The substances that first come over are mobile liquids, a mixture of which goes by the name of naphtha. Towards the end of the distillation the bodies that come over are semisolid : among them is anthracene.
The conversion of alizarin into anthracene at once threw light upon the chemical relations of the former body, and the chemists who had formed anthracene from alizarin next endeavoured secundum artem to reverse this transformation, and get alizarin from anthracene.
It was already known that by boiling anthracene with nitric acid it could be converted into a body containing oxygen, and these chemists recognised in this a body intermediate in composition to anthracene and alizarin- a body, in fact, which may be described as anthracene half-converted into alizarin. This body they renamed, in accordance with the results of the researches they had been carrying on, and called it anthraquinone.*
There being no method known by which it was likely that anthraquinone could be directly converted into alizarin, a change in which it would have to take up as much more oxygen as it already contains, it was acted upon with bromine and converted into a brominated anthraquinone. This new substance was then heated with caustic potash, by which the bromine was removed, but only by having potassium oxide left in its place. This was just what was looked for and wanted : more oxygen had been carried into the composition of the body, and the result was a substance that only differed from alizarin in having two atoms of potassium instead of two additional atoms of hydrogen. The last stage of the process presented no difficulty; the potassium-alizarin had only to be heated with an acid in order to replace the potassium by hydrogen, and alizarin was obtained.
The use of bromine being undesirable for manufacturing purposes, Perkin, the discoverer of the first coal-tar dye, determined to try whether he could not replace its use by that of sulphuric acid. In this attempt he proved ultimately successful; as, however, the steps of the process are otherwise essentially the same as those already described, it is unnecessary to follow them out here.
To those familiar with the use of chemical symbols the information afforded by them is so great that we will here represent by these symbols the composition and relations of anthracene, anthraquinone, and alizarin :
Anthracene . . . . CAH,
• C,H,O, The identity of artificial alizarin with that obtained from madder having been called in question, the last-named chemist has compared the two bodies together, and enumerates the following points of identity, to show the groundlessness of these doubts:
s togeen, the latent obtain
* Quinone is a substance obtained by oxidising an acid called quinic acid found in the Cinchonas, the coffee plant, the Paraguay-tea plant, &c.; and as Graebe and Liebermann found the body obtained from anthracene to belong to the same class as quinone, they indicated this by the term anthraquinone.
Both the natural and artificial bodies crystallize in needles, which are usually curved, especially when small.
When dissolved in caustic alkali, they both form violet solutions of the same tint. .
When applied to mordanted fabrics, they produce exactly the same colours, bearing the treatment with soap equally. They also possess the same tinctorial value.
When dissolved in alcohol, they produce with cupric acetate a purple solution of precisely the same shade of colour.
When examined with the spectroscope, their potassic solutions produce the same absorption bands.
Lastly, the ordinary precipitated artificial alizarin, yields phthalic acid when decomposed with nitric acid, just as alizarin from madder has long been known to do.
There being no other well defined reaction of alizarin, we are, therefore, judging from the above, bound to consider artificial and natural alizarin as identical.
In addition to alizarin another colour, called purpurin, is obtainable from madder. The existence of purpurin has indeed been denied by Schunck, but, after all, this denial has not so much practical importance, as he only contends that the substance obtained and called purpurin is nothing but a combination of alizarin with another substance-verantine, and not an entirely distinct substance. Now the body called purpurin has been considered to be a material ingredient of the finer colours obtained from madder; and if this were the case, artificial alizarin would prove to be only an imperfect substitute for madder. Schunck, however, also denies that anything but alizarin enters into the formation of madder-colours. Then it has been pointed out by Stokes that the spectrum of purpurin is very different from that of alizarin, so that it becomes easy to detect minute quantities of the former in this way; yet on applying this test to fabrics dyed with madder no purpurin can be detected.
According to Perkin there can be no doubt that the more brilliant the colours dyed with madder, the purer is the alizarin in combination with the mordants. This objection therefore to the substitution of artificial alizarin for madder may be safely regarded as of no value whatever.
The difficulty that remains to be solved is as to how anthracene may be got in sufficiently large quantity to make artificial alizarin an economical substitute for madder. Now that there is a special demand for this substance, we may fairly anticipate that manufacturers will, before very long, succeed in producing it in much larger quantities than at present by the distillation of coal. It is already known that at a sufficiently high temperature various other hydrocarbons, alone or mixed together, are capable of being converted into it; so that it is not improbable that the desired increase may be obtained by some modification in the heating of the coal. When the first coaltar dye was made from aniline, the latter substance was literally a rare body; it was not long, however, before a method was discovered by which it could be prepared in large enough quantity to meet the demand for it. So, we may expect, will this soon be the case with anthracene.