Another derivative of rosaniline is the aniline green. It is obtained by dissolving the rosaniline salt in dilute sulphuric acid, adding crude aldehyde (a substance obtained by acting with oxidizing agents on alcohol). The mixture is heated until a sample dissolves in acidulated water with a blue tint; it is poured out into boiling water containing in solution hyposulphite of sodium, boiled, the liquid filtered; and the green dye, if required in the solid state, is precipitated by carbonate of sodium. Aniline green dyes wool and silk, the latter especially, of a magnificent green; perhaps as beautiful a colour as any of the coal-tar series, and one which has the singular advantage among greens of looking as beautiful in artificial light as in daylight. The manner in which this dye was discovered is somewhat curious. It is related by Mr. Perkin of a dyer, named Chirpin, that he was trying to render permanent a blue colouring matter, which had been found could be produced from rosaniline by the action of aldehyde and sulphuric acid. After a number of fruitless attempts at fixing it, he confided his perplexities to a photographic friend, who evidently thought that if it was possible to fix a photograph, anything else might be fixed in like manner, for he recommended his confidant to try hyposulphite of sodium. On making the experiment, however, the dyer did not succeed in fixing his blue, but converted it into the splendid aldehyde green. Like other colouring matters we have described, this is a salt of a colourless base containing sulphur. Like rosaniline, the colourless base takes on the characteristic colour of its salts by merely absorbing carbonic acid from the air.
Again, by a modification of the process for producing the Hofman violets, another green of an entirely different constitution may be obtained. It is bluer in tint than the former, and is much used for cotton and silks, under the name of “iodine green.”
In the manufacture of magenta there is formed a residuum or bye-product, consisting of a resinous, feebly basic substance, from which Nicholson obtained a dye, imparting to silk and wool a gorgeous golden yellow colour. This dye cannot be obtained directly, but is always produced in greater or less quantity when magenta is made on the large scale, and is separated during the purification. By first dyeing the silk or wool with magenta, and then with this dye, which is commercially known as “phosphine,” brilliant scarlet tints are obtained. The yellow colours have been found to be salts of a base termed chrysaniline, a sort of chemical relative of rosaniline, as may be seen in comparing the formulæ which represent their constitution, with which we place also the symbol for another substance obtained by submitting rosaniline to the influence of nascent hydrogen. This body, leucaniline, again yields rosaniline very readily when the hydrogen is removed by oxidizing agents. It will be noticed that the three bodies form a series the members of which differ only by H2, thus indicating their close relationship.
| C20H17N3 | Chrysaniline. |
| C20H19N3 | Rosaniline. |
| C20H21N3 | Leucaniline. |
Some idea will have been obtained from the foregoing particulars of the great colour-supplying capabilities of aniline; but we have not yet exhausted the utility of this interesting substance. It is probable that the letters on the page now under the reader’s eye owe their blackness to an aniline product. For after all the salts furnishing the lovely tints we have mentioned have been extracted, there is in their manufacture a final residuum, and from this an intense black is obtained, which is largely used in the manufacture of printing-ink.
We have mentioned phenol as a substance yielding colours. Phenol is the body now so well known as a disinfectant under the name of “carbolic acid,” a name given to it by its discoverer, Runge, who prepared it from coal-tar, in 1834. Phenol forms colourless crystals, which dissolve to some extent in water, and very readily in alcohol. It is a powerful antiseptic, that is, it arrests the process of putrefaction in animal or vegetable bodies, and it is also highly poisonous. The constitution of phenol is given by the formula C6H5 OH, in which the reader will recognize the same group of atoms already indicated as entering into the aniline derivatives. From some of these phenol may in fact be obtained, and although it cannot be formed directly from benzol, phenol can be made to furnish benzol. When crude phenol is treated with a sulphuric acid and oxalic acid, a substance is obtained which presents itself as a brittle resinous mass of a brown colour, with greenish metallic lustre. This substance is called rosolic acid by chemists, but in commerce it is known as aurine, and is used for dyeing silk of an orange colour, which, however, is not very permanent. But by heating rosolic acid with liquid ammonia, a permanent red dye is procured which has been termed peonine, and has been much used for woollen goods. But it lately had the reputation of exerting a poisonous action, producing blistering and sores when stockings or other articles dyed with it were worn in contact with the skin. It is now, therefore, less extensively employed. Coralline, another body identical with or very similar to the former, is similarly prepared from rosolic acid by heating it with ammonia under pressure.
Again, by heating coralline with aniline, a blue dye, known as “azurine,” or “azuline,” was formerly made in large quantities; but it has been supplanted by the aniline blues already described.
When phenol is acted upon by nitric acid new compounds are produced, standing in the same relation to phenol as nitro-benzol does to benzol. The final result of the action of nitric acid on phenol is picric acid, called also “carbazotic acid,” and, more systematically, “tri-nitro-phenol;” for it is regarded as phenol in which three of the hydrogen atoms have been replaced by the group NO2 thus, C6H2(NO2)3 OH. It forms bright yellow-coloured crystals, and its solution readily imparts a bright pure yellow colour to wool, silk, &c. It received the name of picric acid (πικρος, bitter) from the exceedingly bitter taste of even an extremely diluted solution. It is said that picric acid is employed as an adulterant in bitter ale instead of hops. Now, the colouring power of picric acid is so great, that even the minute quantity which could be used to impart bitterness to beer is recognizable by dipping a piece of white wool into the beer, when, if picric acid be present, the wool acquires a clear yellow tint. Besides its employment as a yellow, it is useful for procuring green tints by combination with the blues. Picric acid again furnishes, by treatment with cyanide of potassium, a deep red colour, consisting of an acid which, when combined with ammonia, furnishes a magnificent colouring material—which is, in fact, murexide, a dye identical with the famous Tyrian purple of the ancients, and formerly obtainable only from certain kinds of shell-fish.
Naphthaline—another of the colour-yielding substances of coal-tar—is, like benzol, a hydro-carbon, but one belonging to quite another chemical series. Its formula is C10H8, and it has an interest to chemists altogether apart from its industrial uses, from having been the subject of the classic researches of the French chemist, Laurent—researches which resulted in the introduction of new and fertile ideas into chemical science, contributing largely to its rapid progress. Naphthaline forms colourless crystals, which, like camphor, slowly volatilize at ordinary temperatures, and are readily distilled in a current of steam. It is thus sufficiently volatile to escape complete deposition in the condensers of the gas-works, and to be partly carried over into the mains, where its collection occasions some trouble. Nitric acid acts upon naphthaline in a manner analogous to that in which it acts on benzol, forming nitro-naphthaline, which, in its turn, submitted to the action of iron filings and acetic acid, is transformed into a base called “naphthylamine.” The salts of naphthylamine are coloured products which, in some cases, have been found available as dyes. There is a crimson colour, and a yellow largely used under the name of “Manchester yellow,” for imparting to silk and wool a gorgeous golden yellow colour. Another coloured derivative of naphthaline, called “carminaphtha,” was discovered by Laurent in the course of his researches.
It would be easy to fill this volume with descriptions of the properties, and modes of preparing the numerous colouring matters that have been obtained from coal-tar products. In order to give the reader an idea of the extent to which the tar products have been made to minister to our sense of the beautiful, a list is here given of the principal colouring matters from these sources that have been employed in the arts. The various names under which a product has been commercially known are in most cases given. It must be understood that the same name is frequently applied to products chemically distinct, and some of the names which appear as synonyms may also in reality indicate different substances.