The readiness with which aniline, in certain of its reactions, produces very brilliant colours, was of course known to chemists from the time Runge produced it from coal tar and gave it the name kyanol. Up to year 1858 it, however, possessed nothing beyond a scientific interest. On the 26th of August 1858, Mr W. H. Perkin obtained a patent for the production of a dye-stuff derived from aniline, which soon became well known as mauve, or Perkin’s purple, as well as by various other names. The discovery of Mr Perkin formed the turning-point in the history of aniline, and was indeed the beginning of a great revolution in the arts and manufactures connected with the dyeing of textile fabrics. The manufacture of aniline dyes was really first begun in France, the French manufacturers acting on the information supplied by Mr Perkin’s patent specification. It immediately spread to all industrial centres, and became one of the most eagerly investigated of all commercial undertakings. A rapid succession of patents were applied for and obtained; new processes and combinations were continually being projected, and a great variety of colours were tried with more or less success as commercial substances. The activity of scientific research kept pace with the energy of manufacturing enterprise, resulting in a rapid improvement of processes, decrease in the cost of the manufacture, and a great increase in the beauty and tinctorial effect of the dyes produced. At the present time every colour, and all tints and shades of colours, are produced from aniline; and, while the processes employed and the combinations formed are very numerous, the names under which the dye-stuffs are sold must be said to be endless.

Of the processes by which aniline may be prepared, that only is available in manufacturing which was devised by Zinin, when he prepared it, under the name of benzidam, from nitrobenzol. Aniline is only found in-very small quantities as a direct product of the distillation of coal tar, but a large proportion of light hydrocarbons are among the products when the temperature ranges between 80° C. and 130° C. The chief of these is benzol, with smaller quantities of toluol and other closely allied substances. These hydrocarbons, when acted upon by strong nitric acid, give up an atom of hydrogen, the place of which is taken by one of nitroxyl (NO 2 ), and benzol is thus transformed into nitrobenzol: i.e., benzol (C 6 H 6 ) + nitric acid (HNO 3 ) = nitrobenzol (C β H 5 NO 2 ) + water (H 2 O). Nitrobenzol is a yellow oily liquid, with the exact flavour of bitter almonds, for which it has long been used as a substitute in fancy soap-making, under the name of essence of mirbane. Its transformation into aniline is now practically effected by submitting it to the reducing influence of nascent hydrogen, by the action of iron and acetic acid. To a mixture of nitrobenzol and strong acetic acid a certain proportion of iron filings is gradually added. Very energetic chemical action is set up with great disengagement of heat. When the reaction is complete the aniline is distilled over by superheated steam. The aniline of commerce thus prepared is not a chemically pure product, but a mixture of aniline and toluidine, just as the commercial benzol operated on is a mixture of benzol with toluol. In the manufacture of dyes it is found essential that certain proportions of both aniline and toluidine should be present in the aniline oil operated on.

The method by which Mr Perkin obtains from aniline the violet colour known as mauve, is by mixing a solution of sulphate of aniline and of potassic dichromate in equivalent proportions, and leaving them for several hours till the resulting reaction is complete. A black precipitate is formed, which is washed free from the potassic sulphate it contains, and then treated with naphtha to dissolve out the resinous matter contained in the mass. The residue consists of the mauve dye, and may be dissolved in alcohol. It has been found to be the sulphate of a base, to which the name mauveine has been given.

The greater proportion of aniline dyes manufactured are now produced directly or indirectly from another basic body, termed rosaniline. About the time of the French-Austrian war in 1859, a second coal-tar dye was introduced into commerce, which became popularly known as aniline red or Magenta, from the battle fought at the period of its introduction. Dr Hofmann’s investigations into the nature and composition of this dye have done more to place the entire industry on a satisfactory basis than any other undertaking. He found that it consisted of a salt of a basic substance, to which he gave the name rosaniline (C 20 H 19 N 3 H 2 O). For the preparation of rosaniline, Hofmann found that a certain proportion of toluidine must be present in the commercial aniline employed. Rosaniline is now entirely manufactured by treating aniline of a known composition with a strong solution of arsenic acid in an iron retort, heated to a temperature of not more than 180° C. The reaction occupies about eight hours; and at the conclusion of the operation a crude mass, consisting of rosaniline arsenite and arseniate, is found, which is next dissolved in water acidulated with hydrochloric acid. Common salt is added to this solution, when a double decomposition takes place. Rosaniline hydrochlorate is formed on the one hand, and sodium arseniate and arsenite on the other. For the preparation of the salts of rosaniline, the base is treated directly with acids.

Rosaniline blues are prepared by acting upon commercial aniline with a salt of rosaniline, such as the acetate, under the influence of a heat of about 190° C., kept up for two hours. The reaction which takes place is the substitution of three atoms of phenyl for three of hydrogen, and hence these blues are chemically salts of triphenylrosani-line. Violet colours, such as Hofmann’s violet, are prepared by an analogous process to that employed for the blues, by treating rosaniline with the iodides of methyl or ethyl, atoms of these radicals taking the place of hydrogen, and forming ethylic or methylic rosaniline. Aniline green is formed by the action of aldehyd on a solution of aniline red in sulphuric acid, and subsequent boiling in a solution of hyposulphite of soda. Aniline yellow, or yellow fuchsine, is formed from a base named chrysaniline, a minor product of the formation of rosaniline; but most of what is termed aniline yellow is prepared from picric acid, a different product of the distillation of coal tar. Various shades of brown and maroon, as well as black colours, are regular commercial products; but they are neither so well understood, nor of such consequence as the reds, blues, and greens.

Aniline colours are employed in the industrial arts for numerous other purposes besides their great use as dyeing materials. Violet ink and other fancy coloured inks are prepared from them. They are used by paper manufacturers for tinting pulps, and for the superficial staining of finished paper. They are likewise used by paper stainers in the printing of wall papers, in the preparation of lithographic inks, and to some extent for water colours. They are largely employed as colouring materials in perfumery, fancy soaps, and cosmetics, besides having many other minor applications.

Concerning these dyes, Dr Hofmann, to whom the industry is so much indebted, wrote, in 1862, while it was yet in its infancy, “Instead of disbursing her annual millions for these substances, England will, beyond question, at no distant day become herself the greatest colourproducing country in the world; nay, by the very strangest of revolutions, she may erelong send her coal-derived blues to indigo-growing India, her distilled crimson to cochinealproducing Mexico, and her fossil substitutes for quercitron and safflower to China, Japan, and the other countries whence these articles are now derived.” It is scarcely needful to say that these bold anticipations have already been fully realised.