Aniline, or AMIDO-BENZENE, was discovered in 1826, as a product of the dry distillation of indigo; hence the name, derived from anil, the Portuguese for indigo. This source has now ceased to be of importance, for, practically, all the aniline now manufactured is obtained from coal-tar. When coal is heated in the manufacture of illuminating gas, a large number of substances are produced, and are obtained as a tarry matter of varying composition. Only a few of these bodies are of commercial importance, the chief being ammonia, carbolic acid, anthracene, naphthalene, pitch, and benzene. It is this last-named substance that yields aniline. If it is treated with strong nitric acid, an intermediate compound, nitrobenzene, , is formed, which, when mixed with acetic acid and iron-filings, yields acetate of aniline. Aniline may also be prepared by passing a mixture of benzene and ammonia through a red-hot tube, after the following reaction:
Aniline may be regarded either as benzene in which one atom of hydrogen has been replaced by the group amidogen, , or as ammonia, , in which one atom of hydrogen has been replaced by the radical phenyl, ; and according as the one or other view is held, it is called amido-benzene or phenylamine (see AMINES).
The pure article is a colourless oily fluid, slightly soluble in water, but readily dissolving in alcohol and ether. It refracts light strongly, and possesses a weak aromatic taste. It boils at . (.), and when pure, has a specific gravity of 1.020. It is a well-marked Base (q.v.), producing numerous crystalline salts, although it has no alkaline action on vegetable colours. It is a powerful narcotic poison, its fumes causing giddiness, and subsequently insensibility, while the body becomes of a livid leaden-blue colour. Taken internally, it soon causes death; and even when respired in small quantity, as by the workmen engaged in its manufacture, it causes severe headaches, nausea, and vomiting.
It is, however, as being the source of the numerous aniline dyes, that this body has become of leading importance.
Aniline unites with acids, forming salts, but these do not constitute the aniline dyes. These consist of various bases obtained by the oxidation of aniline by means of nitric acid, chlorine, arsenic, or other agents. In many instances these bases are quite colourless, and only develop their tints when formed into salts. They may be regarded as Amines (q.v.)—i.e. ammonia in which hydrogen has been replaced by one or more radicals. Thus we have diphenylamine, ; dimethyl aniline, ; methyl ethyl aniline, ; and so on. To refer at length to the various aniline dyes would be impossible, as these now number some hundreds, and we can only indicate the leading varieties. The colours produced by these dyes include every shade and tint, and the list of red or violet compounds would alone exhaust our available space. Fuch sine, which may be taken as typical of the red dyes, is formed when aniline is treated with strong nitric acid; but, in practice, many other chemicals may be substituted for the acid. Blue dyes are produced when aqueous aniline salts are treated with chlorate of potash and hydrochloric acid. Mauveine, a powerful violet dye, was discovered by Perkin in 1856, and this led the way in the manufacture of aniline colours. Perkin produced it by acting on aniline with bichromate of potash. We must pass over the various green, brown, yellow, and gray dyes, merely mentioning that the so-called aniline blacks are usually either very intense greens or blues, appearing black through concentration.
The aniline dyes are noted for their intense colouring power, one part of a rosaniline salt in a million parts of water still possessing a deep crimson colour, and instantly dyeing a skein of silk moistened with vinegar. Even in so dilute a solution as one grain dissolved in 1500 gallons of water, it is capable of dyeing a silk thread immersed in it for twenty-four hours.
Many of the dyes exhibit complementary colours (see LIGHT) when looked at by reflected and transmitted light; thus, the strong solution of the salt above referred to looks a purple red by transmitted, and a brilliant green by reflected light; a fact familiar to the users of an aniline red ink, or an ink for any of the familiar 'graph' copying processes. Here the pen assumes a green shining appearance, quite different from the colour of the ink. Aniline dyes are used as lacquers for cheap toys, being readily soluble in spirit varnish, the well-known 'bronzing liquid' being an example of this. Mixed with gelatin or collodion, and allowed to dry in thin sheets, they furnish the thin transparencies so much used for producing stained glass imitations. They have been also used for colouring wines and sweetmeats, but as arsenic was formerly or is still employed in the manufacture of the red varieties, this practice is not unattended with risk. The use of arsenic has of late been largely abandoned; or, when used, makers take care to eliminate the arsenic at the end of the process, so that the final product is innocuous. Numerous cases of skin-eruptions have been traced to the wearing of red flannel or red stockings dyed by aniline dyes. The readiness with which any housewife can dye articles of clothing or household ornaments has made them great favourites. The chief drawback lies in the fugitive nature of many varieties, but notwithstanding there is a wide field still open to them. The aniline colours are as a whole disapproved from the artistic point of view. Some of them are especially objectionable when used in the same textile fabric along with natural dyes. Notwithstanding this, the introduction of aniline dyes is said to have closed half the dyers' shops in India. A few years ago the Shah of Persia prohibited the importation of these colours into that country. Further details about aniline dyes are given under the article DYEING. Although England and France were first in the field, Germany took up the manufacture with so much zeal and scientific skill that it soon surpassed its competitors, pro- ducing superior shades of colour. Germany is now the headquarters of the industry, its products being of the highest class and the lowest price. The produce in Germany is estimated to be a third greater than that of France, and three times that of England; while the annual value of English aniline colours may amount to a million and a half. See Perkin, 'On the Coal-tar Colours,' in Nature, vol. xxxii.; and The Chemistry of the Coal-tar Colours, by Benedikt (Eng. trans. 1886).