Carbolic acid is liquid, but becomes solid when purified and dried; and as already mentioned the brilliant yellow dye, carbazotic acid, one of the coal tar colours, is a compound radical, in which the peroxide of nitrogen has replaced three equivalents of hydrogen. The other coal tar colours are obtained from the neutral hydro-carbons, that is to say, compounds of hydrogen and carbon, such as benzol, toluol, and other analogous substances.
Benzol, which consists of 12 equivalents of carbon and 6 of hydrogen, is very volatile, boiling at 117° Fahr., and when acted upon by nitric acid, it forms a compound radicle in which one equivalent of oxide of nitrogen takes the place of one of hydrogen. It smells strongly of bitter almonds, and may be used with safety instead of them. When water and iron are mixed with nitro-benzol, the iron combines with the oxygen and forms oxide of iron, and the result is rusted iron and aniline, which is the origin and foundation of the coal tar colours. Now aniline consists of 12 equivalents of carbon, 7 of hydrogen, and 1 of nitrogen. It is a compound radical: it is ammonia in which one equivalent of hydrogen has been replaced by the radical phenyle, consisting of 12 equivalents of carbon and 5 of hydrogen. It may be remarked that in all these chemical operations the quantity of carbon has remained the same.
Aniline is a colourless liquid, and, being an analogue of ammonia, it readily combines with the different acids to form the beautiful coal tar dyes, for which the world is indebted to the brilliant researches of Dr. Hofmann, professor of chemistry.
By combining a solution of the chloride of lime with the colourless liquid aniline, he obtained the beautiful colour mauve, but it could not be used as a dye till it was rendered permanent by his pupil, Mr. Perkins. His next discovery was the rich crimson crystalline dye magenta, which M. Verguin first introduced into trade at Lyons as a dyeing agent. It may be produced by mixing the anhydrous bichloride of tin with aniline and then driving off the excess of aniline by heat. Other metallic chlorides, nitrates, and many oxidizing agents, have the power of converting aniline into magenta; as for example when the two colourless liquids acetic acid and aniline are mixed and heated, a chemical combination takes place in which three atoms of ammonia have coalesced into one, a salt is formed which is the acetate of aniline or magenta. Here two liquids unite to form a solid and as in many other instances the resulting substance has the power of decomposing light which neither of its constituents can do. Magenta has a redder tint than mauve, and on that account it is sometimes called aniline red. Professor Hofmann has discovered quite recently that pure aniline has not the property of producing these colours, but that they originate in an impurity of the aniline called toluidine.