Phenol and Azo dyes are all substances which are of greater interest to the chemical students and to the colour manufacturer than to the ordinary reader.  Many of the names of the bases of various dyes are unknown outside the chemical dyeworks, although each and all have complicated; reactions of their own.  In the reds are rosanilines, toluidine xylidine, &c.; in the blues—­phenyl-rosanilines, diphenylamine, toluidine, aldehyde, &c.; violets—­rosaniline, mauve, phenyl, ethyl, methyl, &c.; greens—­iodine, aniline, leucaniline, chrysotoluidine, aldehyde, toluidine, methyl-anilinine, &c.; yellows and orange—­leucaniline, phenylamine, &c.; browns—­chrysotoluidine, &c.; blacks—­aniline, toluidine, &c.

To take the rosanilines as an instance of the rest.

Aniline red, magenta, azaleine, rubine, solferino, fuchsine, chryaline, roseine, erythrobenzine, and others, are colouring matters in this group which are salts of rosaniline, and which are all recognised in commerce.

The base rosaniline is known chemically by the formula C_{20}H_{l9}N_{3}, and is prepared by heating a mixture of magenta aniline, toluidine, and pseudotoluidine, with arsenic acid and other oxidising agents.  It is important that water should be used in such quantities as to prevent the solution of arsenic acid from depositing crystals on cooling.  Unless carefully crystallised rosaniline will contain a slight proportion of the arseniate, and when articles of clothing are dyed with the salt, it is likely to produce an inflammatory condition of skin, when worn.  Some years ago there was a great outcry against hose and other articles dyed with aniline dyes, owing to the bad effects which were produced, and this has no doubt proved very prejudicial to aniline dyes as a whole.

Again, the base known as mauve, or mauveine, has a composition shown by the formula C_{27}H_{24}N_{4}.  It is produced from the sulphate of aniline by mixing it with a cold saturated solution of bichromate of potash, and allowing the mixture to stand for ten or twelve hours.  A blue-black precipitate is then formed, which, after undergoing a process of purification, is dissolved in alcohol and evaporated to dryness.  A metallic-looking powder is then obtained, which constitutes this all-important base.  Mauve forms with acids a series of well-defined salts and is capable of expelling ammonia from its combinations.  Mauve was the first aniline dye which was produced on a large scale, this being accomplished by Perkin in 1856.

The substance known as carbolic acid is so useful a product of a piece of coal that a description of the method of its production must necessarily have a place here.  It is one of the most powerful antiseptic agents with which we are acquainted, and has strong anaesthetic qualities.  Some useful dyes are also obtained from it.  It is obtained in quantities from coal-tar, that portion of the distillate known as the light oils being its immediate source.