CLASSIFICATION. Artificial colors were for merly classified merely according to the sources from which they were obtained. Thus, many of them, including magenta, 'aniline blue,' ani line green; `aniline yellow,' etc., were grouped together as aniline colors. At present somewhat different systems of classification are used by different authors, but all systems are based ex clusively on the chemical constitution of the dyes.
Many attempts have been made to find a gen eral answer to the question, What must be the chemical nature of a carbon compound in order that it may be a dye? An all-embracing answer to this question has not yet been found. But experience has shown that the true dyestuffs exhibit peculiar groupings of the constituent atoms. Such `chromophore' groupings produce, however, only a tendency toward color, but not necessarily colors; indeed, Many compounds con taining them are perfectly colorless, and the majority of true dyes become colorless if de .
inved of the small amount of oxygen they con tain, although their chromophore groups may not be in the least affected. lf, however, a chromophore group is combined with certain other atomic groups, the result is a dye. For example. the so-called c.-o-group (—N=.N—) is ehromopho•ic; the compound called azohen. zene, although colored red and evidently containing the azo-group, is not a dye; but it becomes one when the so-called amido group (NIL) also is introduced into its mole cule, the compound called amido - azobenzene. being a true dye. If, in
stead of the amido-group, a hydroxyl group (OH) is introduced, the result is again a dye (an orange one). Further, the tints of dyes are produced by variation in the 'substituting' groups which replace hydrogen in the primi tive molecule. Thus, the introduction of the methyl group (CH.,) generally increases the violet tendency; the phenyl group pro duces bluish tints; the naphthyl group a tendency toward brown-red, etc. The relative position of the groups likewise plays a large part in the determination of color. But, as we have already observed, a definite and all-embrac ing rule does not exist. Frequently compounds must enter into combination With a base or an acid before they will fix themselves upon the fibre, and then the tints arc frequently affected by the different bases or acids to a varying de gree. For example, alizarin dyes red with the hydroxide of aluminum, and black with the hy droxide of iron.
For the purposes of the present sketch, the coal-tar colors may be grouped in five classes: viz. the azo-eolors; triphenyl-carbinol deriva tives; quinone derivatives; diphenyl-amine deriv atives; and indigo dyes.