134 MORRIS LOEB 



may not be in the least affected. If, however, a chromo- 

 phore group is combined with certain other atomic groups, 

 the result is a dye. For example, the so-called azo-group 

 ( N=N ) is chromophoric; the compound called azoben- 

 zene, CeH 6 N=N C 6 H 5 , although colored red and evidently 

 containing the azo-group, is not a dye; but it becomes one 

 when the so-called amido-group (NH 2 ) also is introduced into 

 its molecule, the compound C 6 H 5 N=N C 6 H 4 NH 2 , called 

 amido-azobenzene, being a true dye. If, instead 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 primitive molecule. Thus, the in- 

 troduction of the methyl group (CH 3 ) generally increases 

 the violet tendency; the phenyl group (C 6 H 5 ) produces bluish 

 tints; the naphthyl group (CioH 7 ) 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-embracing 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 are frequently affected by the 

 different bases or acids to a varying degree. For example, 

 alizarin dyes red with the hydroxide of aluminum, and black 

 with the hydroxide of iron. 



For the purposes of the present sketch, the coal-tar colors 

 may be grouped in five classes: viz., the azo-colors; triphenyl- 

 carbinol derivatives; quinone derivatives; diphenyl-amine 

 derivatives; and indigo dyes. 



Azo-CoLORS. The characteristic compound of this class 

 is azo-benzene, C6H5N=NCeH5, already mentioned above. 

 We have seen that the introduction of either NH 2 or OH in 

 place of a hydrogen atom produces a coloring matter yellow 



