1915] B. Horowits 163 



(as the substance containing the " chromophore " group is called) 

 may itself be colored, or yield color by the addition of an " auxo 

 chrome" group. For example, benzene itself is colorless; the ad- 

 dition of a " chromophore " group such as — N ^ N — , gives the 

 " chromogen," azo-benzene, which is red. On the other hand, the 

 " chromophore," =C=0, is so weak that benzophenone, CßHg — CO 

 — CgHg, is a colorless " chromogen." In neither case, however, is 

 the true coloring matter, or dye, f ormed, tili the " auxochrome " is 

 added. Thus, amino-benzophenone (yellow) and amino-azoben- 

 zene, each with the " auxochrome " — NH2, are true dyestuffs. 

 The more important " chromophore " groups are 



N^ , N = 0, C = 0, C = C, C = N, N = N 



The first four are decidedly weak in their action. An increase of 

 nitro-groups, instead of increasing color — which is usually the case 

 with increase of chromophoric groups — decreases it; thus, nitro-ben- 

 zene is yellow, but dinitro- and trinitro-benzene are colorless. On 

 the other hand, diphenylethylene, CgHg — CH = CH — CßHg, with 

 a Single C = C group, is colorless but diphenyl-hexatriene, CßHj 



— CH = CH — CH = CH — CH = CH — CßHg, is yellow. The 

 same is true of the carbonyl group. One C = (as in the alde- 

 hydes, for example) is of no effect; and here even the presence 

 of more than one of these groups will not produce color, if they are 

 separated in the molecule. Acetyl-acetone, CH3 — CO — CHg — CO 



— CH3, is colorless, but di-acetyl, CH3 — CO — CO — CH3, is 

 yellow. 



The ring structure seems to have a marked influence on the de- 

 velopment of color. The nitro-paraffins are colorless, whereas a 

 large number of the aromatic nitro-compounds are colored. Tetra- 

 phenyl ethylene, 



CeHsx _ /CeHs 



CeHs/^'^NCeHs 

 is colorless, but bis-diphenylene ethylene. 



C6H4. /C6H4 



C6H4 C6H4 



is red. On the other hand, in the azo group, which is among the 

 strongest of the chromophoric groups, ring structure seems to inter- 



