192 Dr. Carnelley on the Influence of Atomic Arrangement. 



Another point observed by Witt is 



(2) The chromophor exerts its tinctorial influence much more 

 in the salts of a dye-stuff than when the latter is in the free 

 state. 



Thus, nitraniline, nitrophenol, and picric acid, when pure, 

 are pale yellow, whilst their salts are dark orange and some- 

 times red. 



Again, 



(3) Of two dye-stuffs, both possessing in other respects a similar 

 constitution, that has the highest tinctorial power whose salts are 

 the most stable. 



Since most dye-stuffs owe their basicity or acidity to the 

 presence of the NH 2 or OH group, each chromogen may give 

 rise to two colouring compounds. All dye-stuffs, therefore, 

 may be arranged in pairs; and it is worthy of notice that cor- 

 responding dye-stuffs always have a similar colour. Thus, 



Mtrophmol C 6 H 4 .N0 2 .OH | 



Nitroaniline C 6 H 4 .X0 2 .NH 2 J 6 J 



Picric ackl C 6 H 2 .(N0 2 ) 3 .OH | 



Trinitraniline ... C 6 H 2 .(N0 2 ) 3 .NH 2 J J 



Amidoazobenzene NH 2 . C 6 H 4 . !N=N . C G H 5 \ „ 



Oxyazobenzene... OH.C c H 4 .N=N.C 6 H 5 J } ° 



Tetramidotetrazotriphenyl ) , 



Tetroxytetrazotriphenyl ) 



Eosaniline 1 t _, 



Rosolic acid J 



Dioxyanthraquinone or alizarin ) , . , . 



•' ^ > red or violet. 



Diamidoanthraquinone ) 



All the compounds referred to above (except the two anthra- 

 quinone derivatives) contain a nitrogenous chromophor. The 

 dye-stuffs of anthraquinone, however, show that a carbon 

 group may be a chromophor. Neither anthraquinone nor 

 dioxyanthracene are dye-stuffs ; whilst a combination of both, 

 viz. alizarin or dioxyanthraquinone, is a powerful colouring 

 agent. The double ( — CO — ) group is therefore the chromo- 

 phor of alizarin and the related dye-stuffs. These compounds 

 also lose their colouring-properties on the introduction of 

 methyl into the hydroxy 1 groups. If we increase the salt- 

 forming power of alizarin by introducing more hydroxyl 

 groups, or by nitration or amidation, we also increase the 

 tinctorial properties. Further, in alizarin and the related 



