on the Physical Properties of Compounds. 191 



NO, NOa NO, NO, 



SK. 2 



Dye-stuff. 



ff(C a H 3 0) 2 

 Colourless. 



OH 



Dye-stuff. 



OCH 3 



Colourless. 



If, however, we niethylate the NH 2 group of nitro-aniline, 

 we do not destroy its tinctorial properties, because we do not 

 at the same time destroy its basicity. Thus 



SO, 



N0 2 



NH 2 X(CH 3 ) 2 



Basic, and dye-stuff. Basic, and dye-stuff. 



From such considerations as these "Witt has enunciated the 

 following laws: — 



(1) The tinctorial properties of aromatic compounds are con- 

 ditioned by the simultaneous presence of a colour-imparting 

 group (such as a nitro-group) and of a salt-forming group 

 (such as NH 2 or OH group). 



The colouring group he terms the chromophor ; and a com- 

 pound containing a chromophor, and in which a salt-forming 

 radical is still wanting for the formation of a dye-stuff, he calls 

 a chromoqen: thus IST0 2 is the chromophor of C 6 H 4 .N0 2 .NH 2 

 and C 6 H 4 . N0 2 . OH, whilst C 6 H 5 . N0 2 is the chromogen of 

 both compounds. 



Further, the tinctorial properties increase with the number 

 of N0 2 groups present : thus, the nitrodiphenylamines are all 

 acids and dye-stuffs, and their dyeing-power increases with the 

 number of 1ST 2 groups. 



Again, azobenzene, C 6 H 5 . N = N . C H 5 , though of a dark 

 yellow colour, is not a dye-stuff ; nor is benzidine, 



NH 2 .C 6 H 4 .C 6 C 4 .KE 2 , 



though it is a strong base ; but as soon as we combine the 

 properties of the two, by introducing into the latter compound 

 the chromophor — N=N— , or a salt-forming group into the 

 former, we set a series of beautiful dye-stuffs. 



