THE CHEMICAL COMPOSITION OF DYES 93 



The aniline rings are merely particular examples of aryl rings, and 



NHc 



NH. 



NHo 



ci- 



Leitco-para- 

 rosanil'me 



Skeleton-formula for 

 triarylmethane dyes 



Triphenylpara- 

 rosaniline 



all the many dyes that have three such rings held together in the 



+ 



same way, whether any of the rings has an = NH2 group on it or 

 not, are therefore classified together as triarylmethane dyes. In 

 the skeleton-formula for this and other groups of dyes, the auxo- 

 chromes and modifiers are omitted, since these differ from dye to 

 dye. 



Methyl violet is pararosaniline in which four or five of the 

 hydrogens of the amino-groups have been replaced by methyl. 

 This replacement has a strong blueing effect. When all six are 

 replaced, crystal violet is produced; this dye is almost blue. It 

 has already been mentioned (p. 91) that ethyl and aryl groups 

 have still greater blueing effect than methyl. This is well exempli- 

 fied by triphenyl pararosaniline, which, with another closely 

 related dye, constitutes the mixture known as aniline blue SS. 

 Triphenyl pararosaniline is a pure blue cationic dye. It is para- 

 rosaniline in which one of the hydrogens of each of the three 



