THE CHEMICAL COMPOSITION OF DYES 



159 



associated with any of the three nitrogen atoms, or with the central 

 carbon atom, and indeed there are other possible positions. It is 

 the resonance between the particular positions that are available 

 in a molecule that determines the colour. This fact will be strik- 



ci- 



NH, 



NH, 



^ 



?' 



NH2 



Pararosaniline : another resonance position 



ingly illustrated below, by reference to a related dye, crystal 

 violet (p. 168). 



It is accurate to regard the positive charge as belonging to the 

 ion as a whole, and therefore to write the formula in the way 

 shown here. 



NH, NH5 



+ 



[CI]- 



Pararosaniline : another formula 



This kind of symbol will nevertheless not be used in this book, 

 because it is convenient to draw special attention to the chromo- 

 phore. The reader should remember that the formulae that will 

 be used represent, as it were, instantaneous photographs of the 

 dye-ion. He may decide to work out for himself the changes in the 

 position of the electric charge involved in the process of resonance. 



The colour of the dye is very slightly changed if, instead of 

 taking t^vo molecules of aniline and one of paratoluidine, we take 

 one of aniline, one of orthotoluidine, and one of paratoluidine. 

 The dye formed from these constituents is called rosaniline. It will 

 be noticed that it differs from pararosaniline only in the possession 

 of the methyl group introduced with orthotoluidine. 



