General Nature of Dyes 33 



certain general rule which correlates chemical formula with color. 

 In any group of compounds, the simpler ones are converted into 

 the more complex by substitution of radicals for hydrogen atoms. 

 In the dyes the substituents are generally methyl or ethyl groups, 

 or sometimes phenyl groups. Now the general rule is that the 

 larger the number of hydrogen atoms that have been replaced by 

 these groups the deeper the color. The tendency is for the color 

 of the simplest dyes in any group of homologous compounds to be 

 yellow, passing through red to violets and then greens and blues, as 

 the homologs become higher through the introduction of success- 

 ively larger numbers of methyl or other substituting groups. Thus 

 the compound pararosanilin, which is very frequently sold as basic 

 fuchsin is a triphenyl methane, with an amino group attached to 

 each benzene ring, but without any methyl groups; thus: 



H /~\^NH2 



H-X_/-\_C 



/ \_/ \/-\_NH2 

 CI \^/ 



Rosanilin, which is similar in composition, but contains one methyl 

 group attached to one of the benzene rings, 



is a red very similar to pararosanilin but with less of a yellowish 

 cast. Now another methyl group may be introduced into each of 

 the other two benzene rings, and each one successively deepens the 

 shade of red, so that the highest homolog of the series, new fuchsin: 



CH3 



/ 

 /~\_XHo 



_C 



"\ NH> 



CH3 



has a more bluish cast than any of the others. Thus basic fuchsins 

 can vary considerably in their shade according to the proportions 

 in which these four possible components may be mixed. 



It is also possible in another way to deepen the color of pararo- 

 sanilin still further, namely by introducing methyl groups into the 

 amino radicals instead of directly on the benzene rings. Thus the 

 methyl violets are obtained; and the more methyl groups intro- 



