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- 

 duced the bluer the violet, until when all six available hydrogen 

 atoms are thus substituted, crystal violet, the deepest of them all, 

 is obtained. By using three ethyl groups instead of methyl, 

 Hoffman violet or dahlia is formed, which is deeper in color than 

 the trimethyl compound, due to the heavier groups introduced. If 

 three phenyl groups (i.e., the benzene ring (CeHs-) are introduced 

 instead of methyl or ethyl, the color is still further deepened, the 

 resulting dye being spirit blue. Further, it is possible to introduce 

 another methyl group into crystal violet, by addition of methyl 

 iodide (or chloride) to one of the trivalent nitrogen atoms, whereby 

 its valency is increased to five, and a green dye, methyl green, is 

 produced : 



With these facts in mind it will be seen that the grouping of dyes 

 as based upon these chromophores does not classify them in rela- 

 tion to their color. It is a useful classification, however, because it 

 puts together those that have similar chemical structure. The 

 important biological dyes, thus classified, fall into the following 

 groups : 



1. The nitro dves. 



e.g., picric acid. 



2. The azo group. 



e.g., methyl orange, Bismark broicUy orange G, Congo red, 

 Sudan III and Sudan IV. 



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