34 Biological Stains 



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, 

 Hofmann's 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 : 



_C 



CI CH3 



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 : 



A. The nitroso dves. 



e.g., naphthol green B. 



B. The nitro dyes. 



e.g., picric acid. 



C. The azo dyes. 



e.g., methyl orangey Bismarck brown F, orange 0, Congo 

 red, Sudan III and Sudan IV. 



D. The quinone-imine group, including 



1. Indamins; e.g. toluylene blue. 



2. The indophenols 



3. Thiazins; e.g., thionin, toluidine blue, methylene blue. 



4. Oxazins; e.g., brilliant cresyl blue, Nile blue. 



5. Azins, including 



(a) Amino-azins; e.g., neutral red. 



(b) Safranins; e.g., safranin 0, magdala red. 



(c) Indulins; e.g., nigrosin. 



