stain, was true cresyl violet without the reddish dye. True cresyl 

 violet, like this last mentioned product can now be obtained in 

 constant quality in America, and proves to have all the properties 

 needed in this stain. 



Other oxazin dyes sometimes mentioned in connection with 

 histology are: 



Capri Blue. C. I. No. 876. 



Naphthol Blue. C. I. No. 909. Synonym : Neiv blue B, Fast blue 

 SR. Phenylene blue. Meldolas blue. Indin blue 2RD. 



4. THE AZIXS 

 The dyes of the azin group are derivatives of phenazin, 

 C6H4N2Cf,H4, a compound containing two benzene rings linked 

 thru two nitrogen atoms in such a way as to form a third ring. 

 Two formula are possible: 



N_ 



"N 



and 



In the case of the first formula the quinoid ring is the chromo- 



_N— 



phore; in the case of the second formula the azin group itself, | , 



(see p. 17) is assumed to be the chromophore. The quinoid formula 

 is generally preferred today. 



Phenazin is weakly basic, but is not a dye as it does not contain 

 auxochrome groups. In other words, it is a chromogen. Either 

 an -OH group or one or more -NHo groups may be introduced to 

 give it dye properties. The acids and bases are very weak if there 

 is only one auxochrome group present, and their salts are readily 

 decomposed. For this reason some of them are of use as indicators. 

 Strong bases are encountered only among the safranins where basic 

 character is derived not only from the two -NH2 groups but also 

 from one of the azin nitrogen atoms which becomes pentavalent 

 and takes part in salt formation. 



a. 



Amido-azixs or Eurhodins 



If one or more amino groups are introduced into a phenazin, a 

 dye is formed of the class known as eurhodins. They are very weak 

 bases, and therefore weak dyes; but as their salts are readily de- 

 composed with a resulting color change, they form useful indicators. 

 The best known of the group is toluylene red, base : 



CH, 



CH3 



\ 

 / 



N 



NH2 

 _CH3 



53 



