into the usual biological stains, and only these need be considered 

 here. They are as follows : 



BASIC CHROMOPHORES 



1. The azo group, — X = N — , which is found in all azo dyes, of 

 which methyl orange and Bismarck brown are well known ex- 

 amples. In all these dyes, a benzene ring is attached to each 

 nitrogen atom. All the dyes of this group may be looked upon as 

 derivatives of azobenzene, 



X = X 



\/ 



2. The azin group. 



N 



/j\ 



\l/ 



N 



which is found in phenazins. of which neutral red and the safranins 

 are good representatives. The skeleton formula of a safranin is: 



H,X X NHa 



X 



/ \ 



in which x represents the negative ion of a monobasic acid such as 

 hydrochloric, acetic, nitric or sulfuric. This chromophore is 

 capable of variety of rearrangements of its valency bonds, as the 

 bond between the two nitrogen atoms may disappear and the com- 

 pound assume a quinoid structure, as for example the following 

 grouping: 



H.X /\ X /\_XH 



3. The indamin group, — X = , as observed in the indamins, 

 thiazins, and so forth. Methylene blue is the best known repre- 

 sentative of this group. In these dyes, two benzene rings are at- 



17 



