compounds. By a rearrangement of the atoms such a compound 

 is sure to change to a quinoid form, thus : 



OH HO 



I I il 



_N = N_/\ /\_N=.N_/\ 



A compound of this latter structure cannot form salts and does not 

 act as an ordinary dye. It does, however, prove to be soluble in 

 oil and is able to color it by an apparently physical process. Hence 

 the azo-ortho-phenols, or azo-beta-naphthols, like Sudan III and 

 Sudan IV, 



N = N CH3 



N = N 



and 



H3C. 



are important fat staining dyes. 



ORANGE G. C. I. NO. 27 



Synonym: Wool orange 2G. 

 Slightly different grade: Orange GG, GMP. 



S03Na 



/\_N = N 



I I ; 



HO" 



SOjXa 



(An acid dye; absorption maximum about Jf.85) 



This dye is strongly acid because of the two sulphonic groups. 

 It is one of the most valuable plasma stains in histological work. 

 It has great use as a background stain for haematoxylin and other 

 nuclear dyes in cytology. It is frequently employed, both by 

 botanists and zoologists, as a cytoplasmic stain, together with the 

 two nuclear dyes safranin and gential violet in the Flemming triple 

 stain. It is of importance to the pathologist for its use with anilin 

 blue and acid fuchsin in the Mallory connective tissue stain; and 

 is used in various other double and triple staining methods, such 

 as that of Ehrlich-Biondi-Heidenhain, in which it is mixed with 

 methyl green and acid fuchsin. The Erhlich "triacid mixture," 

 also a combination of these same three dyes, is used in staining 

 blood. A further use is Bensley's "neutral gentian," a combination 

 of orange G and gential violet for staining the islands of Langer- 

 hans. 



34 



