STAIN TECHNIQUE 15 



Direct dyes, Colour Index Numbers. 



327. Direct Fast Scarlet 4BS. 



370. Congo Red. 



518. Niagara Sky Blue 6B. 



520. Niagara Sky Blue. 



620. Sun Yellow. 



957. Sulfur Brilliant Blue. 

 1006. Sulfur Green. 

 1012. Sulfur Bordeaux. 



The principal radicals of direct dyes are - NH 2 and - OH, the 

 auxochromes that characterize the basic and acid dyes respectively. 

 A direct dye is applied in neutral solutions containing sodium sul- 

 fate, in ^ per cent acetic acid, or in alkaline solutions. Direct dyes 

 containing alkalies cannot be used on paraffin sections but they are 

 often valuable for the coloring of minute objects mounted entire in 

 glycerin. Solutions of Direct Fast Scarlet 4BS, Niagara Sky 

 Blue, and Sun Yellow should contain both acetic acid and sodium 

 sulfate as chemical assistants. Because Congo Red changes from 

 red to blue in the presence of an acid, only sodium sulfate is used 

 in the stain. The direct dyes provide contrast colors with acid 

 dyes in the staining of animal tissues. Direct dyes are similar to 

 acid dyes and stain the proteins of connective tissues, muscles, 

 blood corpuscles, fungous mycelia, and parenchyma. 



Sulfur dyes, which are direct dyes containing sulfur, are soluble 

 in alkaline solutions of sodium sulfide. They are especially suited 

 to the staining of blue-green algae. These pigments are insoluble 

 in glycerin, whereas the colors of most acid and basic dyes are ex- 

 tracted by this medium. Since the sulfur dyes are decolorized by 

 alcohol or acids they are not used on paraffin sections. 

 Mordant dyes, Colour Index Numbers. 



781. Gallein (acid). 



883. Gallocyanine (direct). 

 1049. Acid Alizarin Green G (acid). 

 1051. Alizarin Cyanine G (direct). 

 1239. Carmine (acid). 

 1246. Haematoxylin (acid). 

 The behavior of mordant dyes parallels the differences noted 



between acid and direct dyes. The acid mordant dyes stain tissues 



