264 DYEING 



orange G, is a sodium salt, the mixture must contain sodium and 

 chloride ions, methyl green cations, and the anions of orange G 

 and acid fuchsine, with perhaps also some undissociated neutral 

 dye. It is not possible to isolate methyl green/orange G and methyl 

 green/acid fuchsine from the Triacid solution as separate, dry 

 substances.^*^ 



When used for blood-smears fixed by heat (or less well by 

 alcohol/ether), the Triacid dye colours chromatin greenish with 

 methyl green, red blood-corpuscles orange with orange G, and 

 eosinophil granules in copper-colour by the action of both the acid 

 dyes; the neutrophil granules are dyed violet, which sharply dis- 

 tinguishes them from everything else in a blood-smear. (See fig. 

 28, A.) 



Ehrlich considered methyl green, methylene blue, and amethyst 

 violet (azine) the most suitable basic dyes for forming neutral 

 compounds. Among the acid dyes he preferred those that had more 

 than one sulphonic group, because they formed neutral dyes that 

 could be dissolved without much difficulty. 



Ehrlich's Triacid and similar fluids are useful in haematology, 

 but it is doubtful whether great interest would have been aroused 

 by neutral dyes had not methylene blue been tried in combination 

 with eosin. No one could have foreseen the extraordinary value 

 of this particular combination, or the complications that would 

 result from the association of two apparently commonplace dyes. 



Before 1891 the nucleus of the malarial parasite had never been 

 seen. A Russian protozoologist, Romanowsky,*^^ set out to tr^" to 

 dye it diflrerentially. Aware of Ehrlich's work on the use of neutral 

 dyes for the difl"erential colouring of blood, he tried a new com- 

 bination of the same sort. He added a 1% aqueous solution of 

 eosin to a saturated aqueous solution of methylene blue, apparently 

 until the acid dye was present in slight excess (in accordance with 

 Ehrlich's practice). On dyeing malarial blood-smears with this, he 

 found a remarkable range of colours in the blood-corpuscles. The 

 dyeing of the parasites was successful beyond any reasonable 

 expectation: for the cytoplasm of the trophozoite was Prussian 

 blue, the nucleus 'carmine-violet'. Thus was the nucleus of the 

 malarial parasite discovered. 



Nearly half a century later the attempt was still being made to 

 disclose the principles underlying Romanowsky's results, and 

 indeed, as we shall see, there is plenty of room for research today. 



