THE BLOOD DYES 263 



orange G. He discovered that the addition of a small quantity of 

 the acid dye in excess allowed the neutral dye to dissolve. 



Such neutral dyes were used by Ehrlich for colouring blood- 

 smears. They were found to be particularly good for bringing out 

 the basiphil constituents in one colour and the acidophil in another, 

 and thus making it easy to distinguish the different kinds of leuco- 

 cytes. Ehrlich attributed this to the fact that the dyeing of the two 

 kinds of constituents was simultaneous instead of successive. The 

 dyeing cations and anions are present in almost exactly equal 

 numbers, and this circumstance must give better indications of 

 basiphilia and acidophilia than any arbitrary mixture or succession 

 of dyes. In addition, the granules of polymorphonuclear leuco- 

 cytes are coloured by both the components of the neutral dye, and 

 that is why Ehrlich called them 'neutrophil'. He regarded the 

 specific dyeing of these granules as an important property of the 

 neutral dves, not to be obtained without their use. 



Methyl green is unusual among triarylmethane dyes in possess- 

 ing two positively charged nitrogen atoms. Orange G possesses two 

 negatively charged sulphonate groups. Thus one molecule of the 

 basic dye could react with one molecule of orange G. Ehrlich, 

 however, was under the mistaken impression that all three nitrogen 

 atoms of methyl green combined with acid dye-radicles when a 

 neutral dye was prepared. He therefore gave the misleading name 

 'Triacid' to the best-known of his neutral dyes, to emphasize his 



CH 



H.Cs 



3 



CHo HqC\ /OH 



+N N 



cr 



c 



II 



N + 



H3C Cri3 CI 



Methyl green 



belief. The solution w^as made by adding aqueous methyl green 

 solution to a mixture of orange G and acid fuchsine in aqueous 

 solution, and then at once adding alcohol and glycerine to prevent 

 precipitation of the complex neutral dye. Since acid fuchsine, like 



