194 DYEING 



Orcein is another amphoteric dye,*^^ much used in its basic state — 

 that is, in strongly acid solution — for the dyeing of chromosomes. ^^^ 

 Haematein is yet another amphoteric dye,*^^ but it is not used as 

 a direct dye in either its basic or acid character, as the colours it 

 produces are feeble and indefinite. The way in which this im- 

 portant dye is used in practice will be explained in chapter ii 

 (p. 207). 



BASIC DYE 



O 

 z 



o 



u- 2 



o 



>■ 



in 



UJ 



PH 



FIG. 25. Diagrammatic representation of the dyeing of collodion by 

 typical basic, amphoteric, and acid dyes. The ordinate is divided 

 into arbitrary units. In general conformity with the data of Seki.*^^ 



It was mentioned above that most tissue-constituents are shown 

 to be amphoteric if tested towards the extremes of pH, and their 

 reactions to dyes are therefore more complicated than those of 

 collodion, which remains negatively charged even in strongly acid 

 solution. More than half a century ago Bethe ^^ tried the efi'ects of 

 adding varying amounts of sodium hydroxide or sulphuric acid to 

 solutions of toluidine blue. He tried these solutions on various 

 mammalian tissues fixed in alcohol. Whereas everything was 

 colourable in alkaline solutions or at neutrality, there was a tend- 

 ency for the various constituents to fail to take the dye as more and 

 more acid solutions were used: one constituent after another dyed 

 more feebly or failed to dye at all. Cytoplasm fell oflP rapidly in 

 capacity to be coloured, chromatin much less rapidly and mucus 

 less rapidly still, while the ground substance of cartilage remained 

 as deeply dyed in the most acid solution tried as at neutrality. 



