234 DYEING 



Permeability 



The permeability of the various tissue-constituents plays a more 

 important role than chemical affinity in determining the differential 

 action of acid dyes in microtechnique. This fact was clear to the 

 genius of Ehrlich right back in 1879. 



It has already been mentioned (p. 193) that Ehrlich had dis- 

 covered a striking fact about the granules of certain leucocytes 

 (eosinophils), namely, that they have a special affinity for acid 

 dyes. These were what he called the 'a' granules. He observed 

 that the *p' granules of certain leucocytes of the rabbit were also 

 acidophil. Leucocytes of this second kind do not occur in man. 

 Ehrlich drew a distinction between those acid dyes (eosin among 

 them) that diffused rapidly, and those (such as nigrosine and in- 

 duline) that diffused slowly. The eosinophil leucocyte could most 

 easily be distinguished from that containing the p granules by 

 using eosin in a single solution with nigrosine or induline. He 

 found that the mixture showed the a granules in the colour of the 

 rapidly diffusing dye, and the p granules in that of the slowly 

 diffusing. It was for this reason that he named the former kind 

 eosinophil. 



'A consequence of this consideration', he wrote, 'is the hypo- 

 thesis that the a granulations are of closer texture (dichter) than 

 the p granulations; that is, that in the former the groups of 

 molecules {Micellen of Naegeli, Syntagmen of Pfeffer) are 

 larger and the intermicellar spaces smaller than in the latter. 

 . . . The molecules of the easily- diffusing eosin penetrate much 

 more quickly into the narrow osmo-regulatory spaces of the a 

 granulations than those of nigrosine, which diffuses with 

 difficulty; and so the micelles of the granulations are already 

 saturated with eosin before the second dyestuff can enter them 

 at all. In contrast to this, the molecules of nigrosine can enter 

 the wider intermicellar spaces of the p granulations and so 

 achieve an important colour-effect.' ^^^ 



Thus Ehrlich showed that while differential dyeing was in some 

 cases caused by the chemical differences between basic and acid 

 dyes, in others it was due to physical factors in dyes and objects. 

 At the time he was 24 or 25 years old (fig. 24, opposite p. 193). 



The differential action of the rapidly and slowly diffusing dyes 

 is reflected in their industrial use. The professional dyer classifies 



