THE DIRECT ATTACHMENT OF DYES TO TISSUES I95 



Pischinger ^^-^ developed Bethe's study by using both basic and 

 acid dyes, instead of basic only. He showed that the effect of both 

 basic and acid dyes tended to fall off sharply towards a particular 

 pH, which he regarded as the iso-electric point of the object dyed. 

 Thus an acid dye would colour a particular object in the tissues in 

 strongly acid solutions, but would scarcely act above a certain 

 pH. A basic dye would act strongly on the same object in alkaline 

 solution, but the affinity between dye and object would fall and 



FIG. 26. Diagrammatic representation of the dyeing of gelatine by 

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

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



I.E. P., iso-electric point. 



nearly disappear at about the same pH as that at which the acid dye 

 failed to act. The position of the iso-electric points of tissue- 

 constituents may indeed be roughly estimated by Pischinger's 

 method, but different pairs of dyes do not give exactly the same 

 results, and Lison ^'^^ prefers to speak of the 'apparent iso-electric 

 point' when referring to information gained in this way. 



Instead of using a basic and an acid dye separately, one may mix 

 them together and judge the position of the iso-electric point of 

 any particular object in a microscopical preparation by noting the 

 pH at which the mixed colour is given. ^^^ Methylene blue and 

 eosin make a good pair for this purpose. 



Methods of this sort help in the histochemical identification of 

 certain tissue-constituents. Carboxyl groups soon lose their nega- 

 tive charge as the pH is lowered, then phosphoric groups, and the 



