THE CAUSES OF DIFFERENTIAL DYEING 



103 



certain tissue-constituents have lost the colour while others 

 retain it. During this regressive process the parts become more 

 distinct from one another than they were when all parts held too 

 much of the dye. The regressive stage of the process is therefore 

 called differentiation. 



Cationic dyes are frequently used regressively. It is customary 

 to use acids as differentiating agents. They act, in principle, in 

 exactly the same way as when they are mixed with the dye: 

 reliance is placed on the ability of the acid to render the am- 

 photeric tissue-constituents basic, and thus make them let go of 

 the cationic (basic) dye. Alkalis can be used in the same way to 

 differentiate acid dyes. 



An alternative way of differentiating is to soak the tissue in a 

 fluid in which the tendency of the dye to remain ionized is 

 reduced or eliminated, but in which the dye is soluble. Ethanol, 

 either absolute or in strong aqueous solution, is often used for this 

 purpose. The dye is rapidly lost from those tissue-constituents 

 that hold little of it. If differentiation be stopped as soon as these 

 constituents appear colourless, the objects that still retain a 

 considerable amount of the dye will contrast strongly with them. 

 The tissue must now be brought quickly into a fluid, such as 

 xylene, in which the dye is insoluble. 



Acidic tissue-constituents, lacking any basic groups, can in 

 certain circumstances be dyed by anionic (acid) dyes. Thus 

 methyl blue, an anionic triarylmethane dye, will colour cellulose 

 cell-walls. Indeed, it is used in the textile industry for dyeing 

 cotton, and is often called cotton blue. The structural formula 

 for cellulose shows the absence of any group that would be 



H OH H.COH 



O-i 



r-O 



H^COH H 



Part of a cellulose molecule 



OH 



