160 CHAPTER XI. 



In some staining processes, however, mordants are inten- 

 tionally resorted to in order to fix the stain. Mordanting has 

 long been employed in some haematein staining processes, such 

 as the iron-alum process of BENDA and M. HEIDENHAIN. More 

 lately it has been resorted to for staining with tar colours, as 

 in the curious t{ inversion " process of RAWITZ. It must be 

 admitted that mordants are in some cases of use by enabling 

 us to fix colouring matter in tissue elements that Avould 

 otherwise be rebellious to staining. And they have in some 

 cases the advantage of affording a very convenient means of 

 regressive staining. For it happens that the colour- com- 

 pounds thrown down in mordanted tissues are in many cases 

 specially soluble in an excess of the mordant ; so that the 

 solution of the mordant itself forms a very appropriate 

 decolourising agent. 



Recognising these advantages, it must still, I think, be said 

 that there seems to be some danger at the present moment 

 that the practice of employing mordants may degenerate into 

 an abuse. For surely the primary use and intention of an 

 histological stain (not of an industrial dye) is, that it should 

 select and r.eveal those elements of tissues that have a natural 

 affinity for its colouring matter. That end is attained in the 

 manner least open to objection by the use of substantive 

 stains, the natural affinities of the tissues and the colouring 

 matter here coming spontaneously and unconstrained into 

 play. Not so in the case of adjective staining. Here the 

 colour is, as it were, forcibly compelled into an unnatural union 

 with all or many of the elements of the tissue, including many 

 which have no natural affinity whatever for the colour. In 

 such preparations (e. g. in the " inversion " stain of RAWITZ) 

 the distinction between chromatic and achromatic elements 

 is obliterated ; and the interpretation of the images afforded 

 by them is open to more serious causes of error than in the 

 case of substantive stains. 



The following substances may be found to act usefully 

 after the manner of mordants, for enhancing* the resistance 

 of many tar colours to the alcohol employed for decolourising, 

 and for producing a stronger stain. 



Iodine : sections may be treated for a few minutes before 

 staining with tincture of iodine. 



Permanganate of potash : see HENNEGUY'S process. 



