STAINING. 159 



202. Substantive and Adjective Staining ; Mordants. In the 

 industry of dyeing, colouring matters are divided into two 

 classes, according to their behaviour with respect to the 

 material to be dyed. Certain dyes are absorbed directly from 

 their solution by the material immersed therein, and combine 

 with it directly. In this case the material is said to be tfith- 

 tttantively dyed, and the colouring matter is called a substantive 

 colouring matter. 



Other dyes do not combine directly with the material to be 

 acted on, but this material must first be charged with some 

 substance known as mordant (generally a metallic salt or 

 hydrate) before it will combine with the colouring matter. 

 These are known as adjective colouring matters.* 



Animal tissues have in general a considerable affinity for 

 colouring matters, taking them up directly from their solutions. 

 In consequence, the majority of histological stains are obtained 

 by substantive staining of the tissues. Still, as has been 

 already pointed out, it seems probable that many of the 

 histological stains that are obtained without intentional 

 mordanting of the tissues, should yet in strictness be 

 attributed to the class of adjective stains. This would be 

 the case whenever there is reason to suppose that the stain 

 obtained results from a combination of the colouring matter 

 with some metallic salt or hydrate that is not a constituent 

 of the living tissue, but has been brought into it by the fixing 

 or hardening reagents, these reagents playing the part of 

 mordants though only intentionally employed for another 

 purpose. This would appear to be the case with the stains, 

 or some of them, obtained after fixation with corrosive 

 sublimate, alum, salts of iron, of platinum, of palladium, of 

 uranium, and, for certain tissue elements and certain colours, 

 chromium. And further, the mordanting substance may not 

 only be present unintentionally in the fixing or hardening 

 agents, it may be present unintentionally, or with imperfect 

 realisation of its import, in the staining solutions themselves. 

 Such is presumably the part played by alum in many of the 

 >tains in which it figures as an ingredient. Iodine also 

 plays in some staining processes a part which seems only 

 explicable on the supposition that it acts as a mordant. 



* For an excellent popular exposition of this subject see BENEDIKT and 

 KM IHT'S 'Chemistry of the Coal-tar Colours' (George Bell and Sons). 



