1 o I PHAPTEU XT. 



combine with it directly. In this case the material is said to 

 be substanfively 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 a mordant (generally a metallic 

 salt or hydrate) before it will combine with the colouring 

 matter. These are known as adjectire colouring matters."* 



Mordants are bodies which have the property of com- 

 bining on the one hand with the elements of tissues and on 

 the other with the colouring principle of the dyes used, 

 forming with the latter insoluble coloured compounds (known 

 as Lakes), which are retained in the tissues. It follows that 

 basic dyes require mordants of an acid character and acid 

 dyes mordants of a basic character. The mordant may be 

 introduced into the tissues either before the dye or at the 

 same time with it. 



It may seem at first sight that the majority of histological 

 stains are obtained by substantive staining of the tissues. 

 But on reflection it will be seen 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. For 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, it must be ad- 

 mitted that these reagents play 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, picric acid, 

 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 bo 

 present unintentionally, or with imperfect realisation of its 

 import, in the staining solutions themselves. Such is un- 

 doubtedly the part played by alum in most of the stains in 



* For an excellent popular exposition of this subject see BENEDIKT 

 and KNECHT'S Chemistry of the Coal-tar Colours (George Bell and 

 Sons). 



