CHAPTER 9 



The Action of Mordants 



up till now we have been concerned with the direct attachment of 

 dyes to tissue-constituents. In some of the most important pro- 

 cesses of dyeing, however, an intermediary or mordant stands 

 between the dye and the tissue. The dye attaches itself to the 

 mordant: the latter (as its name suggests) 'eats into' or grips the 

 tissue. 



The great advantage of mordanting is that the colour is not 

 removable by neutral fluids, whether aqueous or alcoholic. One 

 may therefore colour progressively or regressively until the tissue 

 is properly displayed, and then dehydrate at leisure, or counter- 

 stain as desired. The fact that dehydration may be done as slowly 

 as one pleases is particularly helpful in making whole mounts. As 

 we shall see, it is chiefly chromatin that is coloured by most 

 mordant dyes. Nuclei are crowded together in the epithelial parts 

 of many organs, but are sparse in the connective tissues ; and there 

 may be cavities from which they are absent. Thus the dyeing of 

 chromatin gives clear micro-anatomical pictures in whole mounts, 

 even with quite low powers of the microscope. 



The fastness of mordant dyes makes them suitable for use in 

 aqueous mounts, provided that acidity is avoided. 



All dyes that act with mordants can also be used without them, 

 but if so their efl"ects are quite diff"erent. By no means all dyes can 

 be used with mordants. The chief ones so used in microtechnique 

 are carminic acid and haematein. The chief mordants used with 

 them are salts of aluminium and of ferric iron. 



In the textile industry the chief dyes used with mordants are azo 

 dyes, and the chief mordants are complex basic salts of chromium. 

 The chemistry of the mordanting of azo dyes by chromium has 



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