THE INDIRECT ATTACHMENT OF DYES TO TISSUES 223 



The attachment of the metal to phosphohpids has been discussed 

 in the part of the book that deals with fixation (p. 130). The 

 oxidizer seems to act, partly at least, upon the fraction of the haem- 

 atein that has been taken up by the tissues directly, not in the 

 form of a lake. This technique does not provide a typical example 

 of the action of an oxidizing differentiator on a lake. 



From what has been said it will be clear that a mordant is a salt, 

 the metal of which can combine with certain tissue-constituents 

 and can also be held in the chela-grip of certain acid dyes. It will 

 have suggested itself to the reader that a converse to a mordant 

 might exist, an acidic substance that could be taken up by certain 

 tissue-constituents and could also be linked to basic dyes. Such 

 'converse mordants' do in fact exist. Many basic dyes that are 

 familiar in microtechnique are used in this way in the textile 

 industry. Tannic acid is often used as intermediary between fibre 

 and basic dye. Cotton and linen have a remarkable power of taking 

 up this acid from aqueous solutions, and they can subsequently be 

 coloured by basic dyes, for which they have no direct affinity. The 

 compound between tannic acid and a basic dye has not, however, 

 the chemical complexity of a lake. The word 'mordant' loses some 

 of its meaning if used to include tannic acid and similar substances. 

 There is a suggestion of similarities that do not exist, even in 

 converse form. 



Tannic acid and similar intermediaries are only occasionally 

 used in this way in microtechnique. The reason is this. The tissues 

 consist as a general rule of an amphoteric background with acidic 

 (basiphil) substances (especially chromatin) distributed in it in 

 the form of separate objects. One first colours these separate 

 objects with a basic dye or lake, and then the background with an 

 acid dye. The value of a lake is that it is insoluble and does not 

 dissolve while the acid dye is acting. A converse to a lake would 

 only be useful if the separate objects in cells were basic (acidophil) 

 and we wished to dye them with an insoluble substance before 

 colouring the background with a basic dye. 



Tannic acid is occasionally used in quite a different way, to 

 prevent the escape of a basic dye that has already attached itself to 

 an acidic object. Any dye that began to dissolve out would at once 

 be precipitated. A blood-smear dyed with methylene blue/eosin, 

 for instance, may be treated with a solution of tannic acid,^^^ and 



