STAINING, PRACTICAL AND THEORETICAL 



aurantia or picric acid ; that is, if the basic dyes are united chem- 

 ically with acidic tissue elements. In such a case a dye like aurantia 

 or picric acid could not unite chemically with acidic tissue ele- 

 ments previously stained by the basic dye, because these two 

 dyes are themselves acid and they do not possess any chemical 

 groups which would confer amphoteric properties on their mole- 

 cules. Moreover, these two differentiating dyes can themselves be 

 removed from the tissues by alcohol. It would appear, then, that 

 the coloration of tissues by picric acid or aurantia is purely a 

 physical process. 



(c) A combination of both physical and chemical factors. In the 

 case of differentiation of safranin by picric acid it can easily be 

 shown in vitro that the latter combines with safranin to form a 

 compound, safranin picrate. If a test tube of aqueous picric acid 

 is taken and a few millilitres of i% aqueous safranin added a 

 precipitate of safranin picrate will form: this is insoluble in 

 aqueous picric acid. If another test tube is taken, but this time 

 of alcoholic picric acid, and the same amount of safranin added as 

 was added to the aqueous picric acid, a precipitate of safranin 

 picrate will again be formed: this is insoluble in alcoholic picric 

 acid. It appears reasonable, therefore, to suppose that the 

 differentiating action of picric acid, as far as safranin and other 

 synthetic basic dyes are concerned, is to attract the basic dye 

 away from the tissues, then combine with it to form an insoluble 

 neutral compound which is unable to attach itself to tissues and 

 is washed out in the histological processes that follow. It would 

 appear that this is a case of competition between the acidic 

 tissue elements and the acidic dye (picric acid or aurantia) for 

 possession of the basic dye which is more strongly attracted to the 

 acid dye than to the acidic tissue-elements. 



DECOLORIZING DYES 



As stated elsewhere in this book, certain strongly acid dyes 

 are capable of decolorizing certain tissue-elements that have been 

 stained with certain quinonoid (e.g. triarylmethane) acid dyes. 

 Not only are certain strongly acid dyes capable of decolorizing the 

 tissue-elements, but they have another action. That is, on the 

 quinonoid dye, which may also be decolorized by the strongly 

 acid dye. 



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