STAINING, PRACTICAL AND THEORETICAL 



the case of acid fuchsin decolorized by an acid dye, because the 

 decolorizing agent, unlike that used for preparing Schiff's reagent, 

 is itself coloured; and it imparts its colour to the final solution 

 which contains leuco acid fuchsin united with the decolorizing 

 dye. So long as this compound of leuco acid fuchsin and the other 

 acid dye sun yellow, for instance, remains out of contact with 

 tissues or other stainable material, it holds together, retaining the 

 original colour of the decolorizing dye. If, however, the solution is 

 spotted on filter paper, at first the spot is the same colour as the 

 solution. After a few seconds a colourless fluid exudes from the 

 coloured spot and encircles the spot, spreading outwards. The 

 outer periphery, that is the edge of the zone farthest away from the 

 coloured spot, suddenly changes from colourless to red ; the same 

 colour as that of acid fuchsin. This red coloration then advances 

 inward towards the central spot (which is blue in the case of dianil 

 blue or violet-blue in the case of violamine, or deep yellow in the 

 case of sun yellow G) until the whole of the zone previously 

 occupied by a colourless liquid is coloured red. This may be 

 taken to indicate that the colourless liquid which came out of 

 the central coloured zone was in fact leuco acid fuchsin. This 

 has been confirmed by chemical tests. The behaviour of the 

 compound on filter paper would indicate that the union between 

 the decolorizing dye and the acid fuchsin while strong enough to 

 hold in the absence of competition from tissues or other stainable 

 material (e.g. cellulose) is not strong enough when the acid 

 fuchsin is given an opportunity to escape from the decolorizing 

 dye, as it does on filter paper. It appears that in breaking away 

 from the sun yellow as leuco acid fuchsin, and once out of reach 

 of the decolorizing dye, its colour is restored by atmospheric oxi- 

 dation. The restoration of the red colour of the leuco fuchsin band 

 on filter paper is hastened and intensified by the application of 

 heat. A leuco acid fuchsin of this type might possibly find applica- 

 tion in histochemistry or in biochemistry. Wherever the decoloriz- 

 ing dye can reach the acid fuchsin in tissues, the latter dye will be 

 decolorized. 



Button (1928) states that nigrosin in aqueous solution acts on 

 eosin (of Wright's stain) as a very powerful decolorizer. The dye 

 said to be decolorized in this case (eosin Y) is a carboxylated 

 (acid) dye, devoid of sulphonic groups. It contains no basic 

 groups whatsoever and it does not, therefore, possess any 



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