SECTION ONE 



Light green units chemically with acid fuchsin to produce a 

 blue dye, trifalgic acid. This colour change is brought about 

 through the incorporation of resonance system of the acid fuchsin 

 with that of the light green. This gives the possibility of longer 

 resonance sequence of single and double bonds with consequent 

 deepening of the colour of both component dyes. When this 

 blue dye (trifalgic acid), is applied to tissues, partial ionization 

 takes place, and a proportion of the molecules of the trifalgic 

 acid are split into monofalgic and difalgic acids. These three 

 acids are then taken up by various types of tissue-elements to 

 give a polychrome picture. 



Acid fuchsin and light green belong to the same chromo- 

 phoric group. One might say that here the chromophore of each 

 of the two dyes is compatible with the other. 



When certain strongly acid dyes such as sun yellow, or orange G, 

 which do not belong to the triarylmethane group, are brought into 

 contact with acid fuchsin, under suitable conditions and in the 

 absence of tissues or other stainable material, they unite with the 

 acid fuchsin and bring about its decolorization. The reason 

 appears to be that the chromophore of the acid fuchsin is incom- 

 patible with that of the strongly acid dye whose chromophore is 

 of a different type. 



When the two dyes unite, the resonance sequence of double 

 and single bonds to which acid fuchsin owes its colour, is dis- 

 rupted. On the other hand, the decolorizing dye retains its colour 

 because its chromophore remains undisturbed by the union with 

 acid fuchsin. This type of decolorization is of the Schiff type. 

 Schiff's reagent (leuco basic fuchsin) is, of course, prepared by 

 the action of sulphurous acid on basic fuchsin. The sulphurous 

 acid is generally produced by the action of hydrochloric acid on 

 potassium metabisulphate (page 400), although in some labora- 

 tories SO2 is used instead. This gas is bubbled into the fuchsin 

 solution to form sulphurous acid which unites with the dye in 

 such a way as to disrupt the resonance system of the latter, thereby 

 bringing about its decolorization. Similarly sulphurous acid 

 acts on acid fuchsin and other acid and basic amino-triaryl- 

 methane dyes as well as many other quinonoid dyes. We can see 

 at a glance that the basic fuchsin (or acid fuchsin) has been 

 decolorized by the sulphurous acid, because the solution is 

 colourless, if it has been properly prepared. It is not so obvious in 



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