SECTION ONE 



for a basic dye-ion. Also competition between a basic dye-ion 

 and basic tissue elements for acid dye-ions. 



Another series of compound dyes known to histologists consists 

 of mixtures of acid dyes of the Masson type. But these are not 

 chemical compounds. Then there are mixtures of basic dyes 

 such as methyl green-Pyronin (Pappenheim, 1901 ; Brachet, 1942, 

 1953; Kurnick, 1952, 1955): these also are mixtures, not chemical 

 compounds. 



Although it has long been known to histologists that a pair 

 of suitable dyes, one acid and the other basic, will unite to form 

 a chemical compound dye, it does not appear to have been known 

 until quite recently that certain acid dyes will unite chemically 

 with other acid dyes to produce water-soluble compound dyes 

 which produce polychromatic effects when applied to tissues. 

 Some of these dyes have been isolated in the dry form. However, 

 compound dyes of this type are dealt with elsewhere in this book 

 (pages 76-90) and readers requiring more detailed information 

 should consult the original papers cited above. 



Although it is possible to form chemical compound dyes by 

 the union of many basic dyes with many acid dyes, as well as by the 

 interaction of certain acid dyes with certain other acid dyes, it is 

 not possible to produce chemical compound dyes by the interaction 

 of any basic dye with any other basic dye. The reason for this is 

 that there are no basic dyes, as there are acid dyes, whose molecules 

 contain both reactive basic and acidic groups on the dye-ion itself. 



CLASSIFICATION OF DYES BY MOLECULAR WEIGHT 



It has been suggested (Gurr, i960) that, with certain exceptions, 

 the molecular weights of acid dyes influence their staining effects 

 on tissues. In the work referred to above, tables are given in 

 which some 430 dyes used in microtechnique are arranged in 

 order of molecular weight, irrespective of the dyestuffs groups to 

 which they belong. With the aid of these tables it was observed 

 that those dyes that had been recommended by various authors 

 over the years as satisfactory stains for collagen were acid dyes 

 having molecular weights over 700. Such dyes, it appeared, had 

 been found to be either poor or useless as cytoplasmic stains. It 

 was also noted that the best cytoplasmic stains were acid dyes 

 with molecular weights between about 350 and 590. During the 



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