114 Biological Stains 



violet in color than methylene blue itself but are more selective in 

 their staining action. It is indeed claimed (Scott and French, 

 1924b) that the dye owes much of its value as a nuclear stain to 

 the azures that are present, and that an extremely pure methylene 

 blue is not so satisfactory for such use. A later paper by Haynes 

 (1927) indicates the correctness of such conclusions. 



A methylene blue containing noticeably large proportions of 

 the lower homologs is called polychrome methylene blue. The 

 formation of these oxidation products may be hastened by boiling 

 with alkali, as in Unna's formula. 



Polychrome methylene blue is employed in many blood stains 

 such as Leishmann's and Wright's; in the latter a methylene 

 blue solution containing sodium bicarbonate is heated, and then 

 eosin is added. Eosin enters into chemical combination with the 

 basic dyes present, forming an insoluble compound which pre- 

 cipitates. This precipitate dissolved in methyl alcohol is Wright's 

 stain. (For a more detailed discussion of the subject see Chapter 

 XI). 



Jl5 METHYLENE VIOLET (Bemthseu) 



Methylene violet is formed whenever methylene blue is heated 

 with a fixed alkali or alkali carbonate. It is a feeble base with the 

 formula 



C14H12N2OS; Mol. Wt. 256.312 

 {Absorption maximum 579-581) 



Its preparation from methylene blue is more difficult than that of 

 azure A. A fair yield (30 to 40 per cent) may be obtained by 

 oxidizing methylene blue in dilute ammoniacal solution with 

 potassium chromate and then driving off the ammonia by boiling 

 with the addition of sodium carbonate. It may also be prepared 

 from azure A by boiling this with dilute alkali carbonate. Methy- 

 lene violet precipitates out as needle crystals, insoluble in water. 

 It may be recrystallized from dichlorethane (CH2CI • CH2CI) in 

 which it forms a deep carmine red solution. Although insoluble in 

 water when pure, methylene violet is soluble when mixed with 

 methylene blue or with the azures. It has a rather unusual shaped 

 absorption curve, as shown in graph 3, Fig. 11, p. 105. 



It plays an important part in the nuclear and granule staining 

 of the polychrome methylene blue stains. A definite quantity of 

 this dye is employed in the tetrachrome Iblood stain of MacNeal. 



