Methylene blue is perhaps the stain which the pathologist and 

 bacteriologist would have the greatest difficulty in doing without, 

 and it is of great value to the zoologist as well. It is employed for 

 a greater variety of purposes than any other biological stain except 

 possibly haematoxylin; and for this reason was the first dye to be 

 given a thoro investigation by the Commission. It is used: first, 

 as a nuclear stain in histology, for which purpose its strongly basic 

 character as well as the ease with which it can be applied without 

 over-staining, make it quite valuable; secondly, as a bacterial 

 stain, notably in milk work and in the diagnosis of diphtheria, 

 where it is especially useful because it has an affinity for the bac- 

 terial protoplasm as great as that of the rosanilin dyes, but is less 

 intense, more selective in its action and more subject to differen- 

 tiation; thirdly in the vital staining of nervous tissue, where a non- 

 toxic, basic dye is needed; fourth, in combination with eosin in the 

 blood stains, thanks to the ease with which it can be partly con- 

 verted into other dves like methvlene violet and methvlene azure, 

 and thus acquire polychrome properties; and lastly as an indicator 

 in the Levine eosin-methylene-blue medium for differentiating the 

 colon and aerogenes organisms. 



The polychrome properties just mentioned are quite likely to 

 develop in a methylene blue solution upon standing. Anyone who 

 has had much experience with the stain is familiar with the oc- 

 casional green tones from methylene green, the reddish shades of 

 methylene azure (azure I) and methylene violet. Such a solution 

 is known as "polychrome methylene blue." Its formation is 

 hastened by boiling with alkali. In preparing blood stains the 

 methylene blue solution is treated for this purpose with sodium 

 carbonate, and then eosin is added, which enters into chemical 

 combination with the other dyes present, inasmuch as eosin is an 

 acid dye while methylene blue and its derivatives are basic. The 

 combination of eosin and methylene blue is often spoken of as the 

 eosinate of methylene blue. (For a more detailed discussion of the 

 subject see Chapter VIII.) 



It can be readily understood that an especially pure product is 

 needed when the dye is to be used for vital staining or in blood 

 work. For vital staining the U. S. P. zinc-free dye is always rec- 

 ommended, sometimes with even further purification; altho the 

 recent investigations carried on by the Commission indicate that 

 the U. S. P. product is sufficiently pure. For blood work there is 

 frequently recommended a "methylene blue rectified for blood 

 stains." This grade, however, is generally less pure than the 

 medicinal or U. S. P. grade, and there seems no reason for specify- 

 ing it. The same is true of various other grades such as those 

 denoted BX, BG, etc., which are ordinarily purer than the textile 

 dye, but less pure than the medicinal grade. 



In a recent paper by Scott and French (IQ'^-tb) it is claimed that 

 the specially desired staining properties of methylene blue are 



47 



