associated with the presence of lower homologs, particularly the 

 dimethyl thionins. One of the dimethyl thionins is methylene 

 azure A; hence the statement of Scott and French is merely another 

 way of saying that methylene blue should be partially polychromed 

 in order to have its best staining powers. These lower homologs 

 are generally present to some extent in methylene blue as sug- 

 gested by the minor peak in the absorption curve at 610jLl^t. Scott 

 and French show that a methylene blue may be specially prepared 

 which contains more than usual of these lower homologs and that 

 it is better than the ordinary product for their purposes. How 

 widely their results can be applied to all microscopic uses of methy- 

 lene blue cannot be decided at present. It is interesting that one 

 particular instance has come to the attention of the Commission in 

 which a pure methylene blue was unsatisfactory for a certain 

 neurological procedure, but a crude textile methylene blue proved 

 satisfactory. This may possibly have been due to the presence of 

 other dyes in the impure sample. 



One serious bit of confusion has arisen from the designation 

 ^'methylene blue for bacilli" which was used on a certain type of 

 methylene blue imported before the war. This was the label 

 placed on a certain type of zinc salt, containing a small amount of 

 free chloride. Its designation seemed to imply that it was especially 

 adapted for staining bacteria; recent investigations indicate that 

 it should rather be considered not good enough for any other pur- 

 pose! Even for staining bacteria it is not especially satisfactory; 

 for the most common methylene blue solution of the bacteriologist 

 is the Loeffler formula, in which a certain amount of saturated al- 

 coholic solution is used as a stock. Now, since the zinc salt is 

 nearly insoluble in alcohol, such a stock solution contains little but 

 the free methylene blue chloride present. For all these reasons the 

 discontinuing of this grade of methylene blue is decidedly to be 

 recommended. 



METHYLENE AZURE* 



Synonyms: Azure I. Azure A. Azure B. 



This is one of the components of polychrome methylene blue, 

 first described by Bernthsen (1885). Definite knowledge of its 

 chemical nature was gained by Kehrmann (1906) and Bernthsen 

 (1906). The former showed that there are two azures, the asym- 

 metric dimethyl thionin. Azure A, and trimethyl thionin, Azure B. 

 The symmetrical dimethyl thionin, which he prepared, was found 

 to belong in a quite different category. 



*For the following statements in regard to methylene azure and methylene violet 

 the author is indebted to Dr. W. J. MacNeal. 



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