pared on even more scientific principles. This stain, known as the 

 tetrachrome blood stain, is prepared by mixing definite proportions 

 of methylene blue, methylene violet, methylene azure, and eosin. 

 When first proposed (1922) this stain was to be prepared with a 

 crude methylene azure, the pure product being at that time diffi- 

 cult to prepare and therefore expensive. MacNeal's latest work, 

 however (1925), shows a simple method of preparing methylene 

 azure A (asymmetric di-methyl-thionin), which is apparently the 

 important ingredient of Azure I, and he now specifies azure A in 

 the tetrachrome stain instead of the less definite product methylene 

 azure, as formerly. Azure A for this purpose can already be ob- 

 tained on the market in America. 



Some difficulty was experienced at first in compounding the 

 eosin-methylene-blue blood stains when imported dyes were no 

 longer available. In some cases these difficulties were probably 

 due to poor methylene blue or to poor eosin; but upon investiga- 

 tion the solvent, rather than the dyes themselves, has been found 

 to be most often at fault. As stated above, the precipitated com- 

 pound dye must be dissolved in methyl alcohol; but there are many 

 grades of methyl alcohol and not all are equally suitable for the 

 purpose. Apparently absolute purity is not needed; but two points 

 are very important; the methyl alcohol must be neutral in reac- 

 tion, and it must be free from acetone. In specifying a methyl 

 alcohol for the blood stains, these two properties should be in- 

 sisted upon. Very good methyl alcohol for this special purpose is 

 now on the market. 



Other Compound Stains 



Various other compounds of acid and basic dyes have been used 

 for special purposes. The basic dye employed in these compounds 

 is ordinarily methyl green or methylene blue; but sometimes basic 

 fuchsin, pyronin or rhodamine is used. The most commonly used 

 acid dyes are eosin, orange G and acid fuchsin; but certain others 

 are occasionally employed. Picric acid forms a few useful com- 

 pound dyes, rosanilin picrate (i.e., the compound of basic fuchsin 

 and picric acid) being especially well known as a tissue stain. 



The Pappenheim panoptic triacid stain is a modification of 

 Ehrlich's triacid compound. In this combination methylene blue 

 or methylene azure is substituted for methyl green. It is a tissue 

 stain of use in certain special technics. 



Ehrlich has proposed various other neutral stains, the best 

 known being a compound of acid fuchsin and methylene blue used 

 for staining blood; and a compound of narcein, an acid dye, with 

 two basic dyes pyronin and methyl green or methylene blue. 



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