METHODS OF STAINING. 425 



teria differ greatly, and the method is commonly employed in bac- 

 teriological work to distinguish those species which retain the stain, 

 or are " positive to Gram," from those which are decolorized or 

 " negative to Gram." 



11. Van Giesson's Picric Acid and Acid Fuchsin Stain. 



Aqueous solution of acid fuchsin, 1 per cent., 5 cc. 



Saturated aqueous solution of picric acid, 100 " 



This solution serves to stain fibrous intercellular substances. It 

 is used in the following manner : 



1. Slightly overstain with alum hsematoxylin; e. g., Bohmer's 

 hsematoxylin. 



2. Wash thoroughly in distilled water. 



3. Stain in Van Giesson's dye for five minutes. 



4. Wash in water. 



5. Dehydrate in 95 per cent, alcohol. 



6. Clear in oil of origanum. 



7. Mount in xylol-balsam or xylol-dammar. 



The tissues should have been fixed in a corrosive-sublimate solu- 

 tion or one containing chromates ; e. g., Muller's fluid, Zenker's 

 fluid, or sublimate solution. The connective-tissue fibres are stained 

 red by the acid fuchsin. The reason for overstating with hsema- 

 toxylin is that subsequent treatment with picric acid discharges 

 some of that color. 



12. Benda's Iron-hsematoxylin Stain. This is a powerful stain 

 well adapted to the staining of paraffin-sections that have been 

 affixed to cover-glasses. It stains nuclei and intercellular sub- 

 stances, as well as the protoplasm of cells, various shades of gray, 

 and the color is very permanent. The outline of the method is as 

 follows : 



1. Mordant the sections (after affixing to cover-glasses, if that 

 method is used) in a mixture of equal parts of liquor ferri sul- 

 furici oxydati of the German Pharmacopoeia and distilled water for 

 twenty-four hours. 



2. Rinse in distilled water, and then wash in three changes of 

 tap-water. 



3. Stain in aqueous solution of haematoxylin, prepared by mix- 

 ing 10 drops of a concentrated alcoholic solution of the crystals 

 with 10 cc. of distilled water. Stain for from one-half to twenty- 

 four hours. 



