192 TECHNOLOGY OF BACTERIA. 



cipitation commences. The cover-glass is allowed 

 to float upon this mixture, which may be con- 

 veniently prepared in a watch-glass, for fifteen 

 minutes to half an hour; the side upon which 

 the sputum has been spread is, of course, placed 

 in contact with the staining fluid. The cover is 

 then washed for a few seconds in a strong solu- 

 tion of nitric acid (one part of the commercial 

 acid to two parts of distilled water). After this 

 it must be thoroughly washed in pure water. 

 By this process the stain is removed from every- 

 thing but the tubercle bacilli, which retain the 

 color imparted to them by the first operation. 

 The ground-substance may now be stained so as 

 to give a strong contrast with the bacilli ; brown 

 if the bacilli are violet, or blue if they have been 

 stained red with fuchsin. 



Gills Meihod. The following method of stain- 

 ing the tubercle-bacillus is recommended by Dr. 

 Gibbs, of King's College, London : 



" The great advantage consists in doing away with 

 the use of nitric acid. The stain is made as follows : 

 Take of rosanilin hydrochloride two grammes, methyl 

 blue one gramme ; rub them up in a glass mortar. Then 

 dissolve aniline oil 3 c. c. in rectified spirit 15 c. c. ; add 

 the spirit slowly to the stains until all is dissolved, then 

 slowly add distilled water 15 c. c. ; keep in a stoppered 

 bottle. To use the stain: The sputum having been 

 dried on the cover-glass in the usual manner, a few 

 drops of the stain are poured into a test-tube and 



