STAINING THE TUBERCLE BACILLUS. 131 



METHOD OF STAINING THE TUBERCLE BACILLUS. 

 Select from the sputum of a tuberculous subject oue of 

 the small, white, cheesy masses which it is seen to con- 

 tain. Spread this upon a cover-slip and dry and fix 

 it in the usual way. The slip is now to be taken by 

 its edge with the forceps and the film covered with a 

 few drops of either the solution of Koch-Ehrlich or of 

 Ziehl. It is then held over the gas-flame, at first some 

 distance away, gradually being brought nearer, until 

 the fluid begins to boil. After it has bubbled up once or 

 twice it is removed from the flame, the excess of stain- 

 ing washed away in a stream of water, and it is then 

 immersed in a 30 per cent, solution of nitric acid in 

 water and allowed to remain there until all the color 

 has disappeared. In some cases this takes longer 

 than in others. One can always determine if decoloriza- 

 tion is complete by washing off the acid in a stream of 

 water. If the preparation is still quite colored it should 

 be again immersed in the acid ; if of only a very faint 

 color it may be dipped in alcohol, again washed off 

 in water, and may now be stained with some contrast 

 color. If, for example, the tubercle bacilli have been 

 stained with fuchsin, methylene-blue forms a'good con- 

 trast, stain. In making the contrast stain the steps in 

 the process are exactly those followed in the ordinary 

 staining of cover-slip preparations in general : the 

 slip containing the stained tubercle bacilli is rinsed 

 off carefully in water and a few drops of the methylene- 

 blue solution are placed upon it and allowed to remain 

 for thirty to forty seconds, when it is again rinsed in 

 water and examined microscopically. For the purpose 

 of observing the difference between the behavior of the 

 tubercle bacilli and the other organisms present in the 



