162 BACTERIOLOGY. 



some distance away, gradually being brought nearer 

 until the fluid begins to boil. After it has bubbled 

 once or twice it is removed from the flame, the excess 

 of stain washed away in a stream of water, then im- 

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

 water, and allowed to remain until all color has dis- 

 appeared. This takes longer in some cases than in 

 others. One can always determine if decolorization is 

 complete by washing off the acid in a stream of water. 

 If the preparation is still distinctly colored, it should 

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

 color, it may be dipped in alcohol, again washed in 

 water, and stained with some contrast-color. If, for 

 example, the tubercle bacilli have been stained with 

 fuchsin, methylene-blue forms a good contrast-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 contain- 

 ing the stained tubercle bacilli is carefully rinsed in 

 water, and a few drops of the methylene-blue solution 

 placed upon it and allowed to remain for thirty or 

 forty seconds, when it is again rinsed in water and 

 examined microscopically. For this purpose of observ- 

 ing the difference in behavior of the tubercle bacilli 

 and the other organisms present in the preparation 

 toward this method of staining, it is well to exam- 

 ine the preparation microscopically before the con- 

 trast-stain is made ; then give it the contrast-color, 

 and again examine. It will be seen that before the 

 contrast-color has been given to the preparation the 

 tubercle bacilli are the only stained objects to be 

 made out, and the preparation appears devoid of 

 other organisms ; but upon examining it after it has 



