STAINING THE TUBERCLE BACILLUS. 149 



until the fluid begins to boil. After it has bubbled 

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

 excess of stain washed away in a stream of water, 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 decolorization 

 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 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 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 or 

 forty seconds, when it is again rinsed in water and ex- 

 amined microscopically. For the purpose of observing 

 the difference between the behavior of the tubercle 

 bacilli and the other organisms present in the prepara- 

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

 ine the preparation microscopically before the contrast- 

 stain is made, then remove it, give it the contrast-color, 

 and examine it again. It will be seen that before the 

 contrast-color has been given to the preparation the 

 tubercle bacilli will be the only stained objects to be 

 made out, and the preparation will appear devoid of 

 other organisms; but upon examining it after it has re- 

 ceived the contrast-color a great many other organisms 



