168 BACTERIOLOGY. 



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 asrain 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 

 received the contrast-color a great many other or- 

 ganisms will appear; these take on the second color 

 employed, while the tubercle bacilli retain their orig- 

 inal color. Before decolorization all organisms in the 

 preparation were of the same color, but during the appli- 

 cation of the decolorizing solution all except the tubercle 

 bacilli gave up their color. This micro-chemical charac- 

 teristic, together with reactions to be described, serves 

 to differentiate the tubercle bacillus from other organ- 

 isms with which it might be confounded. A number 

 of different methods have been suggested for the stain- 

 ing of tubercle bacilli, but the original method as em- 

 ployed by Koch is so satisfactory in its results that it is 

 not advisable to substitute others for it. The above 

 differs from the original Koch-Ehrlich method for the 

 staining of tubercle bacilli in sputum only in the occa- 

 sional employment of Ziehl's carbol-fuchsin solution 

 and in the method of heating the preparation with the 

 staining-fluid upon it. 



As Nuttall has pointed out, however, the strong acid 

 decolorizer used in this method can, with advantage, 



