150 BACTERIOLOGY. 



will now appear; these will take on the second color 

 employed, while the tubercle bacilli will retain their 

 original color. Before decolorization all organisms in 

 the preparation were of the same color, but during the 

 application of the decolorizing solution all except the 

 tubercle bacilli gave up their color. This characteristic, 

 together with reactions to be described, as said, 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, be 

 replaced by much more dilute solutions, as a certain 

 number of the bacilli are entirely decolorized by the too 

 energetic action of the strong acids. He recommends 

 the following method of decolorization: after staining 

 the slip or section in the usual way, pass it through 

 three alcohols; it is then to be washed out in a solution 

 composed of 



Water ... .160 c.c. 



Alcohol ... . . 50 c.c. 



Concentrated sulphuric acid . 20 to 30 drops. 



From this it is removed to water and carefully rinsed. 

 The remaining steps in the process are the same as those 

 given in the other methods. 



