ORDINARY STAINING-SOLVTIONS 169 



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 observing 

 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 examine the preparation 

 microscopically before the contrast-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 received 

 the contrast-color a great many other organisms will appear; 

 these take on the second color employed, while the tubercle 

 bacilli retain their original color. Before decolorization 

 all organisms in the preparation were of the same color, 

 bnt during the application of the decolorizing solution all 

 except the tubercle bacilli gave up their color. This micro- 

 chemical characteristic, together with other reactions to 

 be described, serves to differentiate the tubercle bacillus 

 from other organisms with which it might be confounded. 

 A number of different methods have been suggested for the 

 staining of tubercle bacilli, but the original method as 

 employed 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 occasional employ- 

 ment 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 number of the 



