MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 3 



forms for instance, the tubercle-bacillus, investigated by R. 

 Koch, could not be determined with certainty until such a 

 preparation had been made : and, indeed, staining is often 

 necessary in order to detect such bacilli. As an example of 

 the methods of staining, we will enter somewhat more closely 

 into the examination of the tubercle-bacillus^ which led to 

 one of the most important observations made in modern 

 science. Koch gave the following method for its examination : 

 The section of the tissue which contains the bacilli is 

 immersed for 24 hours in a mixture of 200 parts of distilled 

 water, 1 part of concentrated alcoholic solution of methylene 

 blue, and 0-2 part of a 10 per cent, potash solution. By 

 this treatment the section is stained dark blue, and is then 

 immersed, for a quarter of an hour, in a concentrated aqueous 

 solution of vesuvin. The section is now rinsed in distilled 

 water until the blue colour disappears, and a more or less 

 strong brown stain remains ; finally, the section is treated 

 with alcohol, mounted in clove oil, and examined. The cell- 

 nuclei and most species of micrococci are stained brown by 

 this treatment, whereas the tubercle-bacilli assume an intense 

 blue colour. (Of the known species of bacilli, only the 

 bacilli of leprosy behave in the same way; they differ, 

 however, in other respects from those of tuberclosis.) 

 According to Koch, this result depends on the alkaline reaction 

 of the staining solution, since these bacilli never take the 

 stain in acid or neutral solutions ; the neutral solution of 

 another colouring matter entirely removes the first stain, 

 except in the case of the tubercle-bacilli, which retain the 

 original staining. Subsequently, various other methods 

 were proposed for the identification of this micro-organism, 

 the most preferable of which is that of Ehrlich, who used 

 aniline instead of potash. Aniline is a faintly yellow, oily 

 liquid, the saturated aqueous solution of which has the 

 power of taking up more colouring matter than the solution 

 of potash. Ehrlich has also employed mineral acids for 

 decolourising, proceeding on the supposition that the tubercle- 



