MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 3 



for instance, the tubercle bacillus, investigated by EGBERT 

 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 a concentrated alcoholic solution of methylene blue, and 0*2 

 part of a 1 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 vesuvine. 

 The section is now rinsed in distilled water until the blue 

 colour disappears, and a more or less intense 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 tuberculosis.) 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 decolour- 

 ising, proceeding on the supposition that the tubercle bacilli 

 are surrounded by a cell- wall which is only permeable by 

 alkaline liquids. Therefore, when the bacilli, cell-nuclei, 

 plasma, etc., are stained by the alkaline solution, and the first- 

 named are consequently practically indistinguishable in the 



