244 THE AMERICAN MONTHLY TSep 



ed by Ehrlich, who, instead of an alkaline solution of the 

 primary stain, used one of the basic dyes, Gentian Vio- 

 let, or Fuchsin, in a solution of anilin. Ziehl Neelsen still 

 further modified the process by using a carbolized solu- 

 tion of basic Fuchsin, and, it is this last method which is 

 in general use to-day. It will be noticed that the dyes, in 

 that they all belong to the basic group, are the same, the 

 only difference being in the vehicle in which they are dis- 

 solved ; the importance attached to the choice of the ve- 

 hicle will best be appreciated by reference to the process 

 of ordinary industrial dyeing, in which a great number 

 of colors directly stain the material immersed, while others 

 require the presence of some substance, generally a me- 

 tallic salt, or hydrate, technically known as a mordant, 

 before they can be made to give a satisfactory result ; the 

 same holds good in microscopic staining, and the Tuber- 

 cle bacillus, although stained slowly by an aqueous, or al- 

 coholic solution of one of the basic colors, stains much, 

 more rapidly when an alkaline, carbolized, or anilin solu 

 tion is used ; these substances serving as mordants. 



These bacilli, being thus difficult to stain, are equally 

 difficult to decolorize, and, while immersion for a short 

 time in acid or alcohol will effectually decolorize the other 

 elements of the preparation, they still retain their color, 

 and remain unaffected, even when the decolorized ele- 

 ments are again stained with some contrast stain, as in 

 Koch's original demonstration, in which the bacilli were 

 stained blue, the epithelial cells, etc., brown. 



This discovery, opening up as it did the science of Bac- 

 teriology, is one of the greatest triumphs of latter-day 

 staining methods. The country folk in many districts, es- 

 pecially Naples, considered Tuberculosis an infectious dis- 

 ease : already Willemin, of Paris, had proved the same 

 by the vaccination of experimental animals, but efforts 

 hitherto made to demonstrate a disease germ had failed, 

 until Koch succeeded with the above mentioned stain. 



