ANIMALS SUSCEPTIBLE TO TUBERCULOSIS. 307 



in water, and treat the preparation with a saturated 

 solution of methylene-blue in alcohol. If the ques- 

 tionable organism is the tubercle bacillus, it retains its 

 red color; if the smegma bacillus, the red color is dis- 

 solved out by the alcohol and the organism becomes 

 stained blue. 



The differential diagnosis between the tubercle bacil- 

 lus and the lepra bacillus is less satisfactory ; they both 

 take on the same stains and both retain them or give 

 them up under treatment with the same decolorizers. 

 The results of investigations, however, indicate differ- 

 ences in the rate of staining and decolorization, and it is 

 accepted by many of those who have compared the two 

 organisms that the lepra bacillus takes up stain very 

 much more readily than does the tubercle bacillus, 

 often staining perfectly by an exposure of only a few 

 minutes to cold watery solutions of the dyes; but when 

 once stained it retains its color much more tenaciously 

 when acted upon by decolorizing-agents than does the 

 latter organism. 



According to Baumgarten, the lepra bacillus is stained 

 by an exposure of six to seven minutes to a cold, satu- 

 rated watery solution of fuchsin, and retains the stain 

 when subsequently treated with acid alcohol (nitric 

 acid, 1 part; alcohol, 10 parts). By similar treatment 

 for the same length of time the bacillus tuberculosis 

 does not ordinarily become stained. 



These points, particularly what has been said with 

 reference to the smegma bacillus and the bacillus of 

 syphilis, are of much practical importance, and should 

 always be borne in mind in connection with microscopic 

 examination of materials to which these organisms are 

 liable to gain access. It is hardly necessary to say that 



