VARIATIONS OF B. TUBERCULOSIS. 353 



tubercle bacillus does not stain by the ordinary methods. 

 It possesses some peculiarity in its composition that 

 renders it proof against the simpler staining processes. 

 It is therefore necessary that more energetic and pene- 

 trating reagents than the ordinary watery solutions 

 should be employed. Experience has taught us that 

 certain substances not only increase the solubility of the 

 aniline dyes, but by their presence the penetration of 

 the coloring-agents is very much increased. Two of 

 these are aniline oil and carbolic acid. They are 

 employed in the solutions to about the point of satura- 

 tion. (For the methods of staining b. tuberculosis see 

 Chapter on Staining.) 



Under the influence of heat these solutions are seen 

 to stain all bacteria very intensely the tubercle bacilli 

 as well as other forms. If we subject our prepara- 

 tion, which may contain a mixture of tubercle bacilli 

 and other species, to the action of decolorizing-agents, 

 another peculiarity of the tubercle bacillus will be 

 observed. While all other organisms in the prepara- 

 tion give up their color and become invisible, the 

 tubercle bacillus retains it with marked tenacity. It 

 stains with great difficulty ; but once stained it retains 

 the color even under the action of strong decolorizing- 

 agents. 



VARIATIONS OF B. TUBERCULOSIS. Theobald Smith l 

 has called attention to certain very conspicuous differ- 

 ences that may be observed between the bacilli obtained 

 from human and those from bovine tuberculosis ; and in 

 a series of inoculation experiments Ravenel has shown 

 that for a large number of species tubercle bacilli of 

 bovine origin were constantly more virulent than those 

 from human sources. 



i Smith: Transactions of the Association of American Physicians, 

 1896, vol. xi, p. 275. 

 28 



