176 PATHOGENIC BACTERIA. 



in 95 per cent, and absolute alcohol, cleared in xylol, 

 and mounted in Canada balsam. 



A method which has attained great and deserved praise 

 is Unna's. It is as follows: The sections are placed in 

 a dish of twenty- four-hours-old, newly-filtered Ehrlich's 

 solution, and allowed to remain twelve to twenty-four 

 hours at the room-temperature or one to two hours in 

 the incubator. From the stain they are placed in water, 

 where they remain for about ten minutes to wash. They 

 are next immersed in acid (20 per cent, nitric acid) for 

 about two minutes, and become greenish-black. From 

 the acid they are placed in absolute alcohol, and are 

 gently moved to and fro until the pale-blue color returns. 

 They are then washed in three or four changes of clean 

 water until they become almost colorless, and are then 

 removed to the slide by means of a section-lifter. The 

 water is absorbed with filter-paper, and then the slide is 

 heated over a Bunsen burner until the section becomes 

 shining, when it receives a drop of xylol balsam and a 

 cover-glass. 



It is said that sections stained in this manner do not 

 fade as quickly as those stained by Ehrlich's method. 



The tubercle bacillus also stains well by Gram's method, 

 but as this is a general method by which many different 

 bacteria are colored, it is ill adapted for purposes of differ- 

 entiation, especially when the prosecution of the charac- 

 teristic methods is not more difficult. 



So far as is known, the tubercle bacillus is a purely 

 parasitic organism. It has never been found except in 

 the bodies and excretions of animals affected with tuber- 

 culosis, and in dusts of which these are component parts. 

 This purely parasitic nature greatly interferes with the 

 isolation of the organism, which cannot be grown upon 

 the ordinary culture-media. Koch first achieved its arti- 

 ficial cultivation by the use of blood-serum. When 

 planted upon this medium the bacilli are first apparent 

 to the naked eye in about two weeks, and occur in the 

 form of small dry, whitish flakes, not unlike fragments 



