302 PATHOGENIC BACTERIA. 



specimen with strong alcohol, so that this bacillus may- 

 be decolorized. 



It is very difficult to find tubercle bacilli in the feces 

 because of the small number present. 



When the tubercle bacilli are to be sought for in sections 

 of tissue, considerable difficulty is at once encountered, 

 partly because of the thickness of the section and partly 

 because of the presence of nuclei which color intensely. 



Again, Ehrlich's method must be recommended as the 

 most certain and best method of staining a large number 

 of bacilli. 



The sections of tissue, if imbedded in celloidin or par- 

 affin, should be freed from the foreign substances. Like 

 the cover-glasses, they are placed in the stain for twelve 

 to twenty- four hours at a temperature of 37 ° C. Upon 

 removal they are allowed to lie in water for about ten 

 minutes to wash away the excess of stain and to soften 

 the tissue, which often shrinks and becomes brittle. The 

 washing in nitric acid (20 per cent.) which follows may 

 have to be continued for as long as two minutes. Thor- 

 ough washing in 60 per cent, alcohol follows, after which 

 the sections can be counter-stained, washed, dehydrated 

 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 



