STAINING THE TUBERCLE BACILLUS. 179 



as has been said, when this action is accompanied by 

 the decolorizing-properties of alcohol. 



The acid solutions commonly employed are : 



Acetic acid in from 0.1 to 5 per cent, watery solution. 



Nitric acid in from 20 to 30 per cent, watery solu- 

 tion. 



Sulphuric acid in from 5 to 10 per cent, solution in 

 water. 



Hydrochloric acid in from 1 to 3 per cent, sqlution in 

 alcohol. 



NOTE. For details as to the technique of hardening 

 and cutting .sections and staining bacteria in tissues, the 

 student is referred to Mallory and Wright's Pathological 

 Technique. 



METHOD or STAINING THE TUBERCLE BACILLUS. 

 Select from the sputum of a tuberculous subject one 

 of the small, white, cheesy masses which it is seen 

 to contain. Spread this upon a cover-slip, dry and 

 fix it in the usual way. The slip is now to be taken 

 by its edge with forceps and the film covered with a 

 few drops of either the solution of Koch-Ehrlich or 

 that of Ziehl. It is then held over a gas-flame, at first 

 some distance away, gradually being brought nearer 

 until the fluid begins to boil. After it has bubbled 

 once or twice it is removed from the flame, the excess 

 of stain washed away in a stream of water, then im- 

 mersed in a 30 per cent, solution of nitric acid in 

 water, and allowed to remain until all color has dis- 

 ap}>eared. This takes longer in some cases than in 

 others. One can always determine if decolorization is 

 complete by washing off the acid in a stream of water. 

 If the preparation is still distinctly colored, it should 

 be again immersed in the acid ; if of only a very faint 



