142 BACTERIOLOGY. 



ploy old solutions. Solutions older than fourteen days 

 should not be used. 



The three solutions just given may be used for cover- 

 glass preparations in the ordinary way. 



In some manipulations it becomes necessary to stain 

 the bacteria very intensely, so that they may retain their 

 color when exposed to the action of decolorizing agents. 

 These methods are usually employed when it is desirable 

 to deprive surrounding objects or tissues of their color, in 

 order that the stained bacteria may stand out in greater 

 contrast. It is in these cases that the staining solu- 

 tion with which the bacteria are being treated is to be 

 warmed, and in some cases boiled, so as to further 

 increase its penetrating action. When so treated, cer- 

 tain of the bacteria will retain their color, even when 

 exposed to very strong decolorizers. The tubercle 

 bacillus is characterized from all other bacteria, except 

 the bacillus of leprosy, by the tenacity with which it 

 retains it color when treated in this way. It is an 

 organism that is difficult to stain, but when once stained 

 is equally difficult to rob of its color. 



METHOD OF STAINING THE TUBERCLE BACILLUS. 

 Select from the sputum of a tuberculous subject one of 

 the small, white, cheesy masses which it is seen to con- 

 tain. Spread this upon a cover-slip and dry and fix 

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

 its edge with the forceps and the film covered with a 

 few drops of either the solution of Koch-Ehrlich or of 

 Ziehl. It is then held over the gas-flame, at first some 

 distance away, gradually being brought nearer, until the 

 fluid begins to boil. After it has bubbled up once or 

 twice it is removed from the flame, the excess of stain- 

 ing washed away in a stream of water, and it is then 



