148 BACTERIOLOGY. 



The Koch-Ehrlich solution decomposes after having 

 been made for a time, so that it is better to prepare it 

 fresh in small quantities when needed than to employ 

 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-solution 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, certain of the bacteria will retain their color, 

 even when exposed to very strong decolorizers. The 

 tubercle bacillus is distinguished from the great ma- 

 jority of other bacteria by the tenacity with which it 

 retains its 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 

 contain. 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 

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

 some distance away, gradually being brought nearer, 



