166 BACTERIOLOGY 



at the time, until the solution has an opaque appearance, 

 the vessel containing the solution being thoroughly shaken 

 after each addition. It is then filtered through moistened 

 filter-paper until the filtrate is clear. To 100 c.c. of the 

 clear filtrate add 10 c.c. of absolute alcohol and 11 c.c. of 

 the concentrated alcoholic solution of either fuchsin, methyl- 

 ene-blue, or gentian-violet, preferably fuchsin or gentian- 

 violet. 



Ziehl's carbolrfuchsin solution: 



Distilled water . ... 100 c.c. 



Carbolic acid (crystallized) . . 5 grams 



Alcohol . . . 10 c.c. 



Fuchsin in substance .... 1 gram 



Or it may be prepared by adding to a 5 per cent, watery 

 solution of carbolic acid the saturated alcoholic solution of 

 fuchsin until a metallic lustre appears on the surface of the 

 fiuid. 



The Koch-Ehrlich solution decomposes after 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 further to increase its penetrating action. 



