130 BACTERIOLOGY. 



ZiehUs carbolic-fuchsin solution : 



Distilled water 100 c.c. 



Carbolic acid (crystalline) ... 5 grammes. 

 Alcohol . . . . . .10 c.c. 



Fuchsin in substance .... 1 gramme. 



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 fluid. 



Both the Koch-Ehrlich and the Ziehl solutions 

 decompose very quickly after having been made, so 

 that it is better to prepare them when needed in small 

 quantities than to employ old solutions. Solutions older 

 than seven to nine 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 are usually employed for the purpose of depriv- 

 ing 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 djecolorizers. The tubercle 

 bacillus is characterized from all other bacteria, except 

 the bacillus of leprosy, 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. 



