THE STAINING OF BACTERIA. 51 



to the mordant, and those which produce alkalies require the 

 addition of an acid. 



2. Bunge's Method. 



Bunge's method, a modification of Loeffler's method, is as 

 follows : Prepare a saturated solution of tannin in water, and 

 also a 5 per cent, solution of sesquichloride of iron in distilled 

 water ; to 3 parts of the tannin solution add 1 part of the 

 iron solution. To 10 parts of such a mixture add 1 part of 

 concentrated watery solution of fuchsin. This mordant should 

 never be used fresh, but only after it has been exposed to the air 

 for several days. The cover-glass, thoroughly cleaned, is 

 covered over by this mordant for five minutes, after which 

 it is slightly warmed. It is then washed, dried, and stained 

 faintly with a little carbol-fuchsin. 



3. Pitfield's Method. 



The following solution is used as a mordant : 



Tannic acid (10 per cent, solution, 



filtered), 10 parts; 



Corrosive sublimate (saturated aque- 

 ous solution), 5 " ; 



Alum (saturated aqueous solution), 5 " ; 



Carbol-fuchsin, 5 " . 



Let this stand, and pour off the clear fluid. This mordant 

 will keep for one or two weeks. 



The staining fluid is prepared as follows : 



Alum (saturated aqueous solution), 10 parts; 

 Gentian-violet (saturated alcoholic 



solution), 2 " . 



This stain is to be prepared fresh every second or third day. 



The modus operandi is as follows : On an absolutely clean 

 cover-glass make a thin film as already described. Treat the 

 film with the mordant applied cold for twenty-four hours, or 



