THE STAINING OF BACTERIA. 51 



evenly upon it as few as possible of the bacteria to be exam- 

 ined. This is done by diluting with sterilized water a number 

 of times the culture containing them. The cover-glass is dried 

 and fixed in the ordinary way. The following solution, 

 known as a mordant, is then applied : 



Tannic acid (20 per cent, solution in 



water, filtered), 10 parts; 



Cold saturated solution of ferrous 



sulphate, filtered, 5 " ; 



Saturated alcoholic solution of fuchsin, 1 part. 



A few drops of it are placed on the film, and the cover-glass 

 taken up with a pair of forceps and held over the flame of a 

 Bunsen burner until the solution begins to steam, but not 

 allowing the boiling-point to be reached. It is next washed 

 rapidly in water, and then in absolute alcohol. The bacteria 

 are to be stained in anilin- water fuchsin solution in the ordi- 

 nary way. 



Practice has shown, however, that different bacteria behave 

 differently when exposed to this staining, and Locffler himself 

 has modified it to meet these requirements.* Having found 

 that the addition of an alkali favors the staining of flagella 

 in some of the bacteria, he has added to his stain 1 per cent, 

 of sodium hydrate. In other cases, having found that an acid 

 helps to bring out the flagella, he has added to his stain a 

 solution of sulphuric acid in water of such strength that 1 c.c. 

 will neutralize 1 c.c. of the sodium hydrate solution. 



,The following bacteria require an acid solution added to the 

 stain : Bacillus pyocyaneus, the spirillum of Asiatic cholera, 

 Spirillum rubrum, Spirillum concentricum, Spirillum Metch- 

 nikowi. 



The following bacteria require an alkaline addition to the 

 staining solution : Bacillus mesentericus, Micrococcus agUis, 

 Bacillus typhosus, Bacillus subtilisj bacillus of malignant 

 oedema, Bacillus anthracis symptomatic /. 



In a general way one may say that bacteria that produce acid 

 in the media in which they grow require the addition of an alkali 



