THE STAINING OF BACTERIA. 45 



ing and drying a drop of cedar oil is put over the prepa- 

 ration, and the same is examined with the oil-immersion 

 objective without the use of a cover-glass. 



Air-bubbles are often caught between the balsam and the 

 cover-glass. Gentle uniform pressure begun at the centre of 

 the cover-glass and progressively applied toward its periphery 

 will ordinarily remove them. If this should fail, heat care- 

 fully applied until the balsam is quite soft will aid in the 

 riddance of the others. 



IV. The Special Methods of Staining. 



As stated above, this method may be applied for the stain- 

 ing of nearly all bacteria. Some, however, are not so easily 

 stained, and special methods must be resorted to to increase 

 the penetrating power of the dye. The most commonly used 

 will be here described. 



1. Loeffler's Method. 



In this method, instead of using the ordinary watery solu- 

 tion of an anilin dye, Loeffler's alkaline solution of methylene- 

 blue is used. This is prepared as follows : 



Concentrated alcoholic solution 



of methylene-blue, 30 parts; 



Caustic potash solution 



(1:10,000), 100 . 



Mix well and filter. 



This method stains well all the ordinary bacteria, but is 

 specially useful for the staining of the bacillus of diphtheria. 



2. Koch-Ehrlich's Method. 



Anilin water is prepared by adding a few drops of anilin 

 oil, drop by drop, to distilled water in a test-tube, shaking 

 well after the addition of each drop and until the liquid 

 assumes a milky appearance, after which it is filtered through 

 moistened filter-paper until the filtrate is absolutely clear. 



To 100 parts of this clear filtrate of anilin water, 10 parts 



