STAINING BACTERIA. 187 



slightly as to prevent their certain recognition 

 when isolated" (Koch). The aniline colors most 

 used are the methyl- violet, aniline-brown, fuchsin, 

 and methyl-blue. 



An aqueous solution of methyl-violet is perhaps 

 the most generally useful staining fluid ; and in 

 the violet ink sold by the stationers we have a 

 solution ready made, which answers every pur- 

 pose. It usually requires to be filtered. The 

 mode of operating is as follows : The fluid con- 

 taining the bacteria to be stained is spread in as 

 thin a layer as possible, and allowed to dry, upon 

 a thin glass cover. The drying may be hastened 

 by passing the cover-glass, held in forceps, through 

 the flame of an alcohol lamp. A drop or two of 

 the staining-fluid is then poured upon the cover- 

 glass, and after being left a short time is washed 

 away by a gentle stream of water, or by agitating 

 the cover in a glass of clean water. Usually one 

 or two minutes is sufficient time to ensure the 

 staining of the bacteria attached to the cover. 

 For immediate examination, it is now only neces- 

 sary to place the cover on a glass slide over a 

 little drop of distilled water. It is better, how- 

 ever, to support the margin of the cover by means 

 of a circle of white zinc cement, turned in the 

 centre of the slide. This prevents the bacteria 

 from being detached by contact with the slide. If 

 the object is to make a permanent preparation, a 

 drop of some preservative fluid is placed in the 

 shallow cell formed by the circle of cement. A 



