l/U 



19 



STAINING OF BACTERIA. 



Aniline dyes are commonly employed for this purpose, 

 and those which are used most frequently are fuchsine, 

 gentian violet, methyl violet, methylene blue, and vesuvin 

 or Bismarck brown. The first two are quite permanent, 

 stain rapidly and deeply and as a rule are to be preferred. 

 Methylene blue stains slowly and is excellent for special 

 purposes. 



Saturated solutions in absolute or strong alcohol are 

 first prepared and serve as stock. These concentrated 

 solutions are only very rarely used as such. Ordinarily 

 they are diluted with water. For this purpose pour into 

 the small staining bottle (1 oz.), which is provided with a 

 pipette, some of the concentrated solution to a depth of 

 about i inch, then fill the bottle with water. This dilute 

 stain must be perfectly clear from cloud or precipitate, 

 and should not be transparent. The dilute stains do 

 not keep for any length of time owing to the deposit of 

 the dye. In such cases a fresh solution should be made. 



SIMPLE STAINING OF COYER-GLASS PREPARATIONS. 



Place a small drop of water on a cover-glass which is 

 held between the thumb and forefinger of the left hand. 

 The cover-glass must be perfectly clean so that when the 

 drop is subsequently spread over the surface with a plat- 

 inum wire, it will spread in a thin film and not gather into 

 minute globules. The drop of water taken should prefer- 

 ably be so small that when spread in a thin film over the 

 cover-glass, it will dry almost immediately, and thus 

 evenly cover the whole surface. Large drops of water by 

 drying slowly, tend to leave unsightly " shore-lines." 



With a sterilized platinum wire pick up a minute 

 amount of the growth from one of the potato cultures and 

 touch the drop of water cautiously once or twice. Then 

 sterilize the wire, and when cool, spread the drop of water 

 evenly over the whole surface of the cover glass, avoiding, 

 however, contact with the fingers. In a few minutes the 

 water evaporates, and if desired, this may be hastened by 

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