94 PRACTICAL BACTERIOLOGY. 



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and fixed, they may be immediately stained, or they 

 may be put aside, if convenient, for a time, as no 

 change takes place in them if they are left to them- 

 selves. There are a very great many treatises on the 

 subject of staining bacteria ; however, we will content 

 ourselves with a few good methods, which may be 

 used in all cases, and in this chapter will only consider 

 staining with simple aqueous solutions. 



Amongst aniline dyes, only the basic ones are suit- 

 able for staining bacteria, and amongst these, fuch- 

 sine, gentian violet, methyl violet, methylene blue, and 

 vesuvin (Bismarck brown) are the ones most commonly 

 used. These colours are used in aqueous solutions, 

 which may either be prepared afresh each time by dis- 

 solving the dye in water and filtering the solution, or 

 of which permanent concentrated solutions may be 

 made in alcohol, a few drops of which should be 

 added to a vessel containing water. These alcoholic 

 preparations may be kept for any length of time, whilst 

 the aqueous ones, with the exception of that of methy- 

 lene blue, soon spoil. Vesuvin, or Bismarck-brown, 

 can only be dissolved in water, and cannot be pre- 

 served. The solution, in whichever way prepared, 

 should be perfectly dear and transparent in thin 

 layers — that is to say, it should not contain too much 

 of the dye. This can be tested in the following 

 manner. Place a watch glass over a piece of black 

 paper, upon which a white paper cross has been laid. 



