146 BACTERIOLOGY. 



latter should be so clean that, when a drop of water is 

 placed on any of them, and spread over the surface by 

 means of a platinum wire, it will remain spread out as a 

 thin film. If it gathers in minute droplets, which follow 

 the wire and refuse to spread out, the cover-glass is not 

 clean, and is not suitable in that condition for staining pur- 

 poses. The thin layer of fatty matter must be removed, 

 and this is often impossible when the ordinary procedure is 

 followed. The method of obtaining absolutely clean cover- 

 glasses has been described on p. 140. 



Anilin dyes are employed for staining bacteria. It is 

 customary to speak of acid and basic anilin dyes. The acid 

 dyes do not stain bacteria, or but feebly, and are, there- 

 fore, useful as contrast colors. Eosin belongs to this group, 

 and its value will be seen in the double staining of patho- 

 genic bacteria. The basic anilin dyes stain the nuclei of 

 cells and bacteria equally well. They are, therefore, in- 

 variably used when staining bacteria. The basic anilin 

 dyes, most frequently used, are fuchsin, gentian violet, 

 methyl violet, and methylene blue. Occasionally, vesuvin 

 or Bismarck brown, is employed. 



The fuchsin and gentian violet are to be preferred, and 

 used for all ordinary purposes. They stain rapidly and 

 deeply, and the preparations, when properly mounted, will 

 not appreciably alter, even after many years. Methylene 

 blue stains slowly, and is excellent for special purposes. 

 It is not, however, a permanent stain, and such preparations 

 are likely to fade after a short time. 



The dyes mentioned above are dissolved to complete 

 -saturation, in absolute, or in strong alcohol. These con- 

 centrated solutions are never used as such, but serve as a 

 stock, from which the dilute stain which is employed can 

 be readily prepared. 



If the alcoholic solution of the stain is diluted too 

 much, the dye will tend to precipitate, and may yield un- 

 sightly deposits on the cover-glass. It is advisable, there- 



