THE STAINING OF BACTERIA. 43 



or, in case of a solid, a small particle is diluted with sterile 

 water and spread in the same way on the slide or cover-glass. 

 This is allowed to dry in the air, protected from dust, or else 

 is held in a suitable forceps high up over the flame of an 

 alcohol lamp or Bunsen gas-burner until dry, thus forming a 

 thin film on the surface of the glass. After this step has 

 been carefully taken the slide or cover-glass is taken up with 

 a pair of forceps and passed several times through the flame, 

 film-side up, for one-half to one second at each pass, three 

 times in the case of a cover-glass and eight or ten passes in the 

 case of a slide ; which is for the purpose of setting the prepa- 

 ration, or coagulating the albuminoids and fixing them to the 

 glass so that they will not be easily washed away in the sub- 

 sequent procedures. 



After allowing the preparation to cool well, it is ready for 

 the staining reagents. 



II. The Most Commonly Used Stains. 



The basic anilin dyes, such as fuchsin, methylene-blue, 

 gentian- violet or methyl- violet, and Bismarck-brown, are 

 most commonly employed. 



They are made into saturated alcoholic solutions, to be kept 

 in stock, and are freely diluted with water whenever required 

 for use. 



III. The Application of the Dyes. 



Nearly all the known bacteria, with the exceptions to be 

 mentioned later, are readily stained by the watery solutions 

 of any of the basic anilin dyes. The film on the slide or 

 cover-glass, prepared as just described, is covered by a few 

 drops of the stain, or the cover glass, film-side down, is floated 

 in a watch-glass full of the staining solution ; at the end of 

 from one-half to two or three minutes the staining fluid is 

 poured off, the slide or cover-glass washed rapidly in water 

 and then allowed to air-dry ; after which, in the case of 

 cover-glass preparations, they are inverted upon a drop of 

 Canada balsam on a slide and examined with the oil-immer- 

 sion lens ; or, when slides have been prepared, after wash- 



