4O BACTERIOLOGY 



and easily visible without imparting this color to the substance 

 or medium in which they are imbedded. The substances known 

 as aniline dyes are derivatives of coal-tar, but not always of ani- 

 line. These dyes are of great importance in bacteriological 

 work. Their number is very large, but only a few are in common 

 use. It is important to have the purest, and those obtainable 

 from Griibler are reliable. 



It is simplest to classify the aniline dyes as acid or basic. 

 Eosin, picric acid and acid fuchsin are acid dyes; they tend to stain 

 tissues diffusely. Fuchsin, gentian-violet and methylene blue are 

 basic dyes; they have an affinity for the nuclei of tissues and for 

 bacteria; they therefore are the dyes used chiefly in bacteriolog- 

 ical work. The other varieties may be employed as contrast- 

 stains; another contrast-stain frequently used is Bismarck brown. 

 It is best to keep on hand saturated solutions of the aniline dyes 

 in alcohol, which are permanent, but cannot be employed directly 

 for staining. In order to prepare the simple staining solutions, 

 the alcoholic solution is diluted about ten times, or so as to make 

 a liquid which is just transparent in a layer about 12 mm. in 

 thickness, after filtering. These watery solutions deteriorate 

 after a few weeks. 



Fuchsin and gentian-violet stain rapidly and intensely. 

 Methylene blue works more slowly and feebly; it is to be pre- 

 ferred where the bacteria occur in thick or viscid substances, 

 like pus, mucus, and milk. 



Aniline-water Staining Solutions. The intensity with which 

 aniline dyes operate may be increased by adding aniline oil to 

 the solution: 



Aniline oil 5 c.c. 



Water 100 c.c. 



Mix, shake vigorously, filter through wet filter paper. The fluid 

 after filtration should be perfectly clear. Add 



Alcohol 10 c.c. 



Alcoholic solution of fuchsin (or gentian violet, or 



methylene blue) i c.c. 



