28 MANUAL OF BACTERIOLOGY. 



employed should leave the background unstained. This 

 result is best obtained with aqueous solutions of the aniline dyes. 

 These aniline dyes, so called, are derivatives of coal-tar, but 

 not always of aniline. These dyes are of great importance in 

 bacteriological work, since they are used to make stained prepa- 

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

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

 manufactured by Griibler are rehable. 



It is simplest to classify the anihne 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 

 bacteriological work. The other kinds 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, from which watery solutions may 

 be made when needed by adding a few drops of the alcohoHc 

 solution to a small dish filled with water. The alcoholic solution 

 is diluted about ten times, so as to make a liquid which is 

 just transparent in a layer about 12 mm. in thickness, after 

 filtering. 



Fuchsin and gentian- violet operate 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, hke 

 pus, mucus, and milk, and acts more energetically when made 

 slightly alkaline. 



Method of Staining Cover-glass Preparations.— (a) A 

 smear preparation of bacteria having been rnade, dried, and 

 passed through the flame three times in the manner above 

 described, and a watery solution of either fuchsin, gentian- 

 violet or methylene-blue having been prepared, the cover-glass is 

 to be dropped into a dish containing the dye, or the dye may be 

 dropped upon the cover-glass held in the forceps. 



