28 MANUAL OF BACTERIOLOGY. 



vitality of many species. The beginner is, therefore, advised 

 to make his preparations on cover-glasses. 



When very resistant or dangerous pathogenic bacteria are 

 being handled, after fixation by heat upon the slide or cover- 

 glass, the preparation may, if desired, be immersed in i-iooo 

 solution of bichloride of mercury long enough to kill the 

 bacteria, without injuring the preparation or interfering with 

 its staining properties. 



Staining. The bacterial cells are devoid of color, and the 

 object of staining them is to give them artificially some color 

 that would make them distinct and easily visible with the micro- 

 scope. In order that they shall stand out sharply the stain 

 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. They are indispensable in bacterio- 

 logical work. 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 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 methy- 

 lene-blueare 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 solu- 

 tions of the aniline dyes in alcohol, from which watery solu- 

 tions may be made when needed by adding a few drops of the 

 alcoholic 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. 



