EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 39 



writer's laboratory have shown that the ordinary method 

 of fixation in the flame, when applied to bacteria spread 

 upon slides, has little effect on the 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 its 

 staining properties. 



Staining. The staining of bacteria is done for the most 

 part with the aniline dyes. The object of staining bacteria 

 is to give them artificially some color which makes them 

 distinct 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 aniline. These dyes are of great import- 

 ance 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 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 

 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 va- 

 rieties may be employed as contrast-stains; another con- 

 trast-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 alcoholic solution to 

 a small dish filled with water. The alcoholic solution is 

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



