200 BACTERIOLOGY. 



should be kept completely covered with fluid. The 

 bacteria having now been stained, the cover-glass is 

 grasped in the forceps and thoroughly washed in clean 

 water and then dried, first between layers of filter-paper 

 and then in the air. A drop of balsam or water is now 

 placed on a glass slide and the cover-glass placed upon 

 it with the bacterial side down. The preparation is 

 now ready for microscopical examination. 



The Preparation of Sections. Occasionally it is of value 

 to examine the bacteria as they are in the tissues them- 

 selves. These should be obtained as soon as possible 

 after death, so as to prevent any post-mortem changes 

 or increase of the bacteria in them. From the properly 

 selected spots small portions, not larger than one cubic 

 centimetre, are removed and placed in absolute alcohol 

 to harden. These portions, when of nearly the con- 

 sistency of fresh, solid rubber, are removed, and, if the 

 nature of the tissues will allow, fastened to corks or 

 pieces of hard rubber by mucilage. After dry ing, the 

 specimens are replaced in alcohol for twenty-four hours 

 and then cut into thin sections with the microtome. 

 Sometimes the tissues do not become sufficiently dense 

 by this simple method; they must then undergo the 

 process of embedding in celloidin or paraffin. 



The Ordinary Staining Solutions. Simple staining is 

 used for the demonstration of bacteria in general, and 

 is also useful for gaining an idea of the other elements in 

 the preparation. The solutions commonly employed in 

 staining bacteria are the watery solutions of basic aniline 

 dyes fuchsin, gentian-violet, and methylene - blue. 

 These solutions may either be prepared by dissolving 

 the dyes directly in water, or, more usually, by having 

 stock saturated alcoholic solutions of them, from which 



