THE STAINING OF COVER-GLASS FILMS. 



105 



water have a great tendency to decompose. Only small 

 quantities should therefore be prepared at a time. 



The Staining of Cover-glass Films. Films are made 

 from cultures as described above. The cover -glass may 

 be floated on the surface of the 

 stain in a watch-glass, or the cover- 

 glass held in forceps with film side 

 uppermost may have as much stain 

 poured on it as it will hold. When 

 the preparation has been exposed 

 for the requisite time, usually a few 

 minutes, it is well washed in tap 

 water in a bowl, or with distilled - 

 water with such a simple contrivance 

 as that figured (Fig. 40). The figure 

 explains itself. When the film has 

 been washed the surplus of water is 

 drawn off with a piece of filter-paper, 

 the preparation is carefully dried 

 high over a flame, a drop of xylol 

 balsam is applied, and the cover- 

 glass mounted on a slide. Xylol 

 balsam must be used for mounting 

 all bacterial preparations. The 



reason is that xylol causes the colour FlG - 40. -Syphon wash- 

 bottle for distilled water used 



to fade less than any other solvent in wash i n g preparations. 

 of balsam. It is sometimes ad- 

 vantageous to examine films in a drop of water in place 

 of balsam. The films can be subsequently dried and 

 mounted permanently. In the case of tubercle, special 

 stains are necessary (p. 113), but with this exception, 

 practically all bacterial films made from cultures can 

 be stained in this way. Some bacteria, e.g., typhoid, 

 glanders, take up the stains rather slowly, and for these 

 the more intensive stains, red or violet, are to be preferred. 

 Films Q{ fluids from the body (blood, pus, etc.) can be 

 generally stained in the same way, and this is often quite 

 sufficient for diagnostic purposes. The blue dyes are 



