94 MICROSCOPIC METHODS. 



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. 36). This consists of a jar set on a 

 shelf above the bench. From it there proceeds a glass 

 tube with a pointed nozzle by which the contents may be 

 syphoned. The tube has near its lower end a piece of rubber 

 tubing interposed, on which a clamp is fixed. 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 to fade 

 less than any other solvent of balsam. It is sometimes 

 advantageous 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 (v. infra), 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 of 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 

 here preferable, as they do not readily overstain. In 

 the case of such fluids, if the histological elements also 

 claim attention it is best to use a blue which will stain the 

 bacteria and the nuclei of the cells, and then after washing 

 to stain the cellular protoplasm with a one to two per cent 

 watery solution of eosin (which is an acid dye). In the 

 case of films made from urine, where there is little or no 

 albuminous matter present, the bacteria may be imperfectly 

 fixed in the slide, and thus apt to be washed off. In such a 

 case it is well to modify the staining method. A drop 

 of stain is placed on a slide, and the cover-glass, film-side 



