METHODS OF CULTURE AND OF EXAMINATION. 105 



may be considered sufficiently sterile for this purpose) upon 

 the cover-glass. The preparation is permitted to dry in the 

 air, and then, in order to fix it, it is passed three times, 

 with moderate rapidity, through the flame of a spirit-lamp 

 or of a Bunsen burner. In order to obtain satisfactory 

 preparations from cultures it is useful, before staining, to 

 immerse the cover-glasses for from one-half to one minute 

 in from a one to a four per cent, solution of acetic acid. By 

 this means the preparation is cleared, while the bacteria in 

 no wise suffer. The acetic acid is blown off by means of a 

 glass tube, or the cover-slip is simply dried carefully be- 

 tween filter-paper. By means of a pipet or of a small filter 

 (it is usually well to filter the staining solutions before they 

 are used) several drops of the staining solution are placed 

 upon the smeared surface of the cover-slip (conveniently 

 grasped with Cornet's forceps) ; the stain is permitted to 

 exert its influence for a short time, and is then washed off 



ig- 35- Stewart's cover-glass forceps. 



in water. If the staining solution has been placed in a 

 watch-glass, the cover-slip is carefully placed upon the sur- 

 face of the fluid so that it floats thereon with its smeared 

 surface down. As to the length of time that the cover- 

 glasses should be exposed to the influence of the staining 

 solution, no definite statement can be made. This will de- 

 pend upon the thickness of the layer, the concentration of 

 the stain, and the tingibility of the bacteria. In general, 

 from half a minute to a minute is required for staining. . The 

 time of exposure to the action of the staining fluids may be 

 materially shortened by application of heat. For this pur- 

 pose the cover-glass is held with the forceps directly over 

 the flame until the vapor of steam is given off from the 

 staining solution. The stained specimen is thoroughly 

 washed in water, then carefully dried between filter-paper, 

 and mounted in xylol-Canada balsam. If blood-preparations 

 are to be examined for bacteria, it is likewise well to treat 

 the smeared cover-glasses according to the acetic-acid 



