OF THE HUMAN SUBJECT. 47 



such a hanging drop, motile bacteria retaining their power of 

 movement whilst the coloration is proceeding in them. 



To Make a Permanent Preparation of the Hanging Drop. 

 Lastly, a permanent specimen may be made from the same prep- 

 aration by removing the cover-glass, placing it again with the 

 drop upwards on a piece of filter paper beneath a watch-glass or 

 bell-jar, and allowing it to dry ; after which a small drop of 

 Canada balsam (dissolved in xylol) is placed on a slide, a second 

 small drop on the dried film, and the latter then allowed to fall 

 on the slide in the usual manner. Or, after the examination of 

 an unstained hanging drop, this may be allowed to dry in the 

 same way and treated in the manner to be presently described 

 under the dry method. 



The dyeing of hanging drops made from broth cultures (in 

 which case, of course, no water need be placed on the cover - 

 glass) is not quite so satisfactory as those from agar, in conse- 

 quence of an extremely fine colored precipitate appearing in the 

 fluid, probably a proteid present in the broth, which unites with 

 the dye. This precipitate is unconnected with the growth of 

 the bacteria, as it appears if sterile broth is mixed with the 

 diluted dye and examined as a hanging drop. Nevertheless, in 

 such broth preparations when colored, many fields may be found 

 devoid of adventitious granules, which, it may be noted in pass- 

 ing, exhibit lively Brownian movements. 



The examination of jelly that is being liquefied by the 

 growth of an organism is conducted precisely as that of a broth 

 culture ; and admirable preparations may be obtained by color- 

 ing the hanging drop with dilute solution of gentian violet in 

 the manner already described. 



Of all the methods of bacterial microscopy, the hanging 



