V?8 PATHOGENIC BACTERIA 



if a preparation of the typhoid bacillus, e.g., is made from an 

 agar culture of twenty-four hours' growth, in the way described, 

 there is no difficulty in observing the inotility of the microbe, 

 though the movements are not so active as in a broth culture. 

 Examined in the hanging drop, it is to be noted that even non- 

 motile bacteria present marked passive or Brownian move- 

 ments ; these are distinguishable by the fact that no real loco- 

 motion takes place, the movements being of a to-and-fro or 

 oscillatory character. 



Coloration of the Hanging Drop. The hanging drop having 

 been so viewed, may next be advantageously stained. It is not 

 necessary for this purpose to make a new preparation. The 

 cover-glass is removed and turned over on a piece of filter paper, 

 so that the drop is again brought uppermost. 



The best staining fluid is a dilute solution of gentian violet, 

 made by adding .5 c.c. of an aqueous solution of gentian violet 

 (gentian violet 2.25 grammes, distilled water 100 c.c.) to 10 

 cubic centimeters of distilled water. The dilution may be ar- 

 rived at sufficiently well by placing a few drops of the gentian 

 violet solution in a test tube and adding distilled water until the 

 fluid is sufficiently dilute to allow of being seen through when 

 held between the eye and the light. The sterilized 6se is dipped 

 into this, and the drop of dye mixed on the cover- glass with the 

 hanging drop already there ; the cover-glass is then again in- 

 verted and replaced over the hollow slide or upon the blotting- 

 paper of the moist chamber. The ordinary histological method 

 of running in the dye from one side of a wet preparation made 

 without hollow slide or moist chamber, is followed by too much 

 displacement. 



The action of the dye is easily studied in all its grades in 



