PREPARATION OF A HANGING DROP 



75 



Bacteria have two kinds of movement, the so-called 

 Brownian or molecular movement, and true motility. The 

 former may be demonstrated by examining the movement 

 of powdered carmen rubrum in the hanging drop. A very 

 little of the powder is sufficient. Brownian movement 

 is shown more or less by all small particles of insoluble 

 matter (including living non-motile or dead bacteria) in 

 suspension. It is characterized by a vibratory movement 

 affecting the entire field; the relative positions of the 

 insoluble particles are never altered. This type of move- 



FIG. 27. Hanging Drop Slide. (Orig.) 



ment must be distinguished from that of true motility, 

 which is characterized by the progressive movement, more 

 or less rapid, of an organism across the field of the micro- 

 scope, changing its position in the field independently of 

 and in a direction contrary to other organisms present. 



There should be no currents of air entering under the 

 cover-glass and passing through the concavity of the slide 

 nor should there be currents in the liquid. The latter may 

 occur if the organisms have not been well mixed through 

 the drop in the process of preparation. If large numbers 

 of the microorganisms in the drop are moving in one direc- 



