110 BACTERIOLOGY. 



In examining bacteria by this method there is a pos- 

 sibility of error that must be guarded against. All 

 microscopic insoluble particles in suspension in fluids 

 possess a peculiar tremor or vibratory motion, the so- 

 called " Brownian motion." This is very apt to give 

 the impression that the organisms under examination 

 are motile when in truth they are not so, their move- 

 ment in the fluid being due only to this molecular 

 tremor. 



The diflerence between the motion of bodies which are 

 undergoing this naolecular tremor and that possessed by 

 certain living bacteria is that the former particles never 

 move from their place in the field, while the living 

 bacteria alter their position in relation to the surround- 

 ing organisms, and may dart from one position in the 

 field to another. With some cases the true movement 

 of bacteria is very slow and undulating, while in others 

 it is rapid and darting. The molecular tremor may be 

 seen with non-motile and with dead organisms. 



Prepare three hanging-drop preparations, one from 

 a drop of dilute India ink, a second from a culture of 

 micrococci, and a third from a culture of the bacillus of 

 typhoid fever. In what way do they differ ? 



Study of Spoee-foemation. — The hanging-drop 

 method just mentioned is not only employed for the de- 

 tection of the motility of an organism, but for the study 

 of its spore-forming properties. 



Since with aerobic organisms spore-formation occurs, 

 as a rule, only in the presence of oxygen, and is induced 

 more by limitation of the nutrition of the organisms 

 than by any other factor, it is essential that these two 

 points should be borne in mind in preparing the drop 

 cultures in which the process is to be studied; For 



