34 BACTERIOLOGY. 



ing from one place to another. Tiie impulse that creates 

 this real, active motion comes from within the cell, in 

 other words from the living protoplasm, On the other 

 hand, many bacteria will show an apparent motion. The 

 cells move to and fro, trembling as it were, but do not 

 actually change their relative position. The impulse in 

 this case comes from without the cell, which itself remains 

 passive. 



The apparent motility of bacteria is known as Brownian, 

 or molecular, or physical motion. All exceedingly minute 

 objects, when suspended in the air or in a fluid, will show 

 this peculiar swaying or pendulum movement. The mole- 

 cules composing the air or fluid are in constant motion, and 

 consequently strike, again and again, the minute objects 

 that may be in their path. This molecular bombardment 

 of an otherwise motionless bacterial cell causes it to sway 

 to and fro. 



It may at times be very difficult to distinguish between 

 Brownian and real motion. In that case it is necessary 

 to resort to certain experiments. Brownian motion is 

 obviously manifested by dead as well as by living cells. 

 The organism may be destroyed by heating at 60° for one 

 hour, or it may be treated with a germicidal substance such 

 as carbolic acid, or mercuric chloride. If the dead cell 

 exhibits the same motion as the living cell, it is clearly due 

 to purely physical causes. Again, by growing the organism 

 at a constant low temperature of 15° or less it will in many 

 cases become larger than usual, and consequently will 

 respond less readily to the impact of molecules. This pro- 

 cedure is especially useful in doubtful cases. Finally, as a 

 rule, it is possible to distinguish between the two kinds of 

 motion by demonstrating the presence or absence of the 

 characteristic organs of motion. If these are present there 

 can be no doubt of the true motility of the organism. On 

 the other hand, failure to find these motile organs does not 

 prove that they are absent, inasmuch as in some undoubt- 



