50 ORIGIN OF LOWEST ORGANISMS. 



in stationary films,* occurred where the particles were 

 heavy or large ; and that the duration of the move- 

 ment was more and more prolonged, as the par- 

 ticles experimented with, were lighter or more minute. 

 So that, when we have to do with Bacteria, the minute 

 oil globules of milk, or with other similarly light par- 

 ticles, the movements continue for an indefinite time, 

 and are, in part, mere exponents of the molecular 

 unrest of the fluid. They are always capable of being 

 increased or renewed by the incidence of heat or other 

 disturbing agencies. 



In respect of the movements which they may ex- 

 hibit, therefore, really living, though languid, Bacteria, 

 cannot always be discriminated from dead Bacteria. 

 Both may only display mere Brownian movements. 



It becomes obvious, then, that in doubtful cases we 

 ought not to rely very strongly upon the character of 

 their movements, as evidence of the death of Bac- 

 teria although these may frequently be of so extensive 

 a nature as to render it not at all doubtful whether 

 the Bacteria which display them are living. In the 

 experiments which I am about to relate, w r e shall be 

 able to pronounce that the Bacteria are living or dead, 

 by reference to the continuance or cessation of their 

 most essentially vital characteristic. If Bacteria fail 

 to multiply in a suitable fluid, and under suitable con- 



* The specific gravity of the fluid being constant. Where 

 this is dense or viscid, as with glycerine, Brownian movements 

 do not occur at all. 



