324 THE BEGINNINGS OF LIFE. 



also with very minute particles which were gradually 

 precipitated from a hay infusion that had been heated 

 to 3O2F for four hours 1 . Trials with many different 

 substances, indeed, after a time convinced me that the 

 most rapid cessation of Brownian movements in sta- 

 tionary films, occurred where the particles were 

 relatively heavy or large ; and that the duration of the 

 movement was more and more prolonged, as the par- 

 ticles experimented with were lighter or more minute 2 . 

 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 exhibit, 

 therefore, really living, though languid, Bacteria cannot 

 always be discriminated from dead Bacteria. Both may 

 only display mere Brownian movements 3 . 



1 Those of the light particles which come to rest, in such cases, are 

 always in contact with one or other of the contiguous surfaces of glass. 



The specific gravity of the fluid being constant. Where this is dense 

 or viscid, as with glycerine, Brownian movements do not occur at all. 



3 That absence of even customary movements is no certain indication 

 of the non-existence of ' Life,' is admitted by most biologists. The 

 Rev. M. J. Berkeley (Cryptogamic Botany, 1857, p. 92) says : ' It is 

 curious in two such closely-allied algae as Vaucberia sessilis and V. clavata, 

 to find the fruit so very different. The spore of the former is perfectly 

 inactive, while that of the latter revolves by means of delicate cilia 

 covering its whole surface. It is clear, then, that we must not, in these 

 lower cryptogams, attach too much importance to motion.' 



