ORIGIN OF LOWEST ORGANISMS. 49 



which is never manifested by the other. Microsco- 

 pical examination shows that this cloudiness is due to 

 a proportionate increase in the number of Bacteria. 



Is the continuance of the movements of the or- 

 ganisms which had been boiled attributable to their 

 extreme lightness, and to the slight difference between 

 their specific gravity and that of the fluid in which 

 they are immersed ? I soon became convinced that 

 this was one, if not the chief reason, when I found 

 that Bacteria which had been submitted to very much 

 higher temperatures, behaved in precisely the same 

 manner as those which had been merely boiled, and 

 also that other particles which though obviously 

 dead had a similar specific lightness, also continued 

 to exhibit their Brownian movements for days and 

 weeks. This was the case more especially with the 

 minute fat particles in a mounted specimen of boiled 

 milk,* and also with very minute particles which were 

 gradually precipitated \ from a hay infusion that had 

 been heated to 302 F. for four hours. Trials with many 

 different substances, indeed, after a time convinced me 

 that the most rapid cessation of Brownian movements 



* If an unboiled specimen of milk be mounted, a multiplication 

 of living particles takes place here and there amongst the fat 

 globules, just as the multiplication of Bacteria occurs in a vege- 

 table infusion ; but in the boiled specimen no trace of such 

 multiplication can ever be detected. 



f Those particles which come to rest, in such cases, are always 

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



E 



