THE BEGINNINGS Of LIFE. 323 



important difference does, however, soon become ob- 

 vious. The bacteria, which have not been boiled, 

 undergo a most unmistakeable increase within their 

 imprisoned habitat; whilst those which have been 

 boiled, do not increase. The two films may be almost 

 colourless at first (if the 'Bacteria are not very abundant), 

 but after a few days, that composed of unboiled fluid 

 begins to show an obvious and increasing cloudiness, 

 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 

 that other indubitably dead particles which chanced to 

 have 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 



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

 living particles (spherical) takes place here and there amongst the fat 

 globules, just as the multiplication of Bacteria occurs in a vegetable 

 infusion. In a boiled specimen, however, no trace of such multiplication 

 can be detected. 



Y 2 



