EXAMINATION OF LIVING BACTERIA. 23 



by means of whicTi they can immediately be distin- 

 guished from the others. 



If we now take with a needle a small portion of such 

 a bacterium colony from the surface of a potato, and 

 spread it out in a drop of water upon a slide, cover it 

 with a cover-glass, and examine it with a magnifying 

 power of about 500 diameters, and with a small open- 

 ing in the diaphragm, we shall find in most cases a 

 specimen of the genus Micrococcus, or more rarely, a 

 rod-bacterium. The Micrococci are small round cells, 

 which on an average are toVo mm- in diameter ; some- 

 times they are a little larger, sometimes a little smaller, 

 according to the species which happens to be present. 

 They are almost always motionless. Up to now we 

 know very few kinds of Micrococci which, like the Mi- 

 crococcus agilis Cohen, move of their own accord. The 

 trembling movements exhibited by most Micrococci 

 are not manifestations of their own activity, for they 

 are also exhibited by non-living bodies suspended in 

 fluid, such as small particles of carmine. This pheno- 

 menon has not yet been fully explained, but apparently 

 it is caused by the neutralization of the electrical ten- 

 sion of very small bodies. The peculiarity of these 

 so-called Brownian movements consists in this, that 

 the particles., in spite of their continual trembling and 

 dancing, never move from place to place, but remain 

 practically at one point. 



Presently, however, very disturbing movements are 



