EVOLUTION AND MUTABILITY OF 'MATTER': 267 



The observer who for the first time looks through 

 the microscope at a drop of water from the river, from 

 the sea, or from any ordinary source — that is to say, 

 water not specially purified — is struck with surprise 

 and admiration at the motion revealed to him. 

 Infusoria, microscopic articulata, and various micro- 

 organisms people the microscopic field, and animate 

 it by their movements ; but at the same time all sorts 

 of particles are also agitated, particles which cannot 

 be considered as living beings, and which are, in fact, 

 nothing but organic detritus, mineral dust, and debris 

 of every description. Very often the singular move- 

 ments of these granulations, which simulate up to a 

 certain point those of living beings, have perplexed 

 the observer or led him to erroneous conclusions, and 

 the bodies have been taken for animalcules or for 

 bacteria. 



Characters of this Movement. — But it is as a rule 

 quite easy to avoid this confusion. The Brownian 

 movement is a kind of oscillation, a stationary, 

 dancing to-and-fro movement. It is a Saint Vitus's 

 dance on one and the same spot, and is thus dis- 

 tinguished from the movements of displacement 

 customary with animate beings. Each particle has 

 its own special dance. Each one acts on its own 

 account, independently of its neighbour. There is, 

 however, in the execution of these individual oscilla- 

 tions a kind of common and regular character which 

 arises from the fact that their amplitudes differ 

 little from each other. The largest particles are 

 the slowest ; when above four thousandths of a 

 millimetre in diameter, they almost cease to be 

 mobile. The smallest are the most active. When so 

 small as to be barely visible in the microscope, the 



