1 78 Brownian motions. CHAP. 



is not polarised, in every plane at right angles to the 

 direction of the ray ; and the heat of bodies consists 

 of vibrations of their molecules, moving, no doubt, in 

 every direction at once. Sabatier suggests that these 

 motions are in some degree undetermined, and not 

 subject to any rigid law of uniformity ; and he finds 

 traces of the same indeterminism in some motions 

 which are on a sufficiently large scale to be visible 

 under the microscope. One instance of this which 

 he mentions is that of the "Brownian" motions of 

 minute particles suspended in water or other liquids. 1 

 These movements are of very small amplitude, but 

 incessant, of quite sensible rapidity, and in every 

 direction at once. They are well seen in ink, when a 

 drop is placed between two flat pieces of glass ; and it 

 is these motions which prevent ink from losing its 

 properties as such, by the subsidence of the black 

 particles. 



On this subject it is to be remarked that the laws 

 of motion are perfectly simple ; though not mathe- 

 matical in the nature of their evidence for they are 

 proved only by experiment, and have not that self- 

 evidencing character which belongs to mathematical 

 truth yet they are mathematical in form ; though 

 the proof that they are absolutely true is never per- 



1 So named after the eminent botanist, Robert Brown, who 

 first called attention to their importance. Professor Jevons 

 (Quarterly Journal of Science, April 1878) offers what appears 

 to be a satisfactory explanation of these motions, as being due 

 to minute disturbances of electric equilibrium, and analogous to 

 the motions of pith-balls in a well-known electrical experiment. 



