BROWNIAN MOVEMENT 



77 



If a colloidal solution or a fine suspension of gamboge or mastic 

 be kept undisturbed at constant temperature for some time, 

 Pcrrin found that there was a distribution of the colloidal particles 

 under the influence of gravity. At the bottom of the container 

 will be found a denser distribution than at the higher levels. 

 This is exactly similar to the decrease in the density of the atmos- 

 phere with height above sea level, and Einstein argued that the 



FIG. 9. Apparatus for demonstrating cata- 

 phoresis. The deeply shaded lower portion 

 of the U-tube is filled with a colloidal sol, the 

 upper part with ordinary distilled water. On 

 the passage of an electric current the colloid 

 rises towards the electrode of opposite sign to 

 the sol. (Hatschek.) 



FIG. 10. Apparatus for ultramicroscopic 

 observation of the movements of colloids in 

 an electric field (see Part II.). (Hatschek.) 



distribution of suspended particles with height should follow the 

 law which governs the density of the atmosphere with height. 

 Perrin proved by experiment that this was true. In one experi- 

 ment with mastic at four different levels 12/x apart he found 116, 

 146, 170 and 200 particles per unit. For the same levels the follow- 



