MOLECULAR MAGNITUDES 



275 



For the case of a sphere it was calculated by Stokes 

 that the reaction due to viscosity is GTJTCV, where r 

 is radius, v is the velocity, and c is the coefficient of 

 viscosity of the fluid. The acting force which occasions 

 the fall is the space rate of change of the energy and 

 therefore equals the quotient of the left-hand side of 

 equation (1) and h. Hence equating action and 

 reaction gives 



(4/3)irr(D - d)g = farm (2) 



The coefficient c is obtainable from tables of previous 

 determinations or may be found by a separate experi- 

 ment. Stokes' Law, therefore, enabled Perrin to 



FIG. 33. 



determine r from his observations of v. Substitution 

 of this value in equation (1) then gave N. By this 

 experiment he found A r = 6.83Xl0 23 . 



Although this method makes use of the kinetics of 

 visible particles it considers the average particle rather 

 than the Brownian movement of a particular particle. 



