Section III, 1919 [103] Trans. R.S.C. 



A New Method of Weighing Collodial Particles 

 By Professor E. F. Burton, F. R.S.C. 



(Read May Meeting, 1919.) 



Probably the first method suggested for the determination of the 

 size of collodial particles depended on the application of Stokes' Law 

 for the rate of fall of rigid spheres through fluids. Such spheres attain 

 after a very short interval of time a limiting velocity, v, such that the 

 force of friction of the fluid is given by the equation 



F = 67r n a V (1) 



where n = coefficient of viscosity of the fluid 

 a = radius of the particle. 

 For steady motion this force must be equal to the gravitational 

 force acting on the particle immersed in the fluid. The latter force is 

 given by 



4 

 ¥=-Tta? {p-pi) g (2) 



where p and pi are the densities of the material of the particle and of 

 the fluid respectively. Equating these forces we have 



4 



K a.^(p — pi) g = 67r n a V 



from which 



a- = 



9 n V 



(3) 



2 (P-Pi)g 



Unfortunately, this simple formula is of very narrow application to 

 aqueous solutions because the fall of particles less in radius than 

 about 10"^ cm. is masked by the Brownian movement, which, as a 

 result of molecular agitation prevents any settling of the colloidal 

 particle. One can see at any time in the Faraday collection at the 

 Royal Institution, samples of gold colloidal solutions, prepared by 

 Faraday before 1860; these solutions, which are suspensions of metallic 

 gold particles of radius about 10"^ cm. show no sign whatever of 

 settling. 



The method outlined in the present communication may be de- 

 scribed as an artificial production of the settling due to gravitation by 

 super-imposing the motion produced in an electrical field maintained 

 in the liquid medium upon that due to gravitation. 



