ARTICLES 237 



that the Brownian movement ceases immediately before 

 coagulation. The Svedberg ^ has shown that the Brownian 

 movement is unaffected by the presence of electrolytes in solu- 

 tion, and there seems no reason why it should so stop. We 

 have, however, to distinguish between absolute cessation and 

 the rapid falling off of the movement owing to increasing size 

 of the particles, so soon as coagulation has started. It has 

 been stated that the Brownian movement is the manifestation 

 of forces tending to keep the colloid particles in suspension, 

 but it may be pointed out that they may also assist coagula- 

 tion, other conditions being suitable, by bringing the particles 

 into close contact with one another. 



Electrostatic Forces. — In the case of charged particles, electro- 

 static forces will tend to keep them apart, thus aiding the 

 kinetic forces of the solvent molecules. We have seen that 

 the charge is probably due to specific adsorption of ions. A 

 charge on a particle will set up a definite potential difference 

 at its surface, and, assuming the existence of a Helmholtz double 

 layer, we may calculate this in terms of the charge and the 

 size of the particle. If we consider the charge as extending 

 to a depth d on the surface of a sphere, where d is small com- 

 pared with r, the radius of the sphere, we may write for the 

 capacity of the particle, 



47ra 



Where s is the surface area of the particle, and K is the 

 di-electric constant of the medium between the layers. This 

 may possibly be regarded as that of the disperson medium 

 itself. 



Then, if the potential difference be V volts, we can write, 



e =CV 



47ra 



Burton ^ has used this formula to calculate V by combin- 

 ing it with the expression deduced by Lamb ^ for the velocity 

 z; of a charged particle through a liquid under an electric force, 



V 



Xe = 47rr' "q • j 



^ Die Existenz der Molekule, 1915. 



3 Phil. Mag., vol. xi, p. 425, 1906. ^ 



» Ibid., p; 60, 1888. 



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