ELECTROKINETICS 371 



centimeter) rather than in terms of the electrokinetic potential. 

 But if the potential of the particle is desired, one can, by making 

 permissible assumptions in regard to the dielectric constant, the 

 viscosity of the aqueous medium, and the potential gradient, 

 derive the potential directly from the rate of migration simply 

 by multiplying the latter by a factor. Obviously, this factor 

 will have a different value under different conditions. For 

 average laboratory conditions, it has been established as 12.6. 

 If the rate of migration of a droplet of butterfat (in milk) is 

 3.6 /i per second per volt per centimeter (the total potential of 

 the line being 110 volts; the potential gradient in the chamber, 

 6 volts per centimeter; and the actual observed rate, 21.6 m per 

 second), then, by applying the factor 12.6, we obtain 



12.6 X 3.6 = 45 mv., 



which is the potential on the surface of the particle. With 

 standardized equipment and constant temperature, the factor, 

 once established, will hold for subsequent work. 



The foregoing account of the electric properties of colloidal 

 particles ascribes their behavior to their environment, which 

 adds further evidence to the fact that the properties of colloidal 

 systems are determined primarily by the third phase, or interface. 

 Without wholly departing from this viewpoint, there are those 

 who prefer regarding a colloidal particle as a colossal polyvalent 

 ion. Thus, McBain says that the behavior of micelles is exactly 

 like that of the ions of an ordinary electrolyte. This viewpoint 

 is also held by Pauli in regard to both protein molecules (or 

 micelles) and metal particles. Mukherjee states that the migra- 

 tions of ions and of colloidal particles are fundamentally similar, 

 but an ion and a colloidal particle are not identical as chemical 

 entities. There is a similarity between colloidal particles and 

 ions, but the differences are equally pronounced, particularly in 

 that property which best characterizes both, viz., charge. The 

 charge on colloidal particles does not seem to be related to that 

 on the corresponding ions. Metallic ions in general are posi- 

 tively charged, but colloidal dispersions (Bredig dispersals) of 

 them may be positive (as are copper and iron) or negative 

 (as are platinum, gold, and silver). While the sign of the charge 

 of colloidal particles appears to have no relation to that of the 

 ion, the mobility of the relatively large particles is of the same 



