238 SCIENCE PROGRESS 



where X is the gradient of electric potential in liquid. 



/ is a linear magnitude measuring the " facility 

 of slipping " of the particle against the mole- 

 cules of the medium. 

 7) is the coefficient of viscosity of the liquid. 

 e, r, and d as above. 



Whence we may write, 



15= 



" ■•' - E 5 



Lamb gives reasons for considering that / and d are of 

 the same order of magnitude, and they do not probably differ 

 by much. Burton found that the numerical value of V was 

 of the same order of magnitude for each substance investi- 

 gated, and in different media. For platinum in water he found 



V = — 0*031 volt. 



It was generally assumed that, the colloid particles being 

 charged either by friction or adsorption, the opposite charge 

 was borne by the molecules of the solvent. Recently, McBain, 

 Laing, and Titley,^ working on colloidal soap solutions, have 

 attempted to apply an ionic theory to all charged colloids, 

 which seems to have certain experimental grounds for its adop- 

 tion. " The current assumption that the other charge is 

 carried by the solvent is replaced by the hypothesis that free 

 ions of charge equal and opposite to that of the charged colloid 

 are present in sol or gel." In this connection Wilson's electri- 

 cal theory of the colloid state is of interest. Wilson,* working 

 on the assumption that " the colloidal state in sols owes its 

 stability to the formation of a complex between the particles 

 of the disperse phase and certain substances present, or formed 

 in the medium during preparation of the colloid,"^ has derived 

 a relation between the potential difference and the difference 

 in concentration between the ions at the surface layer and in 

 the bulk of the solution. He first deduces, by thermodynamical 

 methods, that " the products of concentration of any pair of 

 diffusible and oppositely charged ions will be equal in surface 

 layer and bulk of solution." He adopts the following nota- 

 tion. Suppose the colloid negatively charged. 



^ Journ. Chem. Soc, vol. cxv, p. 1279, 1919. 



2 Journ. Amer. Chem. Soc, vol. xxxviii, p. 1982, 1916. 



3 Cf. above, and Beans and Eastlake, lac. cit. 



