50 



and the attractive forces acting between the positively charged 

 particles and the hydroxide ions. But Powis produced negatively 

 charged colloidal ferric hydroxide by adding a sol of the common 

 type to a dilute solution of sodium hydroxide. He considered the 

 change in sign of the charge to be due to the adsorption of hydroxidion. 



No reliable method has yet been devised for determining the 

 absolute value of the electrical charge on a colloidal particle. From 

 Burton's data, Lewis made an extremely rough calculation of 

 8 X 10"'' electrostatic units for a platinum particle. Powis calculated 

 a value of about 2 x 10~" for a coarse silver particle. Upon the 

 assumption of the existence of the Helmholtz double-layer at the 

 surface of colloidal particles, the difference of potential between the 

 disperse phase and the medium has been calculated. The voltages 

 found lie almost entirely between — 007 and +0-07. The results 

 of Ellis and of Powis for oil emulsions indicate that for a stable 

 emulsion the absolute value of this voltage must be greater than 

 0-03. Between the values — 0-03 and + 0-03, complete coagulation 

 occurs, but at a velocity apparently independent of the voltage. 



Wilson has shown that the electrical charge on the surface of 

 colloidal particle must cause an unequal distribution of ions between 

 the surface layer of solution surrounding the particles and the bulk 

 of solution, which would, in turn, result in a difference of potential 

 between the two phases. A very important conclusion of this work 

 is that the addition of an electrolyte to a sol, provided no chemical 

 changes follow, must result in a lowering of the absolute value for 

 the potential difference between the two phases, even though there 

 may be no change in the magnitude of the electrical charge on the 

 colloid itself. This is considered to be the explanation of the 

 precipitation of suspensoids by addition of salt. 



This subject is so closely allied to that of electrical endosmose 

 that the report compiled by Briggs (Second Report, p. 26) should be 

 consulted. 



Bibliography. 



Efans. H. T., and Eastlack, H. E. ' The Electrical Synthesis of Colloids 



'J. Am. Chem. Soc.,' 37, 2667 (1915). 

 Burton, E. F. ' The Physical Properties of Colloidal Solutions ' (Longmans, 



Green & Co.). 

 Ellis, R. 'Properties of Oil Emulsions': I Electric Charge; ' Z. Physik. 



Chem.' 78, 321. II. Stability and Size of the Globules, ibid., 80, 597. 



III. Coagulation by Means of Colloidal Solutions, ibid., 89, 145 (1914). 

 Fbenkel, J. ' The Surface Electric Double-Layer of Solid and Liquid Bodies.' 



'Phil. Mag.' 33. 297 (1917). 

 Glixelli, S. ' The Electrical Transference of Gels.' ' KoUoid-Z.' 13, 194. 

 Hakdy, W. B., and Harvey, H. W. ' Surface Electric Charge of Living Cells.' 



•Proc. Royal Soc' London, 84, 217. 

 Hakdy, W. B. ' Differences in Electrical Potential within the Living Cell.' 



'J. Physiol.' 47, 108. 

 VON Hevesy, G. ' The Charge and Dimensions of Ions and Dispersoids,' 



'KoUoid-Z.' 21, 129 (1917). 

 KiMTTRA, M. ' Nature of the Double Layer in Colloidal Particles,' ' Memoirs 



Coll. Science,' and ' Eng. Kyoto Imp. Univ.', 5, 201. 



