HISTORICAL INTRODUCTION 11 



of OH ions. Later he abandoned this view, which, however, 

 is still held by many chemists. 



Another explanation of the coalescence of the particles which 

 have lost their electrical charge was given by Bredig on the basis 

 of surface tension changes. The surface tension at the boundary of 

 a micella and water is diminished when the particles are elec- 

 trically charged and reaches a maximum when the charge is 

 annihilated. Since at the isoelectric point the electrical charges 

 of the particles are nil the surface tension at the boundary of 

 particles and water must be a maximum and as a consequence 

 two isoelectric particles upon coming in contact are forced to 

 coalesce; while the particles will not coalesce when the surface 

 tension is low. 1 



It is, however, doubtful whether the coalescence of the non- 

 charged colloidal particles is due to surface tension effects. 

 Zsigmondy 2 points out that Powis' 3 observations on the precipi- 

 tation of droplets of oil emulsion by salts make it more probable 

 that the coalescence is due to forces of attraction between the 

 droplets, since in commencing flocculation the individual oil 

 globules only adhere to each other without coalescing into larger 

 droplets. 



Colloids can, however, be flocculated by salts even if their solu- 

 tion is not at the isoelectric point. In this case Hardy assumes 

 that the addition of the salt lowers the potential difference 

 between the colloidal particle and the solvent. Schulze, Linder 

 and Picton, as well as Hardy 4 had found that the ion which is 

 responsible for the flocculation has always the opposite sign of 

 charge to the colloidal particle, and moreover, that the coagulative 

 power of the ion increases rapidly with its valency. 5 This rule 

 was considered to strengthen the adsorption theory. 



It was assumed that the micellse possess an electrical charge 



1 MICHAELIS, L., "Die Wasserstoffionenkonzentration," pp. 49-50, Berlin, 

 1914. 



2 ZSIGMONDY, R., "Kolloidchemie," 2nd ed., p. 63, Leipsic, 1918. 



3 Powis, F., Z. physik. Chem., vol. 89, pp. 91, 179, 186, 1915. 



4 HARDY, W. B., Proc. Roy. Soc., vol. 66, p. 110, 1900, J. Physiol, vol. 

 33, p. 251, 1905-06. 



6 For the details and the literature see BURTON, E. F., "The Physical 

 Properties of Colloidal Solutions," 2nd ed., London, New York, Bombay, 

 Calcutta, and Madras, 1921. 



