78 SECTIONAL ADDRESSES. 
that coagulation of a lyophobic hydrosol will occur before the iso-electric 
point is reached, and that Hardy’s famous rule requires revision. 
The following table contains the concentrations (in millimols per 
litre) of certain electrolytes required to reduce the potential of a certain 
hydrocarbon oil emulsion from its ‘ natural ’ value (against pure water) 
of 0.046 volt to the critical value, 0.03 volt :— 
—— | Concentrations Sonate es 
KCl 5 - F 51 2500 
BaCl, . 5 “ 19 95 
AlCl, . A : 0°020 1 
Thch . : é 0°0070 0°35 
These results show the enormous influence of the valency of the 
cation in a series of salts with the same univalent anion, and explain 
in a striking manner the analogous effects in the coagulation of lyophobic 
hydrosols. ‘The exact value of the critical potential and the range 
of the critical zone will depend, of course, on the experimental defini- 
tion of ‘rapid coagulation,’ and on the concentration, nature, and 
degree of dispersion of the hydrosol. It is not to be supposed, there- 
fore, that these critical values are constants except under very definite 
conditions. The fundamental fact is that under given conditions the 
rate of coagulation of the particles of an oil suspension or of a lyophobic 
hydrosol undergoes a relatively sudden and very great increase when 
the interfacial P.D. falls below a certain finite value (positive or 
negative). 
There is not time or space at my disposal to enter into the much 
discussed question as to the inner mechanism of the action whereby 
ions (and electrically charged micelles) set up or vary the potential 
difference in the interfacial layer. According to Freundlich’s original 
theory we must ascribe an independent effect to each ion, which will 
depend on the sign of its charge, its specific adsorbability, and electro- 
valency and the nature of the already existing double layer. A different 
theory was proposed by Freundlich in order to explain the results 
obtained in the electroendosmotic experiments of Elissafoff. According 
to this point of view, the ‘ solid’ surface acts chemically (as an acid, 
base, ampholyte, or salt), whereby it may dissociate off an ion or ions, 
and itself become an ionised surface. Invading foreign ions may then 
alter this ionisation equilibrium; or they may simply combine with 
the ionised surface and form neutral insoluble spots (compare the 
views of Freundlich, Gyemant, and Kolthoff). J. N. Mukherjee has 
‘suggested that ions are attached to the surface by chemical forces, and 
has attempted to work out an electro-kinetic theory of ion adsorption. 
It is probable that surfaces very often do act ionically or chemically, 
and that specific actions of this sort must often be taken into account 
in dealing with the great variety of material presented in the study 
of surface actions. Nevertheless, in the case of the hydrocarbon oil 
‘droplets studied by Ellis and Powis, or the gas-liquid interface studied 
