76 SECTIONAL ADDRESSES. 
concentrations probably causes a very small rise of the P.D., but at 
quite low concentrations its effect is to reduce it, and this effect increases 
rapidly with rising concentration, and is much more marked than in 
the case of potassium chloride. The lowering effect of aluminium 
chloride at low concentrations on the P.D. is much more pronounced 
than in the case of barium chloride, and this effect becomes still greater 
with thorium chloride. Both aluminium chloride and thorium chloride 
at low concentrations reverse the sign of the P.D., the oil side of the 
double layer becoming positive. In these cases the positive charge of 
the oil droplet reaches a maximum with increasing concentration of 
the salt, and then appears to fall slowly towards zero. No second 
reversal of sign has ever been observed. So far as the solid-liquid 
interface is concerned, these results have been in general confirmed by 
the electroendosmotic experiments of G. v. Elissafoff (carried out in 
Freundlich’s laboratory) and by the stream-potential measurements 
of Kruyt. It may also be remarked that Loeb has recently obtained 
similar results in the case of collodion particles, using the micro- 
cataphoresis method. Perhaps the most remarkable result which has 
emerged from these electrical investigations of oil suspensions is the 
relation between the stability of the emulsion and the potential difference 
of the interfacial double layer. The minute oil globules are in constant 
Brownian motion and must frequently collide. Why do the forces 
of cohesion not produce agglomeration or coalescence (coagulation or 
clearing of the emulsion)? We should expect that under determinate 
conditions a certain fraction of these collisions would give rise to coher- 
ence. Is there any other factor besides orientation of path and kinetic 
energy which affects the probability of coherence following an encounter ? 
At distances great in comparison with their own dimensions the electric 
double layers will act practically as closed systems. But when two 
oil drops approach sufficiently near each other the conditions will be 
different, since we must expect a repulsive force when two similarly 
charged outer layers just begin to interpenetrate each other. Hence 
the answer to the question asked above is that the third factor is the 
potential difference or electric density of the interfacial double layer. 
Other things being equal, the probability P of an encounter leading 
to coherence will be a diminishing function of the electric intensity x 
of the similarly constituted double layers, i.e. a will be negative. 
Hence of the total number of encounters in a given small period of 
time the number which lead to coherence should be a maximum at the 
point of zero potential difference (iso-electric point of Hardy). i 
Now the experiments of Powis brought out the very important fact 
that when the interfacial P.D. (whether positive or negative) is above 
a certain value, which was about 0.03 volt for his conditions, the rate 
of coagulation or coherence of the oil drops is relatively small, but 
rapidly increases when the P.D. falls inside the zone —0.03 to +0.03 
volt. Under definite conditions there exist, therefore, what we may, 
speaking broadly, call a critical potential and a critical potential zone. 
When the P.D. is outside this zone the emulsion is comparatively very 
“stable.” Very small concentrations of electrolytes, which, as we 
