B.—CHEMISTRY. fof 
have seen, increase the P.D., increase this stability. As soon as the 
concentration of any electrolyte is sufficient to bring the P.D. into 
the critical zone, the stability of the emulsion undergoes a sudden and 
very marked decrease, and relatively rapid coagulation occurs. Take, 
for example, the case of thorium chloride. On increasing the concen- 
tration we find that the interfacial P.D. traverses successively the 
following regions :— . 
(1) Above the critical value (and negative). __ 
(2) Inside the critical zone (negative and positive). 
(3) Above the critical value (and positive). 
(4) Below the critical value (and positive). 
In exact correspondence with this series we find that the emulsion 
goes through the following states :— 
(1) Stable (oil particles ‘ negative’). 
2) Unstable and flocculating (oil particles negative or positive). 
(3) Stable (oil particles positive). 
(4) Unstable and flocculating (oil particles positive). 
Here we see a very striking analogue and explanation of the pheno- 
mena observed by Joly in studying the effect of aluminium salts on the 
sedimentation of clays, and of the numerous examples of the so-called 
‘irregular series ’ observed in the flocculation of suspensoid hydrosols 
by salts with polyvalent cations. 
As Linder and Picton showed, when two suspensoid hydrosols, one 
negative and the other positive, are mixed, then, depending on the 
ratio, a stable hydrosol (either positive or negative) can be obtained. 
In continuation of this work, W. Biltz demonstrated the existence in 
such cases of a ‘zone of coagulation,’ 7.e. a zone of concentration 
ratios leading to coagulation. A study of the mutual behaviour of a 
negative oil emulsion and the positively charged ferric oxide hydrosol 
provides a complete explanation of this curious phenomenon. When 
increasing amounts of the iron oxide hydrosol are added to the oil 
emulsion it is found that the interfacial P.D. falls to zero, and then 
reyerses its sign, becoming increasingly positive—an action which is 
due to the adsorption of the positively charged micelles at the oil-water 
interface. When the P.D. is above a certain value (positive or nega- 
tive) the system is stable. But within the critical zone a rapid and 
relatively complete mutual coagulation takes place. 
These studies of oil emulsions (and of the glass-water interface), 
by means of the micro-cataphoresis method, have thrown a great deal 
of light on many previously ill-understood points in the theory of 
colloids. If, for example, the P.D. between the particles of a suspen- 
soid hydrosol and the aqueous fluid is not above the critical potential, 
coagulation will occur. But very small concentrations of certain 
electrolytes can raise the P.D. and stabilise the hydrosol. This is the 
explanation of the well-known ‘ peptising’ action. Higher concentra- 
tions of even the same electrolytes will reduce the P.D. below the 
critical potential, and produce flocculation. We see also that rapid 
coagulation will occur before the P.D. becomes zero. This was 
proved for arsenic sulphide hydrosol by Powis. Later experiments 
of Kruyt have confirmed these conclusions. It is obvious, therefore, 
