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B,—CHEMISTRY. 
We have seen that in the case of the air-water surface there exists 
an electrical separation or potential difference in the surface layer, and 
that certain substances can produce pronounced variations, or even 
reversals in sign, of this electrical double. layer. It becomes a matter, 
therefore, of great interest to inquire whether similar phenomena occur 
at the interface between two immiscible liquids, and, if so, to ascertain 
whether such electrical charges or double layers bear any relation to 
the ‘ stability ’ of pure emulsions, or fine dispersions of one liquid in 
another. It is well known that those disperse or finely heterogeneous 
states of matter known as colloidal solutions depend in part for their 
stability on the existence of such electrical potential differences. We 
might expect, therefore, that an investigation of these emulsion systems 
would throw some light on the general theory of what are called ‘ sus- 
pensoid’ or ‘lyophobic’ colloidal states. __ Investigations with these 
objects in view were carried out some years ago in my laboratory by 
R. Ellis and F. Powis. The method employed was to measure directly 
by means of a microscope the motion of minute globules (suspended in 
water) under the influence of a known electric field. This procedure may 
be regarded as an extension and development of the work of Quincke. 
From the measured velocity and potential gradient the interfacial P.D. 
and the electrical charge can be calculated from the theories of Helm- 
holtz, Lamb, and Stokes. The microscopic method has the advantage 
that the P.D. between the aqueous solution and the glass wall (cover 
glass or object glass) can be simultaneously determined It is a remark- 
able fact that the P.D. between various types of hydrocarbon oils (puri- 
fied from acid as far as possible) and water was found to be 0.045—0.053 
yolt, the oil being negative—that is to say, the oil droplet moving towards 
the anode. If we compare this with the value recently calculated by 
McTaggart for the P.D. between an air-bubble and water (deduced from 
a precisely similar type of measurement), namely 0.055 volt, we can 
draw the conclusion that the potential difference is due to an electric 
double layer residing in the surface layer of the water. The oil droplet 
moves, therefore, with an attached negative layer or surface sheet, 
probably determined by hydroxyl ions, this being balanced by a positive 
layer whose charge is determined by hydrogen ions. If hydrochloric 
acid be added to the water the interface P.D. rapidly falls, and appears 
asymptotically to approach zero. If, on the other hand, caustic potash 
be added, the P.D. at first rises, reaches a maximum at a concentration 
of about one thousandth molar, and then falls with increasing concen- 
tration, but nothing like so sharply as in the case of the acid. Similar 
results hold good for the glass-water interface. From the results 
recently obtained by H. R. Kruyt (by means of the stream method) 1t 
is probable that at very low concentrations of acid there also occurs 
an initial increase in the interfacial P.D. The influence of salts is 
yery remarkable. Thus at low concentrations potassium chloride and 
potassium ferrocyanide increase the P.D., whilst at higher concentra- 
‘tions they reduce it, just as in the case of the acid and the alkali. The 
initial increase caused by potassium ferrocyanide is markedly greater 
than that caused by potassium chloride. The effect of the valency of 
the salt cation is very pronounced. Thus barium chloride at very low 
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