eer” 
B.—CHEMISTRY. 71 
aqueous solution reduce the surface P.D. and that this effect runs 
parallel with their influence on the surface tension of water. He also 
finds that acids reduce the P.D. These results may be regarded as a 
corroboration of those obtained by Kenrick. McTaggart has found that 
the nitrates of tri- and tetravalent cations have a powerful effect in not 
only reducing but reversing the P.D. (i.e. the bubble becomes posi- 
tively charged). His experiments also show that polyvalent negative 
ions, such as the ferrocyanide ion, act in the opposite direction to the 
polyvalent cations—i.e. they increase the negative charge on the bubble 
or diminish a previously existing positive one. These results are of 
great interest, inasmuch as they show the powerful effects produced by 
polyvalent ions on the P.D, existing in the surface layer of an aqueous 
solution. As we shall see presently, very similar results have been 
obtained at liquid-liquid and solid-liquid interfaces. But it is of great 
importance to know what happens at the air-liquid interface, since we 
ean largely discount the chemical and physical influence of the gas 
phase. 
Although the electrometric method employed by Kenrick and Thor- 
waldson only gives comparative results (since two interfaces must always 
be simultaneously used), whilst the cataphoresis method gives results 
for a single interface, it is necessary to observe that the electrometric 
method measures the total fall of potential from the bulk of one phase 
to the bulk of another. The cataphoresis method measures what 
Freundlich has called the ‘ electrokinetic’ P.D.—that is to say, the 
potential drop between the limiting surface of the ‘ fixed’ part of the 
_ double layer and the rest of the liquid. The two values need not neces- 
sarily coincide. 
When liquids are sprayed or splashed, or when gases are bubbled 
through liquids, it is known that the gas often acquires an electrical 
charge, whilst the liquid acquires an opposite one (so-called ‘ waterfall ’ 
electrification). Since the pioneer work of Elster, Lenard, J. J. 
Thomson, Kelvin, Maclean, and Galt, very many investigators have 
dealt with this subject (Eve, Christiansen, Bloch, de Broglie, Zwaarde- 
makers, Coehn, &c.). Originally, Lenard thought that the effect was 
due to a ‘ contact electrical’ action between the gas and the liquid, 
whilst J. J. Thomson was inclined to ascribe it to a sort of partial 
chemical action between them. It is known that there are produced 
in the air or gas relatively slow-moving carriers, both positive and 
negative. Lenard has quite recently changed his views, and ascribes 
the origin of these carriers to the tearing off of very small portions of 
the outside layer of the electrical double layer existing in the surface 
of the liquid. It may be mentioned that Kenrick, Thorwaldson, and 
McTaggart came to the conclusion that the surface P.D.’s measured 
by them were not connected, or at all events not connected in any simple 
manner, with the phenomena of waterfall electrification. 
We may say, therefore, that if there be a relation between these 
two types of phenomena, it is a complicated and still largely obscure one. 
The subjects which I have been discussing have an interesting bearing 
on the formation and stability of foams and froths. If air be violently 
churned up with water, only comparatively large bubbles are produced, 
