70 SECTIONAL ADDRESSES. 
with the same type of apparatus by Professor Thorbergur Thorwaldson 
in my laboratory. The general result of these experiments can be 
described in the following terms :— 
Consider the system : 
Aqueous Solution of KCl 
(cone. =c) Air 
A 
The positive potential of A will be equal to that of B. If we now 
add to the solution B a small quantity of a substance S (generally a 
non-electrolyte or weak electrolyte) which has a strong tendency to 
concentrate at the air-B interface, it is found that the positive potential 
of A rises markedly above that of B, the value of the quantity, positive 
potential of A minus that of B, varying with the concentration of S in 
the way that is characteristic of adsorption phenomena. What is the 
interpretation of this phenomenon? If we were to assume that there 
was practically no P.D. at the interface A-air, it would follow that the 
effect of S is to make the positive potential of the bulk of B markedly 
below that of the air. The same result would follow if we were to 
assume that at the interface A-air there exists a P.D. which makes the 
positive potential of the bulk of A markedly below that of the air out- 
side. Both these assumptions would lead to the conclusion that in the 
surface layer of the solution at the A-air interface there must exist either 
no electrical double layer, or else one with its positive half oriented 
towards the air side. Now Quincke has shown that a bubble of air 
in water placed in an electrical potential gradient travels towards the 
anode—i.e. the bubble behaves as if it were negatively charged. From 
this it would follow that the P.D. at the air-water interface is such 
that the negative half lies towards the air side. As an electrolyte such 
as KCl is negatively adsorbed at an air-liquid surface, it is probable 
that a P.D. of the character indicated by Quincke’s experiment exists 
at the A-air interface. If we accept this conclusion, it follows that the 
effect of S is markedly to reduce this P.D. (or to reverse it). Now the 
P.D. at the air-water interface is probably due to the existence of a 
double layer containing hydroxyl ions on the outside and hydrogen ions 
on the inside, or to oriented water molecules regarded as electrical 
bi-poles. If S is a non-electrolyte (or a substance which possesses 
httle self-ionisation), we can understand why its concentration at the 
surface could result in the reduction of this P.D. 
The experiments of Thorwaldson show that a substance such as 
the hydrochloride of methyl violet has a powerful effect on the P.D. at 
the air-water interface. It is probable that in this case the complex 
basic dye cation is drawn into, or ‘ adsorbed ’ in, the outside layer next 
to the air, the result of this being a reduction or possibly reversal of 
the original potential difference. 
Kenrick found that 1f gases such as hydrogen and coal gas be sub- 
stituted for air, there is no effect on the surface P.D. 
Within the last few years H. A. McTaggart has made a number of 
experiments on the electric cataphoresis of gas bubbles in aqueous solu- 
tions and other liquids. He finds that aliphatic acids and alcohols in 
Aqueous Solution of KCl 
(cone. =c) 
