f 38 
X. 
ON THE CONCENTRATION OF SOAP SOLUTION ON THE 
SURFACE OF THE LIQUID. By DAVID HENRY 
HALL, B.A. 
[COMMUNICATED BY PROFESSOR G. F. FITZ GERALD, F.T.C.D., F.R.S.| 
[Read Aprit 19, 1899; Published May 8, 1899.] 
In order to determine whether there is any concentration of soap 
solution in the superficial film of the liquid, the following experi- 
ments were made at the suggestion of Professor Fitz Gerald. 
Lord Rayleigh had observed that the superficial tension of a 
soap solution while the surface was being extended differs from 
that of the surface shortly after it has ceased to be extended. He 
attributed this to a possible concentration of the soapy matter in 
the superficial film which took place after the surface was formed. 
He showed that this large superficial tension of a recently- 
formed surface explained the stability of soap-bubbles and of the 
foam on frothing liquids. When any part of the surface is 
thinned out, the tension then becomes great; and, in consequence, 
the tendency to thin is counteracted, and the film is stable. 
He analysed the foam of a mountain stream, and found much 
more organic matter in it than in the rest of the stream-water. 
This observation is, however, hardly conclusive, and to study the 
matter further, a soap solution was frothed up, and allowed to 
drain for a short time, and then the froth was separated from the 
drained-off liquid, and each separately analysed. The experiments 
were conducted as follows :— 
A large stock of saturated soap solution was prepared by 
allowing distilled water, previously shaken up with pieces of 
