5 I 4 
The British Association . 
[October, 
nels generally.” He described the various theories which 
had previously been put forward to account for the well- 
known fa (51 that the flow of water is not always greatest at 
the surface, and advanced a new theory, which he illustrated 
by various diagrams. 
This was followed by a paper “ On the Pedetic Action of 
Soap,” by Prof. W. Stanley Jevons, which we give in ex - 
tenso 
Since the publication, in the “ Quarterly Journal of 
Science ” for April, 1878, of my paper on Peaesis, or the 
so-called Brownian movement of microscopic particles, it 
has been suggested to me that soap would form a good 
critical substance for experiment in relation to this pheno- 
menon. It is the opinion of Prof. Barrett, and some other 
physicists, that the movement is due to surface tension, 
whereas I believe that chemical and electromotive adtions 
can alone explain the long-continued and extraordinary mo- 
tions exhibited by minute particles of almost all substances 
under proper conditions. Soap considerably reduces the 
tension of water in which it is dissolved, without much 
affedting (as is said) its electric condudtibility. If, then, 
pedesis be due to surface-tension, we should expedt the 
motion to be killed or much lessened when soap is added to 
water. 
Having tried the experiment, I find that the result is of 
the opposite charadter to what Prof. Barrett anticipated. 
With a solution of common soap the pedetic motion be- 
comes considerably more marked than before. I have 
observed this result not only with china clay and some 
other silicates, but also with such comparatively inert sub- 
stances as the red oxide of iron, chalk, and even the heavy 
powder of barium carbonate. The last-named substance — 
one of those we should least expedt to dance about of its 
own accord — gave a beautiful exhibition of the movement 
when mixed with a solution of about 1 per cent of soap, 
and viewed with a magnifying power of 500 or 1000 dia- 
meters. 
The correctness of this result was also tested by observing 
the suspending power of solutions of soap-solution compared 
with water. If a little china clay be diffused through com- 
mon impure water — that, for instance, of the London Water 
Companies — the greater part of the clay will soon be seen 
to collect together in small flocks, and fall to the bottom in 
two or three hours, the water being almost clear. However, 
if a little soap be dissolved in the water, the behaviour 
