178 
Movement of Microscopic Particles 
[April, 
alum has been dissolved in river-water to clear it. I presume 
that we may in the same way explain the use of the Strych- 
nos potatorum, or clearing nut, the seeds of which are used 
in the East Indies for the purpose of clearing muddy water. 
One of the seeds is well rubbed for a minute or two round 
the inside of the vessel containing the water, which is then 
left to settle ; in a short time the impurities fall to the 
bottom. Bitter almonds are employed for the same purpose 
in Egypt, and those of the Kola or Sterculia in Sierra 
Leone. These methods are probably due to the destruction 
of pedesis. 
The connection between pedesis and suspension is simply 
this ; — In the absence of pedesis suspended particles attract 
each other and become aggregated together into little groups, 
which then acquire sufficient weight to force their way down 
through the resisting liquid. This effeCt I have observed 
some hundreds of times, both in and out of the microscope. 
If a little china clay be mixed up with water containing 
i-ioooth or 1-10, oooth part of sulphuric acid, the milky 
liquid will presently assume a flocky curdled appearance, 
and after a time the little flocks will subside, streams of 
clear water making their way up between the interstices. 
The rate of subsidence is doubtless affeCted by other circum- 
stances, such as the specific gravity of the liquid, the 
smallness of the separate particles, and their shape ; but it 
is the aggregation of particles into groups which mainly 
determines precipitation or suspension. As a rain-drop is 
to a cloud particle, so is a group of clay particles to the 
minute suspended particles. Prof. Stokes has shown that 
the resistance experienced by the minute particle in falling 
through a fluid is comparatively enormous, so that there is 
really no further mystery in the falling of curdled precipi- 
tates. But pedetic motion prevents the formation of groups; 
it keeps each minute particle — say i-ioooth of an inch — 
from each other particle, so that each encounters the sepa- 
rate resistance of the fluid. 
Dujardin, indeed, thought that the pedetic motion caused 
particles to diffuse themselves through water. “ (Test lui 
aussi qui repand dans toute une masse d’eau, et y tient en 
suspension les particules desagreges d’un corps animal ou 
vegetal qui se decompose.” In this he is probably wrong, 
for if a layer of clear distilled water be placed above a mix- 
ture of china clay and distilled water no diffusion of the 
clay upwards will be detected. It is difficult to make the 
experiment in an accurate manner ; but if we consider that 
the pedetic motion, as seen in the microscope, is alternating 
