i8 78.] 
Suspended in Liquids. 
1 71 
has not since been used. I have therefore ventured to coin, 
or rather adopt, a new word, and call the motion pedesis, 
from the Greek leaping or bounding, this being the 
correct description of the phenomenon when seen in perfec- 
tion. We also have the advantage of the perfectly classical 
adjedtive pedetic, from the Greek -tt^tlkoq. 
The pedetic movement cannot be better seen than by 
taking a drop of old common ink which has been exposed 
to the air for some weeks, and examining it under thin glass 
with a magnifying power of 500 or 1000 diameters. An 
infinite multitude of minute black particles will be seen, all 
in such rapid motion as to cause a boiling or swarming 
appearance. This is, in fadt, a wonderful sight. In new 
clear ink the particles are fewer and very minute, but all in 
motion, -as far as can be seen. Almost any kind of fine clay 
mixed with pure water shows pedesis, and muddy rain water 
from a macadamised road, if properly examined, will soon 
convince the observer what a common phenomenon it must 
be. Perhaps the best possible exhibition of the motion is to 
be got by grinding up a particle of pumice-stone in an agate 
mortar, and mixing it with distilled water. The minute 
angular particles will be seen under the microscope to leap 
and swarm about with an incessant quivering movement, so 
rapid that it is impossible to follow the course of a particle, 
which probably changes its direction of motion fifteen or 
twenty times in a second. 
It is very difficult to measure or describe the motions 
with accuracy, and they vary much according to circum- 
stances. The distance through which a particle moves at 
any one bound is usually less than i-5oooth part of an inch. 
The motions much depend upon the sizes of the particles. 
Those of a greater diameter than i-5000th of an inch are 
seldom seen to move, and the motion is more marked as the 
particles are smaller. Exceedingly minute particles may 
sometimes be seen literally to skip and dance about, as in 
the case of some minute particles of metallic antimony 
which Mr. Dancer,* the microscopist, of Manchester, 
showed me with an excellent i-8th inch objective. 
There is no apparent lower limit to the size of moving 
particles, and down to the 1-50, oooth or 1-70, oooth part of 
an inch the movement certainly becomes more and more 
remarkable. 
* Mr. Dancer, F.K.A.S., studied this subject at intervals for thirty years, 
but I do not know that he has published any results except those given in the 
Proceedings of the Manchester Literary and Philosophical Society for January 
25th, 1870 (vol. ix., p„ 82). Mr. Dancer found that diamond-dust and graphite 
could be made to show the motion. 
