I 75 
1878.] Suspended in Liquids. 
the cause of motion, because the particles do not come to 
rest after hours. Nevertheless I believe that motions lasting 
for several years are not inconsistent with an electrical 
explanation. 
Prof. Tyndall has quite recently expressed his opinion 
that this motion of particles is due to “ surface-tension,” 
but he cannot be aware of the length of time during which 
the motion will continue. It is obvious that from surface- 
tension we cannot derive a long-continued supply of energy, 
as is required to explain the phenomena. 
One of the most important questions to decide is natu- 
rally what substances exhibit these movements, or do all 
substances exhibit them ? I have tried many experiments 
with a view of answering these questions, and though at 
times there seemed to be particles of the proper size which 
did not move in pure water, subsequent trials often threw a 
doubt upon the conclusion. I hesitate, therefore, to affirm 
that there is any substance which may not be reduced to 
such small particles, and so suspended in water as to show 
pedesis ; but there are certainly great differences in activity. 
Silica, in all its insoluble forms and compounds, is especially 
remarkable for the activity of movement. Clay of several 
kinds, earthenware, fire-brick, common brick, clay slate, 
silicified wood, glass of several kinds, talc, mica, pumice- 
stone, asbestos, basalt, granite, slag from iron and other 
furnaces, — also many silicious minerals, such as topaz, 
tripoli powder, glacier sand, — all exhibit the movement in 
a high degree of intensity ; and these instances are probably 
sufficient to show that all silicious compounds, and accord- 
ingly the greater part of known minerals and rocks, yield 
very adtive particles. But the state of composition is not 
requisite to the effect ; for the purest quartz crystal, agate, 
cornelian, chalcedony, or the finest white sand, when reduced 
to dust, show almost equally adtive motion. On the whole 
the silicates and silica exhibit pedesis in the highest per- 
fection. 
The substances, on the other hand, which seem to be 
least remarkable for pedesis, are oxides and other compounds 
devoid of silica. I may mention especially chalk, fluor spar, 
hematite iron ore, galena, oxide of tin, barium sulphate, 
calcium sulphate, titaniferous sand, bone-dust. Various 
other substances — such as charcoal of several kinds, coal, 
coke, emery powder, amorphous phosphorus — show the 
motion readily. The metals iron, steel, gold, and platinum 
were tried at first without success, probably because it was 
not easy to reduce them to sufficiently minute particles ; 
