Suspended in Liquids. 
T 73 
1878.] 
or to some obscure chemical action between the solid parti- 
cles and the fluid, which is indirectly promoted by heat.” 
Although I believe there is obscure chemical aCtion, my ex- 
periments lead me to think that the effeCt of heat is rather 
the other way, and that increase of temperature decreases 
the motion. I was not able, indeed, to perceive any differ- 
ence produced by warming the microscope plate ; but as 
this is a difficult and uncertain experiment, I tried the 
matter indireCtly. A mixture of charcoal-powder and boiled 
water was surrounded with ice, and a similar mixture was 
placed in boiling water and maintained at ioo° C. At the 
end of one hour the heated mixture had deposited nearly all 
the charcoal, whereas the ice-cold water still had as much 
in suspension after eight hours. A similar experiment with 
china clay gave like results. This is the more surprising 
and conclusive inasmuch as the heated mixture would be the 
most likely to he disturbed by convention currents. It is 
well known that heat causes the aggregation of many preci- 
pitates, and I suspefft that this will in most cases be due to 
the diminution of pedesis, and I shall point out that this is 
probably explained by the increase of eleCtrical conductivity 
of liquids produced by rise of temperature. 
It may still be urged that a liquid while receiving or 
parting with heat might show increased pedesis. To test 
this I took a tube containing china clay and water, and 
during two days frequently suspended it before the fire, 
allowing it to cool in the intervals. An exactly similar tube 
was sunk in sawdust which had been lying for several years 
undisturbed in a wine-cellar. After remaining fifty-two 
hours in nearly perfect darkness and equilibrium of temper- 
ature (about 9° C.), the latter tube was found to contain 
more clay in suspension than the one which had been moved 
about and warmed many times. Even after seven days the 
buried tube still showed a slight cloudiness. This was a 
very striking experiment, and quite convinced me that no 
extrinsic causes are concerned in the matter. 
As it might be thought that the movement is connected 
in some way with the shape of the particles, I compared in 
the microscope the fine needle-shaped particles of asbestos 
dust with the spherical globules of milk, the minute spheres 
of gamboge, the flat particles of talc, the small cubes of 
galena, and the wholly irregular fragments of glass. As 
particles of all these various forms show pedesis, no parti- 
cular form can be essential to its production ; but it seems 
likely that, cceteris paribus, sharp-pointed and irregular par- 
ticles oscillate most intensely and rapidly. Spherical 
