300 
ON BEOWNIAN OR PEDETIC MOTION. 
(/) Or bad conductors, e.g., Sulphur, gamboge. 
[g) They may be transparent, or (li) opaque. 
3rd. — This motion does not depend on the form of the 
particles. 
The question of pedesis is very closely connected with that of 
the settling of finely-divided powders in different menstrua. In 
a paper communicated to the Geological Society of London in 
1876, on the settling of mud, I showed : 1st — What had been 
noticed by previous observers (W. Skey and others), that finely- 
divided matter does not quickly settle in pure water. 2nd — That 
it settles more, quickly in hot than in cold water. 3rd — That 
the rate of settling does not depend on the density of the solu- 
tion, for mud settles more quickly in strong than in weak 
solutions 4th — It does not depend on the chemical action of 
the liquid on the solid, for sulphur follows the same rule as 
other substances. 5th — It follows the same order as the absorp- 
tion of heat when the salt is dissolved, in the solution of which 
the suspended particles settle. 6th — It depends on the 
agglomeration of the particles : when the particles acquire 
sufficient size to have no motion, or a very slow one, they settle 
quickly. This phenomenon is evidently closely allied to pedetic 
motion, and is to be explained by it. 
Pedetic motion depends on, that is, is affected by : — 
1 . The size of the particles. Particles more than -g-p^oth of 
an inch in diameter do not jerk about suddenly, but are some- 
times seen to oscillate slightly. 
2. The specific gravity of the particles. Metals, or particles 
of vermilion, of similar size to particles of silica or gamboge, 
move much more slowly and less frequently. 
• 3. The nature of the liquid. No liquid stops pedesis ; but 
liquids which have a chemical action on the substance do. 
This action may be very slow, still it tends to agglomerate the 
particles. For instance, barium sulphate, when precipitated 
