ON BEOWNIAN OR PEDETIC MOTION. 
301 
from a cold solution, takes a long time to settle ; whereas, when 
warm and in presence of hydrochloric acid, agglomeration soon 
occurs. Iron precipitated as hydrate in presence of salts of 
ammonium, and mud in salt w^ater, are other instances. The 
motion does not cease, but the particles adhere together and 
move very slowly. 
The moving particle may be either liquid or solid ; but the 
motion of one liquid in another has a character of its own. Thus, 
if a little olive oil be shaken to an emulsion with a large quan- 
tity of water, the minute drops move, but slowly and not with 
a jerky motion. Similarly, a few drops of water mixed with a 
large volume of olive or other oil, display the same character of 
motion. 
This motion cannot be attributed, to currents in the 
liquid, for its nature is such as to preclude this explanation. It 
is in no sense regular, or in one direction. 
I have thought it worth while to compare the relative size of 
such particles with those estimated for molecules, and likewise 
the amplitude of their motion with that of molecular vibration. 
The diameter of a molecule, according to Sir W. Thomson, 
lies between the millionth and ten-millionth of a millimeter. 
The diameter of an active particle is about or below the two- 
thousandth of a millimeter. With this size the pedetic motion 
is slow and infrequent. If w^e take the larger diameter for the 
molecule, then diameter of molecule is greater than that of 
particle as 1 is to 500, and the mass, supposing them to be of 
equal specific gravity, as 1 to 125 millions. 
. If molecules do not coalesce and move as a whole, then they 
would appear to have no possible power of giving motion to a 
mass so much larger than themselves. But that molecules 
have arrangement is probable, owing to the power which some 
liquids possess of rotating the plane of polarised light. 
Clerk-Maxwell supposed for some time that the attraction of 
