THE CHARGE ON AN ION 
47 
Suppose a small particle, not too small to be visible with a 
powerful microscope yet not too large in comparison with the 
size of the molecules to be affected by collision with them, is 
suspended in water. This particle will, by the kinetic theory, 
be bombarded on all sides by the molecules of the water moving 
under their velocities of thermal agitation. On an average the 
number striking the particle will be the same in all directions 
and the average momentum communicated to the particle in 
any direction will be zero. Since, however, the collisions are 
governed by the law of probability there will, if the interval 
of time considered be sufficiently small, be an excess of momentum 
in one direction or the other, and if the mass of the particle is 
not too large compared with the masses of the colliding mole 
cules it will be given an irregular kind of motion which may 
be observed under a microscope. 
This motion has been known for many years as the Brownian 
movement and is always to be observed in suspensions of fine 
particles in a liquid. 
Experiments have shewn that the Brownian movements 
are independent of any currents in the liquid, or of any 
external vibrations. They occur with particles of any nature 
providing that the size is suitable, and they do not diminish 
with lapse of time. For example, Brownian movements 
can be observed among the fine particles enclosed in those 
liquid cavities which are often found in specimens of quartz and 
which have been sealed up for many millions of years. The ex 
planation just given is the only one consistent with all these facts. 
The particles in Brownian movement are thus sharing the 
thermal agitations of the molecules of the surrounding liquid 
in exactly the same way that the molecules of a heavy vapour 
share the motion of the molecules of a light gas in which they 
may be placed. In other words, from the point of view of the 
kinetic theory the particles in a suspension behave like the 
molecules of a dilute gas of very high molecular weight, each 
particle functioning as a single molecule. We can, therefore, 
apply the gas laws to them, and from observations on these 
visible particles determine the various unknown constants in 
the gas equations.