Every Brownian particle is then moving about, according to Einstein’s assumption, with a mean energy of motion along each axis equal to
This motion is due to molecular bombardment, and in order to write an equation for the motion at any instant of a particle subjected to such forces we need only to know (1) the value
of the
-component of all the blows struck by the molecules at that instant, and (2) the resistance offered by the medium to the motion of the particle through it. This last quantity we have set equal to
and have found that in the case of the motion of oil droplets through a gas
