  The periodic Lorentz gas is the dynamical system corresponding to the free
motion of a point particle in a periodic system of fixed spherical obstacles of
radius $r$ centered at the integer points, assuming all collisions of the
particle with the obstacles to be elastic. In this Note, we study this motion
on time intervals of order $1/r$ and in the limit as $r\to 0^+$, in the case of
two space dimensions.
