  The Boltzmann-Enskog equation for a hard sphere gas is known to have so
called microscopic solutions, i.e., solutions of the form of time-evolving
empirical measures of a finite number of hard spheres. However, the precise
mathematical meaning of these solutions should be discussed, since the formal
substitution of empirical measures into the equation is not well-defined. Here
we give a rigorous mathematical meaning to the microscopic solutions to the
Boltzmann-Enskog equation by means of a suitable series representation.
