  The virial expansion of a gas is a correction to the ideal gas law that is
usually discussed in advanced courses in statistical mechanics. In this note we
outline this derivation in a manner suitable for advanced undergraduate and
introductory graduate classroom presentations. We introduce a physically
meaningful interpretation of the virial expansion that has heretofore escaped
attention, by showing that the virial series is actually an expansion in a
parameter that is the ratio of the effective volume of a molecule to its mean
volume. Using this interpretation we show why under normal conditions ordinary
gases such as O_2 and N_2 can be regarded as ideal gases.
