  A simple assumption of an emergence in gas of small atomic clusters
consisting of $c$ particles each, leads to a phase separation (first order
transition). It reveals itself by an emergence of ``forbidden'' density range
starting at a certain temperature. Defining this latter value as the critical
temperature predicts existence of an interval with anomalous heat capacity
behaviour $c_p\propto\Delta T^{-1/c}$. The value $c=13$ suggested in literature
yields the heat capacity exponent $\alpha=0.077$.
