modeled ISM included a cold dense phase (T < 300 K), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T ~ 10⁴ K), consisting of rarefied neutral and ionized gas. added a dynamic third phase that represented the very hot (T ~ 10⁶ K) gas that had been shock heated by supernovae and constituted most of the volume of the ISM.These phases are the temperatures where heating and cooling can reach a stable equilibrium. Their paper formed the basis for further study over the subsequent three decades. However, the relative proportions of the phases and their subdivisions are still not well understood.