  Using simple and general arguments we propose an effective Hamiltonian for
the description of low-energy pure QCD. The Hamiltonian is a function of
spatially constant collective modes. Its eigenstates can be organized into
bands classified by the irreducible representations of an O(8) group. The
latter also determine parity and charge conjugation of the states. The energy
spectrum agrees well with the glueball spectrum as measured on the lattice, and
in particular the level ordering with respect to spin is naturally explained.
