  Impulse formulations of Hall magnetohydrodynamic (MHD) equations are
developed. The Lagrange invariance of a generalized ion magnetic helicity is
established for Hall MHD. The physical implications of this Lagrange invariant
are discussed. The discussion is then extended to compressible Hall MHD.
Compressibility effects are shown not to affect, as to be expected, the
physical implications of the generalized ion magnetic helicity Lagrange
invariant.
