  Ferromagnetic metals and spin-polarized $^{3}$He are spin 1/2 systems with
the same macroscopic symmetry, and thus should have macroscopic magnetic
dynamics with the same structure. Using Onsager's irreversible thermodynamics,
we develop a theory for these systems that contains two relaxation times (one
for the magnetization $\vec{M}$ and the other for the spin current
$\vec{J}_{i}$), a magnetic compressibility, and a mean-field parameter.
Currently spintronics data on metallic ferromagnets are analyzed using a
complex decay length from a theory employing a diffusion constant, a lifetime,
and a mean-field parameter. The present theory leads to a complex decay length
with the same structure. On neglecting decay of $\vec{M}$, the present theory
applies to liquids and gases. For macroscopic equations the particle statistics
is not relevant, so the theory also applies to bosons. The theory predicts a
longitudinal spin wave whose velocity we estimate for liquid $^{3}$He and for
paramagnetic metals; but such a wave should also occur for ferromagnets and for
gases.
