  We report the behaviour of mixtures of fine bronze and glass spheres under
sinusoidal vertical vibration. Depending upon the ratio of their diameters and
the amplitude and frequency of the vibration, we observe the formation of sharp
separation boundaries between glass-rich and bronze-rich phases, the absence of
gross convection which would mix these phases, and a number of oscillatory and
non-periodic behaviours. These phenomena are related to the differential air
damping of the glass and bronze grains, disappearing completely in the absence
of air.
