  We report the first analyses of SDO/AIA observations of the formation of a
quiescent polar crown prominence in a coronal cavity. The He II 304 \AA\ (log
T_{max} ~ 4.8 K) data show both the gradual disappearance of the prominence due
to vertical drainage and lateral transport of plasma followed by the formation
of a new prominence some 12 hours later. The formation of the prominence is
preceded by the appearance of a bright emission "cloud" in the central region
of the coronal cavity. The peak brightness of the cloud progressively shifts in
time from the Fe XIV 211 \AA\ channel, through the Fe XII 193 \AA\ channel, to
the Fe IX 171 \AA\ channel (log T_{max} ~ 6.2, 6.1, 5.8 K, respectively) while
simultaneously decreasing in altitude. Filter ratio analysis estimates the
initial temperature of the cloud in the cavity to be approximately log T \sim
6.25 K with evidence of cooling over time. The subsequent growth of the
prominence is accompanied by darkening of the cavity in the 211 \AA\ channel.
The observations imply the possibility of prominence formation via in situ
condensation of hot plasma from the coronal cavity, in support of the proposed
process of magneto-thermal convection in coronal magnetic flux ropes.
