  The dust clouds in the torus of the quasar are irradiated by the central
source, and the clouds at the inner radius of the torus re-radiate mostly in
the near-infrared (NIR) wavebands. The ratio of the near-infrared luminosity to
the bolometric luminosity L_NIR/L_bol can therefore reflect the torus geometry
to some extent. We find a significant correlation between the ratio of the
near-infrared luminosity to the bolometric luminosity L_NIR/L_bol and the
central black hole mass M_bh for the Palomar-Green(PG) quasars, whereas no
correlation is found between the Eddington ratio L_bol/L_Edd and the ratio
L_NIR/L_bol. Similar correlations are found for the mid-infrared and
far-infrared cases. It may imply that the torus geometry, i.e., the solid angle
subtended by the dust torus as seen from the central source, does not evolve
with the accretion rate. The correlation of the solid angle subtended by the
torus with the central black hole mass M_bh implies that the formation of the
dust torus is likely regulated by the central black hole mass. We find that the
torus thickness H increases with quasar bolometric luminosities, which is
different from the constant torus thickness H with luminosity assumed in the
receding torus model. The mean covering factor of the dust clouds at the inner
radius of the torus derived from the IR emission data is ~0.39 for PG quasars.
The average relative thickness H/R of the tori in the PG quasars derived from
the ratios of the infrared to bolometric luminosities is ~0.9. We suggest that
the further IR observations on a larger quasar sample including more fainter
quasars by the Spitzer Space Telescope will help understand the physics of the
dust tori in quasars.
