  We present 2.5-11 micron spectrophotometric & imaging ISO observations of 28
Sf1, 29 Sf2 & 1 normal galaxy. The Sf1 & Sf2 MIR spectra are statistically
different: Sf1 have a strong power-law continuum of index -0.84+/-0.24 & weak
PAH emission bands. Sf2s have a weak continuum & very strong PAH emission
features with EW (equivalent widths) up to 7.2 micron. On the other hand, the
Sf1 and Sf2 PAH luminosities do not differ statistically and the 7 micron
continuum is ~8 times less luminous in Sf2s than in Sf1s. The PAH emission is
unrelated to the nuclear activity & arises in the bulge ISM. PAH EW are thus a
sensitive nuclear redenning indicator. These results are consistent with
unified schemes & imply that the Sf2 MIR nuclear continuum is, on the average,
extinguished by 92+/-37 visual magnitudes whereas it is directly visible in
Sf1s. The dispersion in Sf2's PAH EW is consistent with the expected spread in
viewing angles. Those Sf2s with PAH EW > 5 micron suffer from > 125 magnitudes
of extinction and are invariably weak X-ray sources. Such Sf2s represent the
highly inclined objects where our line of sight intercepts the full extent of
the torus. About 1/3rd of the Sf2s have PAH EW <= 2 micron, in the range of
Sf1s. Among them, those which have been observed in spectropolarimetry or IR
spectroscopy invariably display "hidden" broad lines. As proposed by Heisler et
al (1997), such Sf2s are most likely seen at grazing incidence such that one
has a direct view of both the "reflecting screen" and the torus inner wall
emitting the MIR continuum. Our observations thus constrain the screen & the
torus inner wall to be spatially co-located. Finally, the 9.7 micron Silicate
feature appears weakly in emission in Sf1s, implying that the torus vertical
thickness cannot significantly exceed E+24 cm-2
