  We present a uniform analysis of all mid-infrared $R\approx90$ spectra of
field M5--T9 dwarfs obtained with the Spitzer Infrared Spectrograph (IRS). The
sample contains 113 spectra out of which 12 belong to late-M dwarfs, 69 to L
dwarfs, and 32 to T dwarfs. Sixty-eight of these spectra are presented for the
first time. We measure strengths of the main absorption bands in the IRS
spectra, namely H$_2$O at 6.25 $\mu$m, CH$_4$ at 7.65 $\mu$m, NH$_3$ at 10.5
$\mu$m, and silicates over 8--11 $\mu$m. Water absorption is present in all
spectra and strengthens with spectral type. The onset of methane and ammonia
occurs at the L8 and T2.5 types, respectively, although ammonia can be
detectable as early as T1.5. Silicate absorption sets in at spectral type L2,
is on average the strongest in L4--L6 dwarfs, and disappears past L8. However,
silicate absorption can also be absent from the spectra at any L subtype. We
find a positive correlation between the silicate absorption strength and the
excess (deviation from median) near-infrared colour at a given L subtype, which
supports the idea that variations of silicate cloud thickness produce the
observed colour scatter in L dwarfs. We also find that variable L3--L7 dwarfs
are twice more likely to have above-average silicate absorption than
non-variables. The ensemble of results solidifies the evidence for silicate
condensate clouds in the atmospheres of L dwarfs, and for the first time
observationally establishes their emergence and sedimentation between effective
temperatures of $\approx$2000 K and $\approx$1300 K, respectively.
