Transmission and emission spectra of clouds and layers of fine particulate samples of quartz, magnesium oxide, and aluminum oxide in the 6.5-35-??m wavelength range are presented. They demonstrate that the behavior of layers of particles constitutes a good analogue for a cloud of particles; that individual micrometer-sized particles emit most where they absorb most; that as the size of the particle is increased, the emission features reverse polarity and the spectrum approaches that of one obtained from a polished plate; and that as the particle layer-thickness increases, radiative interaction becomes increasingly important so that the emission maximum shifts from the strongest to weaker features, or produces a maximum at the Christiansen wavelength.
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Infrared spectral behavior of fine particulate solids