Recent models of the hydrogen recombination line emission from
 HII regions show two effects due to dust internal to the nebula.
 These are (a) reduction in the Lyman optical depth due to dust
 absorption and consequent modification of the spectrum from
 Case B towards Case A, and (b) excitation and stimulated emission
 of upper levels via re-cycled thermal IR photons. The
 departure of the emitted spectrum from Case B should be
 measureable and, since the quantitative effect depends on the
 dust content and its optical properties, would provide the first
 opportunity to study the internal dust.

 The combined deviation in line flux due to the two processes
 described above occurs over a range of principle
 quantum numbers (n, depending on dust properties), but is
 most significant for n = 8 to 40. Other processes also modify
 the line emission from Case B, , principally dust extinction at
 short wavelengths and self-stimulated emission at long
 wavelengths. These latter processes must first be characterised
 in order to study the deviations due to internal dust.
 Consequently we have embarked on a campaign of multi-wavelength
 measurements of hydrogen recombination line fluxes, recently
 obtaining near-IR (UKIRT), sub-mm (n = 26 - 30; JCMT) and
 mm (n = 40 - 70; Nobeyama) data. We now wish to complete this
 data set by obtaining measurements in the mid-IR with ISO/SWS
 of five/six lines with n = 4 - 9.

 This data will allow us to quantify the dust extinction and
 intrinsic ionization rate and , in conjunction with the
 existing data, investigate the dust content and its properties
 via the physical mechanisms described.