Transmission measurements have been used to determine absorption coefficients over the wavelength range 1.5-7.5 μ, and interferometric determinations made of the refractive index from 7-20 μ. It is shown that the following four factors contribute significantly to the dispersion: (i) short-wave absorption band centred in the visible region, (ii) tail absorption band, (iii) free carrier absorption, (iv) Restrahlen band. The `characteristic' value of refractive index occurs at approximately 10 μ, where it is 3.96.A more accurate calculation is made of the radiation lifetime. Using these data, τ_R=0.79 μsec.