We wish to perform neutron diffraction measurements on glass samples of SiO2 and 30CaO.70SiO2 containing both natural oxygen and 18O. The oxygen scattering lengths are almost identical, so that the differences will arise almost entirely from the change in thermal broadening which results from the change in oxygen mass. Thus this gives us an ideal opportunity to study the effect of thermal motion on the results of a diffraction pattern, and to investigate the applicability of the correlated Debye model recently developed for diffraction from crystalline materials.