As a method to experimentally study aerodynamic drag the terminal settling velocities of aerosolized silica and glass microspheres (with sizes of 1-44 mu m) have been measured at various gas pressures in the range 0.6-10 mbar and using various gas compositions including He, Ar, Xe, Air, CO2, and water vapour. Within the molecular scattering regime, i.e. where the Knudsen number Kn > 10 and up to 500, reasonable agreement has been found with the model of Epstein (1924). Values of the scattering parameter delta were observed to be within the expected range of 1-1.44 for respectively specular/evaporative-diffuse molecular scattering processes. However, delta was seen to depend upon gas composition, specifically for H2O; delta = 0.96 +/- 0.07, CO2; delta = 1.16 +/- 0.07, noble gasses; delta = 1.25-1.44 and in the case of Air delta = 1.18 +/- 0.07 which is not in agreement with that conventionally used of around 1.3-1.4. These observations imply disagreement with the concept of a general or universal model of drag for all particle surfaces and atmospheres. Similarly, for Kn < 10 these results were not well described by conventional models such as the semi-empirical expression of Knudsen-Weber (1911) or the general law of fall proposed by Millikan (1923b).