Measurement of the specific surface area of fine powders. A comparison of the 'gas-adsorption' and 'air-permeability' methods

Measurements of specific surface areas of nineteen powders of various chemical and physical types have been made ( a ) by adsorption of nitrogen on to the powders at 77° K and ( b ) by the air‐permeability method described by Rigden. The results obtained by the two methods were appreciably different, the areas obtained by the adsorption method being in all cases the larger. It is suggested that the discrepancy between the two sets of measurements may be due in part to the existence of an internal surface in some of the powders, which was not measured by the air‐permeability method.