Types II, IV and VI can be measured by BET method. (interaction adsorptiv- adsorbent > adsorptiv-adsorbate). Types III, V have weak interactions between gas. Brunauer–Emmett–Teller (BET) theory aims to explain the physical adsorption of gas . The BET method is widely used in surface science for the calculation of Concept · Derivation · Applications · Activated Carbon. Process according to one or more of Claims 1 to 3, characterised in that SiO₂- containing material having a specific surface area, measured by the BET method.