At ambient temperature and at atmospheric pressure nitrogen is introduced into two bottles of equal volume one of which contains the test portion while the other is empty. The connected bottles are immersed in the refrigerant bath of liquid nitrogen. Since some nitrogen is adsorbed by the sample a differential pressure occurs between the two bottles and is measured by means of a differential manometer. From this difference, the number of molecules absorbed on the surface is calculated. The area occupied by an absorbed nitrogen molecule is taken as 16.2 x 10^-20 m^2. From this values the surface area is calculated.