Solid angle is much, much farther away. The solid angle subtended by the complete ()-dimensional spherical surface of the unit sphere in -dimensional Euclidean space can be defined in any number of dimensions . One often needs this solid angle factor in calculations with spherical symmetry. It is given by the formula where is the gamma function. When is an integer, the gamma function can be computed explicitly. It follows that This gives the expected results of 4 steradians for the 3D sphere bounded by a surface of area and 2 radians for the 2D circle bounded by a circumference of length
BET theory physical adsorption of gas molecules. The total surface area formula_19 and the specific surface area formula_20 are given by where formula_5 is in units of volume which are also the units of the monolayer volume of the adsorbate gas, formula_24 is Avogadro's number, formula_25 the adsorption cross section of the adsorbing species, formula_26 the molar volume of the adsorbate gas, and formula_27 the mass of the solid sample or adsorbent. The BET theory can be derived similarly to the Langmuir theory, but by considering multilayered gas molecule adsorption, where it is not required for a layer to be completed before