A cubic box with infinitely hard walls having volume V. contains a non-relativistic classical ideal gas consisting of Nmolecules, each of which has mass m. spin- and magnitude of magnetic moment u. The partition function of the gas in uniform external magnetic field B = Bon in their equilibrium at temperature T is given by 4 2 cosh Bo kBT 3 2mk T [v (2 mkg)*( h [2 ( 2V cosh ) 3 20 (1) (2 x mk z T) 2V cosh h 2mkgT uBo KBT 3 ) coch() cosh 7N