Theory of powders
T he num ber density o f a pow der lies betw een 10° ml 1 and 1016т Г [. A lthough much less than the 1022 o f atom s, this range takes on e into the region w here Van der W aals forces and plain electric charges becom e dom inant. A lthough w e do not n eed to refer to a density, w e have in mind the range 103 to 106, i.e . friction is im portant but attractive forces are not dom inant. For bulk sam ples, the num bers are still very large so that statistical argum ents are entirely appropriate.