In Physics classes at college I was taught that atmospheric pressure is due to the weight of the air above a body. I understand this and all related equations and concepts. But just the other day I was reading up on gas laws in a chemistry book. In that book it stated that atmospheric pressure was due to the "kinetic theory of gases". That is atmospheric pressure is caused by the collisions of air molecules with objects, exerting a force (and subsequentially a pressure) on a body. (In the same way that gas exerts pressure on the walls of a propane container). Clearly this contradicts what I was taught at college. The question is "What acctually is the cause of atmospheric pressure ... weight of air or kinetic theory?" Also if kinetic theory is not the cause of atmospheric pressure how does it apply to gas in a container and why does it apply only to contained gas and not gas in the atmosphere? Many thanks in advance, Jonny.