The ideas that solids are composed of compact arrays of atoms, while gases are composed of atoms or molecules in very rapid translational motion, are so obvious that we accept them nowadays without question; in teaching textbooks they are stated as if they were axioms. In its most elementary form, without any sophisticated calculations about the distribution of velocities, with only the one assumption that the impacts of the molecules on the walls of the containing vessel produce the pressure, a very simple calculation gives the equationpv = 13mNc2wheremandNare the mass of a molecule and the number per unit volume, andcis a velocity;pis the pressure andVthe volume of the gas.