the theory that the minute particles of all matter are in constant motion and that the temperature of a substance is dependent on the velocity of this motion, increased motion being accompanied by increased temperature: according to the kinetic theory of gases, the elasticity, diffusion, pressure, and other physical properties of a gas are due to the rapid motion in straight lines of its molecules, to their impacts against each other and the walls of the container, to weak cohesive forces between molecules, etc.