n (1864) Either of two theories in physics based on the fact that the minute particles of a substance are in vigorous motion. The first theory is that the particles of a gas move in straight lines with high average velocity, continually encounter one another and thus change their individual velocities and directions, and cause pressure by their impact against the walls of a container. Also known as the Kinetic Theory of Gases. The second theory is that the temperature of a substance increases with an increase in either the average kinetic energy of the particles or the average potential energy of separation (as in fusion) of the particles or in both when heat is added. (Connors KA (1990) Chemial kinetics. Wiley, New York) Also known as the Kinetic Theory of Heat.