The Pressure of the Atmosphere

By pressure we mean the force or weight acting on unit area. From many years of observations and experiments scientists have formulated a theory about the structure of gases; the theory is known as the Kinetic Theory of Gases. The theory draws a picture of a gas as being made up of a large number of very small particles, which are called molecules. The actual volume of the molecules is very small compared with the volume of the gas. They are moving at high speeds in all directions in straight lines, and collide both with each other and with the walls of the containing vessel. Since the molecules are perfectly elastic no energy is lost in these collisions. The continual bombardment of the walls of the combining vessel creates the pressure which the gas exerts on these walls. The pressure will defend upon the number of impacts, on unit area per second. If the volume of the vessel increased, the number of impacts on unit area per second increases and so the pressure decreases. Conversely the pressure will increase as the number of impacts increases if the volume of the vessel decreases.