Boyle's Gas Law
For a fixed mass of gas at constant temperature the product of pressure and volume is constant. Therefore, if the pressure increases, the volume decreases and visa versa. For example, if the volume if halved, then the pressure is doubled. If the temperature is held constant, it becomes an isothermal process. Discovered by Robert Boyle (1627-1691), an Irish physicist and chemist and co-founder of the Royal Society.
A volume of gas enclosed in a cylinder with a movable piston.
Halving the volume leads to twice the rate of collisions and a doubling of the pressure.