If you press the piston of a syringe containing a certain number of gas molecules, the space of the gas molecules is reduced. Therefore, the gas molecules collide with the syringe wall more and more. As a result, the pressure of the gas in the syringe increases. Conversely, pulling the piston of the syringe widens the space in which the gas molecules can move. In this case, the pressure of the gas is reduced because the number of gas molecules colliding with the wall of the syringe is reduced.
By doubling the pressure of the gas, the volume of the gas is reduced to ‘1/2’, and when the pressure of the gas is increased three times, the volume of the gas is reduced to ‘1/3’. In 1662, a British scientist, Boyle (1627-1691), found that gas pressure and volume were inversely proportional at constant temperature. This is called ‘Boyle’s Law’.