Robert Boyle investigated the relationship between the volume of a fixed mass of gas and its pressure under conditions of constant temperature. 
Robert Boyle

Boyle's law
Robert Boyle found that for a given number of moles of gas the volume of the gas is proportional to its pressure provided the temperature is kept constant.
This can be expressed as:
The pressure of a gas derived from collisions of the gas particles with the walls of the container. If the volume is decreased then the gas particles are resticted to less space and there are more collisions per second at the walls and consequently a greater pressure is exerted.
Boyles law allows calculation of new volume and/or pressure conditions (at constant temperature) in, for example, a piston or gas syringe.
Example: Calculate the pressure when 100cm^{3} of gas at atmospheric pressure is compressed to 10cm^{3} at constant temperature. Boyle's law states that PV is constant therefore the initial product of the pressure times the volume must equal the final product of the pressure times the volume, i.e.: P_{1}V_{1} = P_{2}V_{2} P_{1} = 1.01 x 10^{5} Pa, V_{1} = 0.1dm^{3}, V_{2} = 0.01dm^{3}, P_{2} = ? P_{2} = (1.01 x 10^{5} x 0.1)/0.01 P_{2} = 1.01 x 10^{6} Pa 