Boyle's Law demonstration. Close-up of apparatus used to test Boyle's Law, one of the Gas Laws. Boyle's Law (first proposed by Robert Boyle in 1662) states that for a fixed mass of gas at a fixed temperature, PV=k where P is pressure, V is volume and k is a constant. That is, the product of the pressure and volume remains constant. This equipment has a sample of gas trapped at the top of the tube above a liquid (brown). The scale markings allow the volume of this gas to be measured. To test the law, the liquid is pressurised to compress the gas, with the scale allowing volume to be recorded.