Boyle's Law demonstration. Image 2 of 2. 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. The pressure in the glass bulb has been reduced, hence the volume occupied by air in the marshmallows is increased, so the marshmallows have expanded. For the marshmallows before expansion, see A350/157.