Boyle's Law (sometimes referred to as the Boyle-Mariotte Law) states that the absolute pressure and volume of a given mass of confined gas are inversely proportional, if the temperature remains unchanged within a closed system . That is to say, the product of pressure and volume is a constant for a given mass of confined gas as long as the temperature is constant. The law was named after chemist and physicist Robert Boyle, who published the original law in 1662. Boyle showed that the volume of air trapped by a liquid in the closed short limb of a J-shaped tube decreased in exact proportion to the pressure produced by the liquid in the long part of the tube.

The trapped air acted much like a spring, exerting a force opposing its compression. Boyle called this effect "the spring of the air" and published his results in a pamphlet with that title. The difference between the heights of the two mercury columns gives the pressure (76 cm = 1 atm), and the volume of the air is calculated from the length of the air column and the tubing diameter.

The law itself can be stated as follows: for a fixed amount of an ideal gas kept at a fixed temperature, P (pressure) and V (volume) are inversely proportional (when one doubles, the other halves).

Remember that these relations hold true only if the number of molecules n and the temperature are both constant. This is a relation of inverse proportionality; any change in the pressure is exactly compensated by an opposing change in the volume. As the pressure decreases toward zero, the volume will increase without limit. Conversely, as the pressure is increased, the volume decreases (but it can never reach zero). There will be a separate P-V plot for each temperature; a single P-V plot is therefore called an isotherm. Here is a graph with some isotherms for one mole of an ideal gas at several different temperatures:

Each plot has the shape of a hyperbola, where the locus of all points has the property xy = a.