Boyle's law

Boyle's law states that, for a fixed quantity of gas held at constant temperature, the pressure P and volume V satisfy PV = constant. Halve the volume and the pressure doubles; plotted as P against 1/V the relationship is a straight line through the origin. It is the isothermal special case of the ideal gas law.

Robert Boyle established the law in 1662 using a J-shaped glass tube partly filled with mercury, with help from his assistant Robert Hooke. By adding mercury to the open arm he compressed the air trapped in the sealed arm and recorded how its volume shrank as the pressure rose. Boyle described the trapped air as behaving like a coiled spring — the 'spring of the air' — that pushed back ever harder as it was squeezed.

Microscopically the law follows because pressure is the rate of molecular momentum delivered to the walls per unit area; compress the gas into half the space at the same temperature and molecules strike the walls twice as often, doubling the pressure. Boyle's law holds for real gases only approximately, deviating where attraction or finite molecular size matter.

Published by Robert Boyle in 1662 in an appendix to New Experiments Physico-Mechanical; known on the European continent as Mariotte's law after Edme Mariotte, who stated it independently in 1679 and added the constant-temperature condition explicitly.