The volume (V) of a given mass of gas at a constant temperature is inversely proportional to its pressure (p), i.e. pV=constant. This is true only for an ideal gas. This law was discovered in 1662 by Robert Boyle. On the continent of Europe it is known as Mariotte’s law after Edmé Mariotte (1620–84), who discovered it independently in 1676. See also gas laws.