Boyle's law

Boyle's law is a special case of the ideal gas law from which one may calculate either the pressure or the volume of gas. It was developed by Robert Boyle in the 1660s and describes an inverse relationship between the pressure (P) and the volume (V) of a fixed amount of gas at a fixed temperature. This law is only valid if the temperature and the amount of gas is held constant.

The law reads:
 * $$PV = \textrm{constant}\,$$

at fixed temperature and fixed amount of gas.