The constant R in this equation is known as the universal gas constant. PV=nRT Boyle's law (named for Robert Boyle) states that, at constant temperature, the pressure P of a gas varies inversely with its volume V, or PV = k, where k is a constant. Charles's law (named for J.-A.-C. Charles, 1746-1823) states that, at constant pressure, the volume V of a gas is directly proportional to its absolute (Kelvin) temperature T, or V/T = k. These two laws can be combined to form a single generalization of the behaviour of gases known as an equation of state, PV = nRT, where n is the number of gram-moles of a gas and R is called the universal gas constant. Though this law describes the behaviour of an ideal gas, it closely approximates the behaviour of real gases. The universal gas constant has an empirical value which depends only upon the units in which the pressure and volume are measured. The best available value of the universal gas constant is: (1) 8.3143510 J mol^-1 K^-1, (2) or 8.3143510 kPa L (or dm3) mol^-1 K^-1, (3) or 8.3143510 Pa m3 mol^-1 K^-1, (4) or 0.08206 L atm mol^-1 K^-1