The volume of a fixed mass of gas at constant pressure expands by a constant fraction of its volume at 0°C for each Celsius degree or kelvin its temperature is raised. For any ideal gas the fraction is approximately 1/273. This can be expressed by the equation V = V₀ (1 + t/273), where V₀ is the volume at 0°C and V is its volume at t°C. This is equivalent to the statement that the volume of a fixed mass of gas at constant pressure is proportional to its thermodynamic temperature, V = kT, where k is a constant. The law resulted from experiments begun around 1787 by Jacques Charles but was properly established only by the more accurate results published in 1802 by Joseph Gay-Lussac. Thus the law is also known as Gay-Lussac’s law. An equation similar to that given above applies to pressures for ideal gases: p = p₀ (1 + t/273), a relationship known as Charles’ law of pressures. See also gas laws.