Permanent gas

A gas which can be liquefied only at very low temperatures, and was formerly believed to be incapable of liquefaction; specifically a gas that is above its critical point, and so is impossible to liquefy by pressure alone. Nitrogen and Oxygen, the major constituents of air, are both permanent gases. Water vapour can be readily condensed at room temperature. It is not a permanent gas and is removed from a vacuum system under evacuation much more slowly than Nitrogen and Oxygen.