 When viewed from a Newtonian perspective, and assuming that the vessel containing the gas is itself massless, the mass of the vessel of gas is simply equal to the sum of the masses of the molecules. From a relativistic point of view, this last assertion states incorrectly that the rest-mass of the vessel of gas is equal to the sum of the rest-masses of the molecules. Yet, from a relativistic point of view, the rest-mass of the vessel of gas is equal to the sum of the rest-masses of the molecules plus the kinetic energy of the molecules divided by \(c^2\). Since, according to the Kinetic Theory of Gases, the temperature of the gas is proportional to the average kinetic energy of its molecules, if the gas temperature increases or decreases, the rest-mass of the vessel of gas increases or decreases accordingly by a tiny amount. 