 For the purposes of this kind of discussion, an ideal gas is composed of molecules that are treated as idealized point particles to each of which we assign a mass (i.e., a rest-mass). The molecules are treated as moving uniformly, i.e., with constant velocity, and as interacting only in perfectly elastic collisions. Consequently, if we consider a sample of gas contained in a massless vessel, the only energy “carried” by the constituent elements is the kinetic energy of the molecules. 