A sample of gas, originally treated as a single quantum system, is now described in terms of its molecular constituents, starting with the case of a single radiating molecule in an equilibrium bath of perturbers. First, the isolated radiator is considered, as if the bath had been deactivated, allowing a discussion of how its internal energy and angular momentum may change when, in the presence of an electromagnetic field , a radiant transition takes place, and of how the transition amplitude may be reduced under the Wigner–Eckart theorem. Then, the interaction between radiator and bath is reinstated, but the initial correlations between the two are neglected, so that a separate average over the bath may be taken. There is then an examination of various approximations that may be of use elsewhere. These are the restrictions to collisions that are binary in nature, the possibility that a collision may be said to follow a classical trajectory, and the validity of treating it under the impact approximation, which carries a restriction to the core region of a spectral line, but offers a great simplification when collisions may be regarded as very brief, well-separated events.