Intensities of spectra of individual atoms or molecules typically vary linearly 
with the numerical gas density. However, if gas densities are sufficiently 
increased, quite generally contributions may also be observed that vary as density
squared, cubed... These are the collision-induced spectra of two-body (and 
quite possibly three-body,...) collisional complexes. The collision-induced spectra
have sometimes been separated from the continua of individual atoms and 
molecules, based on the characteristic density dependences. In other words, a 
virial expansion in terms of powers of the numerical gas density is often 
observable, just as this is widely known for the virial expansion of the equation 
of state of compressed gases. The first term of the expansion, which is linear 
in density, represents the ideal gas (or "ordinary) spectra where these 
exist. (This first term vanishes for the infrared inactive gases,) And
the quadratic, cubic,... terms of the virial expansions arise from optical 
transitions of binary, ternary,... intermolecular complexes, which are 
(often unjustifyably) neglected in the ideal gas approximation of spectroscopy.