Abstract
The aim of this lecture has been to show how dilute gases made up of atoms or matter-light composite particles can be used to test the various properties hitherto studied theoretically. We first returned to the classical field formalism to determine the position of the transition point in terms of usual thermodynamic variables, such as total density, temperature or chemical potential. We then described a series of recent experiments carried out on gases of bosonic or fermionic atoms (but which, in the latter case, can form diatomic molecules that are therefore bosons). These experiments tested both the position of the critical point and the properties of the gas on either side of it. We conclude with a description of some similar experiments, this time on cavity polaritons.
