  We derive a non-local Nambu-Jona-Lasinio model from QCD with a form factor
exactly obtained in the infrared limit. With this model, having all the
parameters properly fixed through those of QCD, we consider the case of finite
temperature and compute the solution of the gap equation at small temperature
and small momenta and zero chemical potential. Taking the quark masses to be
zero, it is proved that the theory undergoes a phase transition with a critical
temperature exactly determined. These results prove unequivocally that the
picture of the vacuum of QCD as a liquid of instantons is a very good
approximation.
