  Dynamics of vibro-fluidised granular gas is investigated experimentally using
the transfer of grains from a compartment through a horizontal slit at a given
height h . It is demonstrated that the transfer rate j varies linearly with the
grain number N in the box when N remains small; however j(N) becomes strongly
non linear as soon as the number n of layers is larger than 0.3; dj/dN becomes
negative for n>0.4.It is found also that the maximum of j(N) increases slightly
with the acceleration af_ of the vibration which excites the granular gas.
Using dynamical system theory, dynamics equations are written, and a critical
bifurcation is found, which explains the existence of a condensation and of a
phase transition. This explains how the pseudo " Maxwell's demon" works in
granular gases. This experiments contradicts recent modeling . Pacs # : 5.40 ;
45.70 ; 62.20 ; 83.70.Fn
