Abstract : A dynamical description of the multifragmentation process shows that mechanical instabilities are responsible for spinodal decomposition of a nuclear system that spend sufficient time in the low-density region. The Xe+Sn system at 32 MeV/A incident energy, measured with INDRA multidetector, was chosen to prove experimentally this hypothesis. High-performance techniques in energy calibration of Silicon detectors and CsI(Tl) scintillators were developed in order to exploit the excellent detection qualities of the multidetector. For the first time, the contribution of delta rays to the light output emitted by scintillators was quantitatively derived. Multifragmentation events coming from a system composed by almost all of nucleons of the reaction entrance channel were selected using detection completeness and event shape criteria. The dynamical model BoB realistically simulates the spinodal instabilities. This model reproduces all of dynamic and static observables. More exclusive comparisons were made to constrain again the model. Reduced velocity correlation functions were studied and gave information about the topology of the fragments at freeze-out. Charge correlation of the fragments showed that a small proportion of events (0.1 %) emits equal-sized fragments. This was interpreted as a fossil signature of the spinodal decomposition in a finite system. Indirectly, this is a proof of a first order phase transition associated to the multifragmentation of hot nuclei.