  The unbound nucleus $^{18}$Na, the intermediate nucleus in the two-proton
radioactivity of $^{19}$Mg, was studied by the measurement of the resonant
elastic scattering reaction $^{17}$Ne(p,$^{17}$Ne)p performed at 4 A.MeV.
Spectroscopic properties of the low-lying states were obtained in a R-matrix
analysis of the excitation function. Using these new results, we show that the
lifetime of the $^{19}$Mg radioactivity can be understood assuming a sequential
emission of two protons via low energy tails of $^{18}$Na resonances.
