  We discuss the diffusion phenomenon in the parabolic and hyperbolic regimes.
New effects related to the finite velocity of the diffusion process are
predicted, that can partially explain the strange behavior associated to
adsorption phenomenon. For sake of simplicity, the analysis is performed by
considering a sample in the shape of a slab limited by two perfectly blocking
surfaces, in such a manner that the problem is one-dimensional in the space.
Two cases are investigated. In the former, the initial distribution of the
diffusing particles is assumed of gaussian type, centered around the symmetry
surface in the middle of the sample. In the latter, the initial distribution is
localized close to the limiting surfaces. In both cases, we show that the
evolution toward to the equilibrium distribution is not monotonic. In
particular, close to the limiting surfaces the bulk density of diffusing
particles present maxima and minima related to the finite velocity of the
diffusion process connected to the second order time derivative in the partial
differential equation describing the evolution of the bulk density in the
sample.
