Temporal behaviour of the slow wave total energy density for a prominence plasma (a) with a constant temperature (continuous line); (b) with a temperature which decreases with time (dashed line); (c) with a temperature that decreases with time and radiative damping of the perturbations included (dotted line). H = 1 × 10^-6 W m^-3, τ = 3000 s, T_0i = 10 000 K, k_z = 3 × 10^-7 m^-1. The total energy density has been normalized with respect to its initial value.