The Evolution of Interstellar Clouds. I.

The consequences of the depletion of heavy elements on grains are examined in the two-phase model of the interstellar medium. It is found that, if depletion is important in the cloud phase, then clouds left to evolve in the absence of external dynamical perturbations must heat up gradually, and eventually destroy themselves on timescales of the order of 3 x 1O years.