The total interparticle potential is investigated numerically when the repulsive Coulomb electrostatic potential of a double layer is included. A modified stability ratio is presented that takes into account the effects of both the hydrodynamic interactions and the distribution of the thermal energies of Brownian particles. Because of the Born repulsive forces of the electron clouds, the primary coagulation is reversible, as is the secondary coagulation. In addition to that for Brownian coagulation, the stability ratio for the case of weak gravitational/strong Brownian coagulation is obtained.