Three phenomena involved in the perikinetic coagulation process were studied with regard to their ability to account for the influence of gravity on the coagulation rate : (1) surface charge effects on the pressure in the gap between two approaching particles, through the streaming potential generated by the squeezing flow; (2) free convection induced by temperature changes in the surroundings; (3) surface roughness and hydration force effects on the interaction force between two particles. In a separate paper flow interactions between two identical spheres are shown to be insufficient to explain the influence of gravity on coagulation rate. Of these phenomena studied, a combination of surface roughness and free convection can explain the influence of gravity on coagulation.