The periodic generation of bubbles by injection of a gas at constant flow rate through an orifice at the bottom of a quiescent liquid of high viscosity is investigated numerically. The volume of the bubbles is determined as a function of a capillary number and a Bond number in the absence of inertial effects. Pairs of bubbles coalesce in the vicinity of the orifice when the capillary number is higher than a critical value that depends on the Bond number, but the final volume of the bubbles still follows a well-known scaling law at large capillary numbers. Qualitative experimental visualizations are presented that display the sequences of detachment and coalescence computed numerically.