The dc electrical breakdown of long vapour bubbles in capillaries generated by external heating of the capillary is studied. The fluid columns in the capillary on both sides of the vapour bubble serve as electrodes for the electrical discharges inside the bubble. When a dc voltage is applied across a static vapour bubble, a self-pulsing corona regime is observed, which is followed by a transient spark. The sparking or breakdown voltage of the vapour bubble decreases with increasing conductivity of the NaCl solution. After the transient spark a constricted glow-like discharge is observed in the bubble whilst the bubble expands due to the energy deposition of the discharge. Electrical and optical emission properties of the corona, spark and glow-like regime are discussed.