A mercury capsule for dispensing mercury into an arc discharge lamp comprises a tubular metal member having a main body portion, a sealed end portion immediately adjacent the main body portion and means for sealing the end portion. The sealing means includes a substantially undulating configuration containing a predetermined number of undulations to enable rupture of the mercury capsule at an elevated temperature in accordance with the number of undulations. In a preferred embodiment of the invention, the mercury capsule is formed from a metal cup having a larger diameter skirted portion and a smaller diameter portion.