Thanks MateHey look, someone up as early as I!
Did any glass shards go missing in the depths? I doubt mercury would be an issue, as the vast majority of heaters use electronics or bimetallics to drive the thermostat.
Just make sure all the glass is out of the tank. Every little bit! Use a flashlight at different angles and see if you see any shimmering.
In the future, I recommend you use something like Marineland's Stealth heater. It has a shatter-proof plastic casing, instead of glass.