"Storeria dekayi, commonly known as the brown snake"
First line from wikipedia. That's...a really descriptive common name.
As for humanely killing mice, sulfur hexafluoride would probably be better. Lungs detect CO2 levels, not oxygen. A high concentration of CO2 will cause your brain to panic as it thinks it's suffocating, as it is in this case. (CO2 mixed with oxygen has been used therapeutically to inducing panic). Any gas other than CO2 and your lungs don't notice and you (mice being the you in this case) will lose unconsciousness and die without panic or fear.
Probably prohibitively expensive, though. They trick the mice into sleeping with lower CO2 levels and then increase it to toxic levels.
And now for humor, Neil Patrick Harris on sulfur hexafluoride.