Muleshue, as Coinneach stated, mercury is liquid at room temperatures. The recoil dampers utilize the mercury sloshing around the tube to reduce recoil, some law of physics that I barely understand. Some shooters have used lead in the stock or even in the magazine tube to add weight and reduce recoil in autos. More weight =less recoil.
I wouldn't want to have the mercury leak out of the tube, but guys I know who use the mercury filled recoil dampers swear by them. One guy I know has three of them on his AL390, two in the stock and one on the magazine tube cap. His gun is lefthanded, so I havent tried to shoot it, but have shot another with two dampers and it definitely was soft shooting.
BTW, I just noticed that seminole does work on Benellis, maybe they can take the bite out of Michael64's M1.
Geoff Ross