Is that a dangerous form of mercury ? Or are all forms of mercury dangerous ?? Is that the same mercury in the fillings?

Surely they must absorb some of it through their hands

http://www.youtube.com/watch?v=BOnDOtnd2no&feature=related

Not that video, but in another video someone has it in his hand and is playing with it.

http://www.youtube.com/watch?v=31CE2BYicyU&feature=related

Also, does it leave traces behind on the glass?, it seems very complete and doesn't leave droplets left in the beaker like water would ?