I just found out a fab fact on the [highly recommended] medical history podcast Sawbones. This episode is about the 18th century American doctor Benjamin Rush. He loved bloodletting, and is said to have been largely responsible for the deaths of George Washington and Ben Franklin.
He equipped the Lewis and Clark expedition with mercury laxative pills. And he gave them so many and they used them so enthusiastically that the best way for modern researchers to track the route of the expedition is the reliable trail of mercury they left behind.
So Sawbones - its free, look it up on the iTunes store.