Just on a historical note, I remember reading that the treating of syphilis with compounds of mercury was discovered by chance.

In the late Middle Ages it was thought that diseases were caused astrologically by positions of the stars and planets. I can't quite remember the details but it was something to do with the conjunction of certain planets in Scorpio, which was the sign thought to be responsible for the genitalia- someone got the idea of treating the disease with a compound on mercury, because that planet wasn't in that particular conjunction, and it was the first cure found.

I probably haven't got the technical details right as it was a long time ago, but the story is basically true.