Mercury is the 80th element in the periodic table. Although it has a rather toxic reputation nowadays, it has been used in numerous applications from thermometers to barometers, but not any more. It's is a member of the transition metals. Although it's relatively rare, it's pretty cheap. Mercury is one of only 2 elements that's a liquid at room temperature. Of all the elements, it expands most with increasing temperature, which is why it was used in thermometers. Most of the applications of Mercury up until very recently have been banned internationally due to its toxic nature.