How was it first discovered that hot air could rise?

The history of balloon flight dates to 1783, when two French brothers, Joseph and Etienne Montgolfier, discovered that filling a bag with hot air would cause it to rise. They demonstrated their principle at Annonay, France, on June 4, 1783, with an unmanned balloon made of linen and pape

Not a  mixture of two common elements that can be separated by physical means.