<Li> Mercury and its compounds have a reputation for toxicity but on a scale of 1 to 10, dimethylmercury ((CH) Hg) (abbr . DMM), a volatile colourless liquid, has been described as a 15 . It is so dangerous that scientists have been encouraged to use less toxic mercury compounds wherever possible . In 1997, Karen Wetterhahn, a professor of chemistry specialising in toxic metal exposure, died of mercury poisoning ten months after a few drops of DMM landed on her "protective" latex gloves . Although Wetterhahn had been following the then published procedures for handling this compound, it passed through her gloves and skin within seconds . It is now known that DMM is exceptionally permeable to (ordinary) gloves, skin and tissues . And its toxicity is such that less than one - tenth of a ml applied to the skin will be seriously toxic . </Li> <Dl> <Dd> <Ul> <Li> The expression, to "go down like a lead balloon" is anchored in the common view of lead as a dense, heavy metal--being nearly as dense as mercury . However, it is possible to construct a balloon made of lead foil, filled with a helium and air mixture, which will float and be buoyant enough to carry a small load . </Li> </Ul> </Dd> </Dl> <Dd> <Ul> <Li> The expression, to "go down like a lead balloon" is anchored in the common view of lead as a dense, heavy metal--being nearly as dense as mercury . However, it is possible to construct a balloon made of lead foil, filled with a helium and air mixture, which will float and be buoyant enough to carry a small load . </Li> </Ul> </Dd> <Ul> <Li> The expression, to "go down like a lead balloon" is anchored in the common view of lead as a dense, heavy metal--being nearly as dense as mercury . However, it is possible to construct a balloon made of lead foil, filled with a helium and air mixture, which will float and be buoyant enough to carry a small load . </Li> </Ul>

A strong contrast between dark and light is a typical characteristic of which period