Mercury

Category: transition metal Atomic number: 80 Colour: greyish silver Melting point: −39°C (−38°F) Boiling point: 357°C (674°F) First identified: ancient civilizations

The vivid red mineral cinnabar (also known as vermilion) was extensively traded millennia ago; in the Near East it was used as a rouge, and it had other colouring uses, such as in the amazing seventh-century Mayan Tomb of the Red Queen, which contains a sarcophagus and burial objects covered in bright red powder made from cinnabar. It was also well known that you could extract ‘quicksilver’ from cinnabar, a substance that ‘dissolved’ gold; theoretically, it could be used to extract the latter from other minerals – in particular, it would enable one to gather it more rapidly from river deposits in this way.

Except that the latter part wasn’t quite right. Cinnabar is mercury sulphide, and the mercury can be extracted by heating it and collecting the evaporated metal. (The chemical symbol Hg comes from the Greek hydrargyrum, meaning liquid silver.) However, gold doesn’t dissolve in liquid mercury – instead, the two metals amalgamate, forming a kind of compound, at an unusually low temperature. When you subsequently heat the mixture, the mercury evaporates leaving gold behind.

Mercury had a much better reputation in the past. The alchemists saw it as a primary form of matter upon which all other metals depended; the Romans and Greeks used it in medication and the Chinese believed that a mercury cocktail could guarantee long life.

Of course, we know now that mercury, the only metal which is liquid at room temperature, is toxic and that all these practices were sorely misguided. The Mad Hatter in Alice’s Adventures in Wonderland is inspired by the mental derangement caused by the use of mercury nitrate in hat production. And one of mercury’s most dangerous forms – methylmercury – can accumulate in fish, which will then make anyone who eats them very ill indeed.

Many former uses of mercury have been gradually phased out. In the past, you’d have found it in most thermometers, dental amalgams for fillings, fishing floats and paint pigments. It is still used in some chemical production methods today, but for the most part the allure that people felt towards this fascinating metal in the past has been replaced by great caution today.