Short answers, pulled from the story.
Mercury is a chemical element with the symbol Hg and atomic number 80, commonly known as quicksilver. It is a heavy, silvery d-block metal and the only metallic element known to be liquid at standard temperature and pressure.
Mercury is called quicksilver because of its liquid, shiny, silver-like properties. The symbol Hg comes from hydrargyrum, a romanized form of the ancient Greek hydrargyros, a compound word meaning water-silver. The English name mercury comes from the planet Mercury, named for the Roman god of speed.
Exposure to mercury and mercury-containing organic compounds is toxic to the nervous system, immune system, and kidneys. Mercury vapor is the primary hazard of elemental mercury, while organic forms such as methylmercury are the most toxic and accumulate in fish through biomagnification. Symptoms include tremors, impaired cognition, sensory loss, and lack of coordination.
The Minamata disaster was the dumping of waste mercury compounds into Minamata Bay, Japan, between 1932 and 1968. It is estimated that over 3,000 people suffered deformities, severe mercury poisoning, or death from what became known as Minamata disease, caused by methylmercury concentrating in fish.
Mercury is used in thermometers, barometers, sphygmomanometers, fluorescent and mercury-vapor lamps, switches, and dental amalgam. It served as a propellant in early ion engines, as the medium in delay-line computer memory, and in the mercury cell process to produce chlorine and caustic soda. Many uses are being phased out due to toxicity.
Mercury was the first known superconductor, discovered in 1911 by Heike Kamerlingh Onnes. He achieved this by cooling mercury below 4 K, shortly after the discovery and production of liquid helium.
The phrase mad as a hatter comes from the felt-hat trade, where from the mid-18th to mid-19th centuries hatters used mercuric nitrate in a process called carroting. The toxic vapors caused psychological symptoms of mercury poisoning. The United States Public Health Service banned mercury in the felt industry in December 1941.