Mercury

Mercury has more meanings than you can shake a stick at. Here are some of the more relevant and major examples.

The god
The original use of "Mercury" as a name was for the Roman version of the Greek god Hermes, who was the "Messenger of the Gods" and known for his great speed. Both the planet and the element (and, most likely, the make of Ford automobile) are named after the Roman god Mercury.

The planet
Main article: Mercury (planet)

Mercury is the planet that is closest to the Sun. It is also the smallest planet in the Solar System, now that Pluto has been demoted to reclassified as a "dwarf planet". Since it is the innermost planet, it has the quickest orbital period (88 days), and its surface temperature ranges between boiling and freezing; the former is due to the planet's close proximity to the Sun and the latter is due to the thin atmosphere letting heat escape, in contrast to Venus's very thick atmosphere which keeps much of the heat in. Most importantly, the orbit of Mercury cannot be accurately predicted using the laws of mechanics derived by Newton and it wasn't until Einstein's general relativity, which explains gravity as distortions in spacetime, that the orbit could be predicted accurately. Scientists had previously attributed a partially-molten core to be the source of the anomaly in Mercury's orbit.

The orbit of Mercury has the greatest eccentricity (0.2056, cf. Earth 0.017) of all the planets in the Solar System.

In terms of surface features, Mercury is a battered world covered by craters of every size, with some hints of (likely past) volcanism as well as some large rips that are likely the result of the planet contracting after formation.

Paradoxically, going to Mercury is hard despite what one could naively think (ie: simply hurl the probe towards the Sun and let Mercury catch it when the former is close to the latter) as one must cancel a whole lot of kinetic energy to enter into orbit, this explaining why to date just two space missions have explored the planet: the "Mariner 10" (which was just a fly-by that never entered Mercurial orbit) and "MESSENGER".

The music
Mercury, the Winged Messenger is the scherzo section of Gustav Holst's suite of tone poems, "The Planets".

"Mercury Poisoning" by Graham Parker

Also, of course, there is the lead singer of the band Queen, Freddie Mercury (RIP). He picked his name because his sun sign was Virgo with Mercury rising.

The element
The metallic element mercury — also known as quicksilver ("quick" in the sense of "living") — is unique in being a liquid metal at room temperature, which is defined as somewhere in the region of 295K. What makes this trait truly unique is not merely that it is a metal that is liquid at room temperature, but also that it does not "wet" glass (meaning it doesn't stick to glass as water does). Gallium is a metal whose melting point is almost exactly room temperature, allowing one to melt it with one's own hand's body heat, but it does wet glass, and it tends to supercool; also, like water, gallium expands slightly when it is solidified. Cesium will be liquid on a hot day or at body temperature above 301K, but reacts quite violently with water; bromine is also a liquid at room temperature but it is a halogen, not a metal, and like cesium it is quite reactive in its pure form. Mercury's atomic number is 80 and symbol is Hg after its Latin name of hydrargyrum, meaning "watery" (i.e. liquid) silver. It has been used extensively in thermometers, manometers (hence the pressure measurement of "millimeters of mercury" or "mmHg") and electrical switches. With moderate exposure it is also a neurotoxin, particularly when organic mercury compounds like methylmercury are formed. Historically, mercury compounds were used in hat making; the neurological effects of mercury exposure might have resulted in the expression "mad as a hatter".

However, there does seem to be a connection between mercury and homosexuality in aquatic birds. (Cue homophobic fundamentalists protesting the element, if they find out about this.)

The mercury militia (or, actual dangers vs. woo)
The "mercury militia" is a term coined by Orac of Respectful Insolence to describe a collective of quacks and their supporters who believe just about anything containing the slightest bit of mercury is toxic or deadly. Mercury is, of course, dangerous in certain doses. The operative word being "doses". Different chemical forms of mercury also have different levels of toxicity; elemental mercury is pretty toxic, and dimethylmercury is gram-for-gram one of the most poisonous substances known, but the mercury in thiomersal is relatively harmless.

The general logic of the "mercury militia" types is that mercury can be dangerous, therefore anything containing mercury is dangerous. If you grew up in a house with a mercury thermometer in the window, it's probably time to start guzzling chlorella supplements! This is generally a failure to understand the golden rule of toxicology: "The dose makes the poison" (or the dose-response curve, to be technically correct). This line of reasoning is common to many types of "chemophobia". Here is a brief overview of real dangers versus fear-mongering:

Real

 * Shattering mercury thermometers. Don't try this at home, kids.
 * Mercury emissions from power plants.
 * Mercury in fish. However, this is often blown out of proportion by quacks who insist nearly any consumption of fish will lead to mercury poisoning. In reality, the danger is related to what kind of fish you eat, how much of it you eat, and if your mother is pregnant with you while eating it (see the FDA guidelines below).
 * Broken CFL light bulbs. Another favorite of quacks, who will claim that using fluorescent bulbs will lead to mercury poisoning. This is false — the CFLs are only dangerous when broken, although as in many cases, the danger is in the dose, and the dose of mercury vapor from one or two broken CFLs really isn't enough to panic about most of the time. Fluorescent lights rely on a tiny amount of ionized mercury to conduct electricity through their tubes, although even in large, commercial-sized bulbs it's only a milligram amount. To give a sense of perspective, a mercury thermometer provides enough Hg for over 100 CFLs.
 * Batteries, certain electronic equipment, and certain antique devices.
 * is staggeringly toxic and can diffuse with ease through rubber gloves. Incredibly, it was considered as a possible rocket propellant in the 1950s, but its true danger wasn't revealed until the 1997 death of chemist from a tiny drop of the stuff. Fortunately, you're not likely to run into it outside of chem labs, and even those have mostly phased out its use, due to the aforementioned death.
 * The Mercury Maze. A freaking toy!
 * . It's worth noting that this was used in medicine up to the early twentieth century!
 * Vermilion, a red pigment, is made of mercury sulfide. This, alongside other pigments like paris green (arsenic), cadmium yellow, and galena (lead which was used for gray or black), means that many of the great paintings of the Renaissance are severely toxic.
 * Mercury nitrate, which was long used by milliners to turn rabbit hides into felt for hats. This is where the phrase "mad as a hatter" comes from.

Woo



 * Its use in a compound used to preserve vaccines, thiomersal, has led to one common element of vaccine hysteria, even though the compound has been shown not to be bioaccumulative and isn't used as extensively as anti-vax campaigners lead people to think.
 * Mercury amalgam fillings are the basis for most dental woo.
 * Mercury as a quack cure has been consigned to the dustbin of history for the most part, but it's still used in traditional Chinese (Qing Fen, 輕粉), Ayurvedic medicine, and curanderismo.
 * Mercury as a mainstream medical treatment for 2nd-stage syphilis, popular when syphilis was first introduced to the Old World near the end of the Renaissance. The cautionary saying went: "A night with Venus, a lifetime with Mercury."
 * Lactose pills made from that theoretically contain magic memories of mercury are used in homeopathy to treat "bone pains and right-sided face pain [with] a night-time aggravation."

Mercury and crankery
The mercury militia is responsible for a wide range of crank ideas purporting to diagnose and cure mercury poisoning. These include:
 * Filling extraction kits and methods (again, see dental woo).
 * Autism as a symptom of mercury poisoning.
 * Use of chelation for conditions that are not mercury poisoning, e.g. autism.
 * Use of hair analysis for diagnosing mercury poisoning.
 * Use of a "provoking agent" prior to urine testing to discover "hidden body stores". This temporarily raises heavy metals in the urine and jeopardizes the results of legitimate urine tests.
 * "Cleansing" methods such as colonic irrigation and detox diets. These diets often include algal supplements such as chlorella and or are a form of liquid fast such as the lemonade diet.
 * Lupron therapy (see also the Geier family).

There's also "red mercury", which is claimed to be a code name for a substance used in triggering compact nuclear devices. In actuality, it probably doesn't exist and was probably made up as bait to track people trying to obtain nuclear weapons materials on the black market. The name might have been inspired by mercury-containing substances like mercury fulminate, a contact explosive used in ammunition cartridges, and mercury (II) thiocyanate or "Pharaoh's Serpent", a substance with similar but more dramatic properties to black snake fireworks, which are generally considered a safer alternative. Whether the probably fictional "red mercury" is actually supposed to be a mercury compound is anyone's guess.

Some other fun
Ancient Chinese doctors believed that consuming mercury would prolong your life. Qin Shi Huang, the first emperor of the Qin dynasty, tried to achieve immortality by taking a concoction of mercury and powdered jade. Instead, he got severe mercury poisoning, his liver stopped working, and he eventually became brain-dead. Despite it being common knowledge that mercury elixirs were toxic, the practice continued for roughly two thousand years, with numerous emperors, nobles, scholars and other Chinese officials falling prey to