Elemental mercury is a slippery substance.
In the earth’s crust, it anchors itself by bonding with other elements, creating materials like the rough coppery rock cinnabar, a crystalline combination of mercury and sulfur. Once cinnabar, or other metallic ores, are mined and crushed, mercury can be easily extracted. Then the warmer above-ground temperatures, the decrease in pressure, cause pure mercury to become a very odd liquid metal.
Unlike a drop of water, a drop of mercury touched by a finger does not wet the skin. Instead, it breaks into smaller drops, tiny glittering balls that skitter away, that break into ever-smaller balls if touched again. That rapid scattering had prompted alchemists to nickname the metal quicksilver, and to formally name it Mercury, for the fleet-footed Roman messenger god.
In the periodic table of elements it’s called “Hg”, which comes from the Latin word hydrargyrum (liquid silver). The same silver-ball formation also explains why mercury is less acutely dangerous in its elemental form. Pure mercury is strikingly self-contained. High surface tension keeps the liquid balled up, preventing it from puddling outward like other liquids or from readily soaking into its environment. That same tension also keeps pure mercury from being easily absorbed by the body.
A few people, mostly in the mid-19th century, actually swallowed a gleaming cupful, believing it would cure constipation. The element apparently slipped right through; the mercury-drinkers showed no signs of acute sickness, although many complained of developing extremely sore mouths. It became obvious though, that even with such limited exposure, elementary mercury wasn’t entirely harmless. The mercury drinkers didn’t necessarily become sick immediately but some of them developed cancers, referred to as “mercurial tumors” in the medical textbooks
The element poses more direct risks when it bonds with other materials. Compounds such bichloride of mercury (HgCl2), once used as antiseptics, are no longer available as medicines because they proved so acutely poisonous. Essentially a mercury-loaded salt, the very structure of mercury bichloride – also called corrosive sublimate – allows it to be readily absorbed by the body.
The mercury risk we know best today is from a compound called methylmercury, which combines a methyl group (carbon attached to a cluster of hydrogen atoms) with a mercury atom. (The formula is CH3Hg+.) This is the form of mercury that results from burning of fossil fuels, like coal, which contain a mixture of natural occurring elements. It’s methylmercury that we’re now finding in the food chain, for instance in long-lived fish such as tuna and sea bass.
In the end, as we mix up mercury, it serves as another reminder that we’re constantly, inadvertently, running chemistry experiments. The planet becomes our laboratory and its lifeforms – including ourselves – the guinea pigs.