Magnesium

Magnesium was isolated by Humphry Davy in 1808, another product of his electrochemical campaign on the alkaline earths. Its name traces to Magnesia, a district of Thessaly in ancient Greece whose mineral wealth also lent its name to manganese and, through the lodestone, to the magnet. Two magnesia earths were distinguished by early chemists: magnesia alba, the white magnesium carbonate, and magnesia nigra, a black manganese ore. Magnesium is a light, silvery metal that burns with a brilliant white flame. The lightest of the common structural metals, it is used in aircraft and vehicle alloys and in flares and fireworks, while magnesium is also the atom at the centre of the chlorophyll molecule, making it indispensable to photosynthesis and thus to almost all life on Earth.

Because it burns so fiercely, hot enough to ignite underwater and too bright to look at directly, magnesium was adopted for military incendiary devices and flares in both World Wars, a stark counterpart to its gentler role lighting early photographic flashbulbs. In the body magnesium is the essential partner of ATP, the molecule that carries chemical energy in every living cell, so that the same element driving photosynthesis in plants is simultaneously required for hundreds of enzyme reactions in animals, making it one of a handful of elements indispensable on both sides of the food chain.

Magnesium sits at the centre of one of chemistry's tangled etymologies. The place-name Magnesia gave rise to three distinct words: magnet, from the lodestone said to be found there; manganese, from a black ore; and magnesium, from the white earth magnesia alba, long used medicinally as a laxative and antacid (milk of magnesia, Epsom salts). The element has no planetary or alchemical role of its own, being isolated only in the nineteenth century, but the mineral magnesia alba was a familiar apothecary's substance well before Davy's battery separated the metal within it. Magnesium's brilliant white flame later made it the light of early flash photography.

Epsom salts take their own name from the town of Epsom in Surrey, where a bitter mineral spring was discovered in the early seventeenth century and swiftly became a fashionable spa cure, its purgative and, later, supposed muscle-soothing properties promoted well beyond what evidence supports; magnesium sulfate baths remain a folk remedy today though the sulfate ion barely crosses the skin. Milk of magnesia, magnesium hydroxide suspended in water, is a still-familiar over-the-counter antacid and laxative, so that two of magnesium's oldest medicinal names, one a spa town and one a description of appearance, both survive in the modern pharmacy.